Comparative studies of structure, spectroscopic properties and intensity parameters of tetragonal rare earth vanadate nanophosphors doped with Eu(III)

Article abstract of DOI:10.1016/j.jallcom.2018.01.095

Hydrothermal method was applied in order to synthesize nanocrystalline YVO₄, LaVO₄ and GdVO₄ materials doped with Eu³⁺ ions. The conditions of synthesis were chosen to allow control of the process based on precipitation reaction in an autoclave, at elevated temperature and pressure. The prepared materials crystallized as single phase spherical-like nanocrystals of the tetragonal I4₁/amd structure. The average size of the particles was in the range of 7–10 nm in the YVO₄- and GdVO₄-based products and about 32 nm when LaVO₄ was the host compound. The excitation spectra of the materials prepared revealed a broad and intense band in the UV region. The band resulted from charge transfer phenomena: excitation of the VO₄³⁺ groups was followed by the energy transfer to Eu³⁺ ions. Intense, red emission of the samples was a result of electronic transitions in Eu³⁺ dopant ions. The theoretical Judd-Ofelt intensity parameters Ω_λ, obtained using the novel approach to the calculation of Eu-O bond stretching force constant and subsequently charge factors, were compared to the experimental Ω_λ. Forced electric dipole part of Ω_λ was calculated from scratch (using Eu³⁺ coordination geometry in REVO₄ from DFT calculations), while a single parameter in the dynamic coupling part was fitted to the experimental data. The issues related to the force constant calculation are discussed. Crucial influence of crystal lattice distortions on Ω_λ and Eu³⁺ emission intensities of the materials was shown.

Full text of DOI:10.1016/j.jallcom.2018.01.095

Accepted Manuscript
Comparative studies of structure, spectroscopic properties and intensity parameters
of tetragonal rare earth vanadate nanophosphors doped with Eu(III)
Tomasz Grzyb, Agata Szczeszak, Andrii Shyichuk, Renaldo Tenorio Moura, Jr.,
Albano Neto Carneiro Neto, Nina Andrzejewska, Oscar Loureiro Malta, Stefan Lis
PII:
S0925-8388(18)30096-3
10.1016/j.jallcom.2018.01.095
JALCOM 44559
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 24 September 2017
Revised Date: 5 January 2018
Accepted Date: 7 January 2018
Please cite this article as: T. Grzyb, A. Szczeszak, A. Shyichuk, R.T. Moura Jr., A.N.C. Neto, N.
Andrzejewska, O.L. Malta, S. Lis, Comparative studies of structure, spectroscopic properties and
intensity parameters of tetragonal rare earth vanadate nanophosphors doped with Eu(III), Journal of
Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.095.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Comparative studies of structure, spectroscopic properties and intensity parameters of
tetragonal rare earth vanadate nanophosphors doped with Eu(III)
Tomasz Grzyb,1 Agata Szczeszak,1 Andrii Shyichuk,2 Renaldo Tenorio Moura Jr.,3 Albano Neto Carneiro
Neto,4 Nina Andrzejewska,1 Oscar Loureiro Malta,4,5 Stefan Lis1*
1. Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614
Poznan, Poland
2. Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
3. Department of Chemistry and Physics, Center of Agrarian Sciences, Federal University of Paraíba,
Campus II − University city, 58397-000 Areia, PB, Brazil
4. Department of Fundamental Chemistry, Federal University of Pernambuco, Prof. Moraes Rego Ave.,
University city, CEP 50,10 740-560 Recife, PE, Brazil
5. Department of Chemistry, Federal University of Paraíba/CCEN, University city, João Pessoa, PB 58051,
Brazil
*E-mail: blis@amu.edu.pl
ABSTRACT
Hydrothermal method was applied in order to synthesize nanocrystalline YVO4, LaVO4 and GdVO4
materials doped with Eu3+ ions. The conditions of synthesis were chosen to allow control of the process
based on precipitation reaction in an autoclave, at elevated temperature and pressure. The prepared materials
crystallized as single phase spherical-like nanocrystals of the tetragonal I41/amd structure. The average size
of the particles was in the range of 7-10 nm in the YVO4- and GdVO4-based products and about 32 nm when
LaVO4 was the host compound. The excitation spectra of the materials prepared revealed a broad and intense
3+
band in the UV region. The band resulted from charge transfer phenomena: excitation of the VO4 groups
was followed by the energy transfer to Eu3+ ions. Intense, red emission of the samples was a result of
electronic transitions in Eu3+ dopant ions. The theoretical Judd-Ofelt intensity parameters ꢀλ, obtained using
the novel approach to the calculation of Eu-O bond stretching force constant and subsequently charge
factors, were compared to the experimental ꢀλ. Forced electric dipole part of ꢀλ was calculated from scratch
(using Eu3+ coordination geometry in REVO4 from DFT calculations), while a single parameter in the
dynamic coupling part was fitted to the experimental data. The issues related to the force constant calculation
are discussed. Crucial influence of crystal lattice distortions on ꢀλ and Eu3+ emission intensities of the
materials was shown.
Keywords: Rare earth vanadates, Hydrothermal method, Nanomaterials, Luminescence, Judd-Ofelt intensity
parameters, Crystal distortions
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1. Introduction
Rare earth orthovanadates (REVO4) are intensively investigated inorganic materials. The interest in the
properties of REVO4, in particular in their nanosized forms, has been evidenced in a number of papers
devoted to them and published over the last few years [1-12]. These materials have been found attractive in
many areas. REVO4 materials co-doped with luminescent lanthanide (Ln3+) ions, e.g. Eu3+ or Tb3+, can be
applied as phosphors for light emitting diodes (LEDs), transparent light-emitting layers for displays or
phosphors for other purposes [13-15]. Besides, not only down-conversion luminescence but also up-
conversion one can be obtained from REVO4 materials, when doped with near infrared absorbing Yb3+ ions
and luminescent, Er3+, Ho3+ or Tm3+ co-dopants [1,12,16-18]. Because of their properties, Ln3+-doped
REVO4 materials can be applied in biomedical applications, such as bioimaging, and in design of multimodal
probes or cancer therapy [5,19,20]. Another important area in which the properties of orthovanadates can be
utilized is forensic science, where luminescence signals help to image fingerprints [21].
Recently developed methods for the synthesis of REVO4-based materials allowed the preparation of
nanocrystalline materials with various morphology and surface properties [3,11,12,18,22-24]. Also,
advanced, multimodal, magnetic-luminescent nanomaterials often utilizing the core/shell structures were
based on REVO4 compounds [19,25,26]. Other fields in which REVO4 can be applied include photocatalysis,
production of ceramics, sensors, solar cells and mesoporous materials [4,6,27-30]. REVO4 nanoparticles
were found to be non-toxic, which is their additional advantage [9,19,31]. The studies of structural and
spectroscopic properties of REVO4 are important and of current interest [3,13,19,32,33].
Pure and nanocrystalline REVO4 can be obtained by sol-gel methods, co-precipitation in the presence of
organic compounds, by reverse micelle method or by solvo- and hydrothermal methods [4,7,20,31,34-37].
Usually, the obtained products require additional annealing at high temperature in order to properly
crystallize or to remove organic residues [27,34,37]. Hydrothermal methods could give the desired products,
but the reaction usually takes longer than several hours [31,36,38].
The most recent papers on REVO4 concern, among others, luminescent YVO4:Eu3+ shells on
SiO2 core (with reduced cost of the obtained phosphor in respect to a whole-YVO4:Eu3+ phosphors)
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[39], the YVO4:Eu3+ phosphor with photonic structure [40], and LaVO4:Eu3+ microspheres with a
single hole [41]. Several papers emphasize a significant role of the precursor solution pH in control
of the product morphology (using hydrothermal synthesis) [42,43]: LaVO4:Eu3+ microspheres with
a hole (pH=4), submicrometer-length nanowhiskers (pH=7), thick nanorods (pH=11) or bulky
microparticles (pH=13) can be obtained [43]. However, the hydrothermal method [41-49] remains
the most popular REVO4:Ln3+ synthesis route.
Herein, we present a way for the synthesis of rare earths vanadates by microwave-assisted hydrothermal
method, featuring a short reaction time, no need for pH adjustment and the use of rare earth acetates
(instead of typically used nitrates). The presented method resulted in single-phased nanocrystalline
products, and no additional annealing was necessary. The purity of the products resulted from
selected reaction conditions and the lack of additional substances such as organic co-reagents or
sodium. The presented method resulted in single-phased nanocrystalline products, obtained in a relatively
short reaction time. The as prepared products were as desired and additional annealing was not necessary.
The purity of the products resulted from the appropriately selected reaction conditions and from the lack of
additional substances such as organic co-reagents. The presented structural and spectroscopic results
revealed interesting behavior of Eu3+-doped REVO4, dependent on the host compound and concentration of
the dopant ions.
Despite the great interest in rare earths vanadates in the material sciences, not many theoretical studies
have been performed so far. Such analyses could be useful in improvement of the spectroscopic properties
REVO4-based materials. Therefore, it is important to study the behavior of luminescent dopant ions,
particularly Eu3+ ions, in order to better understand the basic processes in these materials, such as the
influence of nanostructure on defects and distortions within the Eu3+ surrounding. The effects of the type of
host compound and the concentration of Eu3+ dopant ions on final luminescence properties were studied. In
scientific literature, no detailed comparison has been made between the YVO4, LaVO4 and GdVO4 host
compounds doped with Eu3+ ions in wide range of concentrations. Previously, the REVO4:10%Eu
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phosphors (RE = Y, La, Gd) have been briefly compared [50], without any theoretical analysis of
their photoluminescence properties.
Noteworthy, LaVO4 can crystalize in either monoclinic (monazite-type, m-LaVO4) or tetragonal
(zircon-type, t-LaVO4) phase. While the t-LaVO4 is considered a better host for Eu3+ ions [43-45], it
is thermodynamically less stable (metastable) in respect to m-LaVO4 [44, 51]. Tetragonal LaVO4
can be obtained under elevated temperatures and pressures [51], as well as via hydrothermal route
[41], while increase in the Eu3+ dopant content results in stabilization of t-LaVO4:Eu3+ phase [51].
The rare earths tetragonal vanadates are also peculiar from the theoretical point of view. It has been
recently shown that highly symmetric environment of RE3+ ions in these materials must result in a very low
5
intensity of the hypersensitive D0-7F2 transition of Eu3+, which contradicts the experimental data [38]. In
5
particular, the D0-7F2 transition is the most intense in the emission spectra of REVO4:Eu3+. This
inconsistency leads to a conclusion that the local symmetry of Eu3+ coordination sphere in t-REVO4 must be
somehow reduced (considering the fact that X-ray diffraction, XRD data confirm the assumed crystal
structure). A possible explanation [37], assuming the occurrence of crystal distortions was proposed on the
basis of ab initio density functional theory (DFT) studies and from-scratch calculations of the Judd-Ofelt
intensity parameters ꢀλ. In the theoretical part of this paper, we present a further analysis of the subject and
propose a new numerical solution to the problem of decomposition of the force constants and charge factors
calculation.
Similarly, as in our previous paper [38], the theoretical analysis in this work was performed within the
Bond Overlap Polarizability Model (BOPM) [52]. In the model, the charge donation factor g (the one that
enters in the forced electric dipole part of transition rates) can be calculated from the elastic force constants
that correspond to the stretching of chemical bonds of the central Ln3+ ion to the ions in its coordination
sphere. As before, we used displacements of the central ion to obtain the force constants. Consequently, no
bonds other than Eu-O were involved. Displacement of an atom results in deformation of all of its bonds;
consequently, the second derivative of the energy of such displacement over the displacement would
correspond to collective-action (effective) force constants. In this paper, we propose an alternative (more
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robust and universal) way to decompose the effective force constants into force constants of individual
bonds.
2. Experimental
2.1. Synthesis
The starting materials, Y2O3, La2O3, Gd2O3 and Eu2O3 (Stanford Materials, 99.99%), were dissolved in
concentrated acetic acid (Chempur, pure p.a., Poland); 0.25 M RE(CH3COO)3 and 0.25 M Eu(CH3COO)3
solutions were obtained, respectively. NH4VO3 (REACHIM, 99%) was dissolved in distilled water and
prepared as 0.5M solution. Then, the stoichiometric amounts (based on the formula RE1-xEuxVO4, with x = 0,
0.005, 0.01, 0.02, 0.05, 0.1, 0.15 and 0.2) of appropriate acetate salts were mixed, and a 25 molar % excess
of the NH4VO3 solution (80oC) was added dropwise upon vigorous stirring. Finally, distilled water was
added to the above mixture to get 70 mL as the total volume of the solution (pH 6.5). The as-obtained
mixture was transferred to an autoclave, heated and held at 180oC for 3 h. Afterward, it was slowly cooled to
room temperature. The synthesized nanocrystals were collected by centrifugation and washed several times
with ethanol. The as-prepared yellowish powder was dried at 80oC for 8 h in air.
2.2. Apparatus
XRD patterns were recorded using a Bruker AXS D8 Advance diffractometer in Bragg-Brentano
geometry, with Cu Kα1 radiation (λ = 1. 5406 Å) in the 2Θ range from 6 to 60 degrees. Transmission electron
microscopy (TEM) images were recorded using FEI Tecnai G2 20 X-TWIN transmission electron
microscope which used accelerating voltage of 200 kV. Excitation and emission spectra were recorded on a
Hitachi F-7000 fluorescence spectrophotometer which was also used for radiative lifetime measurements at
liquid nitrogen temperature (77 K). All spectra were recorded for the dried samples and corrected for the
instrumental response. The presented excitation spectra were normalized for the maximum intensity, whereas
the emission spectra were normalized for the 5D0-7F1 transition band.
2.3. DFT calculations
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Cambridge Serial Total Energy Package (CASTEP) [53] plane-wave DFT code and Perdew-Burke-
Ernzerhof exchange-correlation functional were used [54]. Plane wave cutoff was set to 500 eV. The
calculations were performed using ultrasoft pseudopotentials supplied with the code. CASTEP was used to
optimize the crystal structure (unit cells) of tetragonal YVO4, LaVO4 and GdVO4. Full list of CASTEP
settings applied is given in the supplementary materials. Then, the optimized unit cells were replicated to
create 2×2×2 supercells. In the supercells, single Eu atom was introduced at the Y/La/Gd site, replacing the
original atom. Cell vectors of such structures were relaxed using CASTEP. Then, the relaxed supercells were
used to prepare input structures for the force constant calculations. Namely, the Eu atom was displaced along
each of its bonds to the coordination oxygens, by -0.1, -0.05, 0.05 and 0.1 A. This procedure resulted in the
total of 4ꢁ8 = 32 structures (plus one with zero displacement, common to all bonds) for each material.
CASTEP was used to calculate total energies, without any further geometry changes. The total energies were
plotted as a function of the bond lengths, for each bond. The functions E = aR2 + bR + c were fitted to the
data, for each bond. As the elastic force constant k equals d2E/dR2, the effective force constants, arising from
collective deformation of all Eu bonds upon its displacements (also referred to as the raw force constants
below), were obtained as kef = 2a.
2.4. Experimental Judd-Ofelt parameters and refraction indices
Although typically the refraction index is used in the form of a single value, it depends on the
wavelength. In the Ln3+ theoretical modeling, it enters the equations that connect the intensity parameters and
the actual transition intensities. In this paper, the experimental (phenomenological) Judd-Ofelt intensity
parameters ꢀλ were calculated using the traditional methodology [55]:
3hc3 A0
Ωeλxp.
=
λ
2
2
(1)
n0
(
n02 + 2
)
4e2ω 3
7 Fλ U ( 5D0
λ
)
λ
λ
9
5
where n0λ is the refractive index at the barycenter wavelength of the D0-7Fλ transition. Note that the
traditional approach uses simply n, independently of the wavelength. This modified approach results in ꢀλ
parameters that differ by several % from those obtained with a single value of n. The n0λ indices were
calculated using CASTEP and unit cells of the three vanadates in question. On the one hand, we could have
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used the refractive indices from literature. However, as one of the goals of this study was to compare the
three matrices, we decided to use the calculated values. In such a way, the error introduced by the refractive
index is systematically the same for all wavelengths and all the three materials.
The refractive indices were calculated (in the form of n as a function of wavelength) by Materials Studio
suite from dielectric functions calculated with CASTEP. Scissor operators were applied to match the
calculated bandgaps with the experimental data. In this way, we obtained the refractive indices that were
quite close to those reported in literature. Please refer to Supplementary materials for more details.
2.5. Force constant calculations
The recently proposed method for the force constant calculation was based on analytical derivatives of
projections of elastic forces that act on the central atom [38]. In the method, the bonds were treated as
mechanical springs. In the method presented here, the same considerations apply, while the method itself is
based on the idea of finite displacement. Namely, it compares the forces in the initial (equilibrium) state of
the central ion (point O, Fig. 1) and in the state in which the central ion is displaced by some lengths ꢂi along
the i-th bond (to the point Oi, Fig. 1).
In this method, the value of finite displacement is a constant parameter, ꢂ (in units of length, Å). For a
particular i-th ligand atom, the central ion is displaced towards it, along with the corresponding bond, by the
ꢁ
length of ꢂ. Such a displacement can be represented by a ꢀꢂvector, which has a length of ꢂ and is collinear
with the corresponding ꢃꢁ vectors:
ꢂ
ꢃꢁ
ꢀꢂ = ꢂ ⋅ ꢀ
ꢁ
(2)
ꢃ
ꢂ
Displacement of the central atom causes elastic reaction from all bonds, proportional to their
ꢁ
deformations. A change in the length of j-th bond, caused by ꢀꢂis given by:
ꢀji = ꢃ − ꢃ
(3)
ji
ꢄ
ꢁ
Here, rj is the equilibrium length of j-th bond, while rji is its length when ꢀꢂ is applied, and it equals to the
length of ꢃꢁ :
ji
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ꢁ
(4)
(5)
ꢃꢁ = ꢃꢁ − ꢀꢂ
ji
ꢄ
ꢁ
Let’s assume that elastic response of j-th bond to the ꢀꢂdisplacement is given by:
ꢁ
ꢁ
ꢅ = −ꢆꢄꢀji
ji
ꢁ
ꢁ
ji
The ꢀjivector must be collinear with ꢃꢁ . A decrease in rj (rji<rj) must result in ꢅ in the direction opposite to
ji
ꢁ
ꢃꢁ . Moreover, an increase in rj (rji>rj) must result in ꢅ towards the ligand atom Lj. Consequently:
ji
ji
ꢃꢁ
ꢃꢁ
ji
ꢁ
ꢀji = −ꢀ
ji ꢃ
=
ji ꢇꢃ − ꢃ ꢈ
(6)
(7)
ꢄ
ji
ꢃ
ji
ji
ꢁ
ꢁ
ji
Using ꢀji, a projection of ꢅ on ri can be found:
ꢁ
ꢁ
ꢀji ⋅ ꢃꢁ
ꢂ
ꢅ ⋅ ꢃꢁ
ji
ꢂ
ꢅꢉ =
= −ꢆꢄ
ji
ꢃ
ꢃ
ꢂ
ꢂ
ꢁ
Finally, the value of total force caused by ꢀꢂcan be described as:
ꢁ
ꢀji ⋅ ꢃꢁ
ꢂ
ꢅꢉ = ꢊ −ꢆꢄ
= −ꢆꢂefꢀꢂ
(8)
ꢂ
ꢃ
ꢂ
ꢄ
where kj are the searched force constants of the individual bonds. Note the minus signs before kj and kief in
equation (8).
As 1 <= i, j <= N, there would be N equations of this kind, with N components of the sum. The equations
make a set which can be solved for kj. We prefer using the matrix method, Ax = b. The values of ꢆꢂefꢀꢂform
the b vector, while the values of kj are obtained as x. The i,j-th matrix element of the coefficient matrix A is
ꢁ
ꢀji ⋅ ꢃꢁ ꢋꢃ ,
ꢂ
ꢂ
We used ꢂ = 0.05 Å, the same as the displacement step applied in the DFT calculations. Additionally,
both negative and positive values (-0.05 and 0.05) were used, in separate calculations, and the solution was
obtained as the mean of the two. The effective force constants were averaged for symmetric bonds. Check
the discussion section for details.
3. Results and discussion
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Hydrothermal method is a convenient way to prepare single phase product, and it allows control of
reaction conditions. It is especially important for materials which can crystallize in several phases, like
REVO4. Other widely used methods are the solvothermal method, solid state reactions, precipitation or sol-
gel method [20,27,28,34,56-59]. Application of the above mentioned methods for the synthesis of REVO4
usually resulted in getting microcrystalline materials. In our experiments, the appropriately chosen synthesis
conditions, such as pH, reaction time and reactants concentrations allowed the synthesis of nanocrystalline
materials. The applied initial reaction conditions prevented partial precipitation of products before
introducing the reactants in the hydrothermal reactor. In this way, the uncontrolled growth of nanocrystals
was suppressed. Another important factor was the use of ammonium vanadate as a precipitating agent
instead of sodium vanadate previously used for the reactions of this type [60]. In our opinion, sodium ions,
Na+, are able to incorporate into the crystal structure of the products, influencing final luminescence
properties. Hydrothermal method was used previously by Van Deaun et al. in order to synthesize monoclinic
microcrystalline LaVO4 (additional thermal treatment was applied to obtain a tetragonal structure) [32] by
Zhang et al. who obtained tetragonal LaVO4 microcrystals or by Dong et al. who synthesized tetragonal
nanoparticles of GdVO4 [31]. YVO4 was also synthesized by this method by Miao et al, however the
products were microcrystalline [17]. Herein we present another approach to REVO4 preparation, different
from the so far proposed ones [38].
3.1. Structure and morphology
The XRD patterns of our products are presented in Fig. 2a-c and compared with the database reference
by Joint Committee on Powder Diffraction Standards (JCPDS). The prepared REVO4 materials crystallized
in tetragonal crystal system with I41/amd space group. No effect of doping on the crystal structure was
observed in the used range of concentrations. The XRD patterns of YVO4 and GdVO4-based materials were
similar. They showed wide peaks, in contrast to LaVO4:Eu3+ samples exhibiting relatively narrow XRD
peaks. This suggests a formation of larger crystallites in LaVO4-based materials. The increase in number of
Eu3+ ions in LaVO4 host compound resulted in the formation of smaller nanocrystals, manifested as the
widening of XRD peaks with growing amount of Eu3+ ions in the structure (see Fig. 2b). As the conditions of
performed reactions were the same, only the properties of the chosen ions could have influenced the
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morphology of the obtained products. The La3+ ion has a larger ionic radius (in an eight-coordinated crystal
site) than the two remaining ions, Y3+ and Gd3+ [61]: r(Y3+) = 1.019 Å, r(La3+) = 1.16 Å and r(Gd3+) = 1.053
Å. The Eu3+ dopant ions are smaller than La3+, r(Eu3+) = 1.066 Å. Therefore, replacement of La3+ ions with
Eu3+ ones in the LaVO4 structure resulted in decreasing nanocrystals sizes. As a consequence of the largest
ionic radius of La3+ ions, the LaVO4 compound has the largest crystal cell volume of the three investigated
host compounds. The observed correlation between the crystal cell volume and the nanocrystalline size
confirms the assumption that crystal cell volume influences the nanocrystals size (see TEM images in Fig.
3). Moreover, the XRD patterns of the LaVO4:x%Eu3+ (x = 0-2) nanomaterials showed small additional lines
indicating the impurities in the samples. These lines correspond to the foreign phase, which is monazite-type
LaVO4 (JCPDS No. 53-0367). Besides the tetragonal space group, the LaVO4 often crystallize in the
monoclinic structure, which is also suitable for the Ln3+ ions' doping [62]. However, as the increase in Eu3+
content results in increased stability of tetragonal LaVO4:Eu3+ phase [51], the monazite-type impurity was
not detected in samples with more than 2% of Eu3+.
TEM images, presented in Fig. 3 are in accordance with the above discussed observations and confirm
the conclusions drawn from the XRD patterns. The prepared nanocrystals of REVO4 are very small, with the
average sizes not exceeding 10 nm for YVO4 and GdVO4 compounds. The histograms presented, also show a
narrow distribution of nanocrystals sizes. LaVO4 nanocrystals grow three times larger than those of YVO4
and GdVO4, with their diameter of around 30 nm. This was also reflected in their XRD patterns discussed
above. Doping with Eu3+ ions at the molar concentration of 5%, had a negligible effect on the size of
nanocrystals presented in TEM images. Smaller, YVO4 and GdVO4 nanocrystals were found to agglomerate
notably while LaVO4-based materials made relatively well separated nanocrystals. Probably, some of the
particles carry electric charge, and smaller particles undergo a strong mutual electrostatic attraction, while
the larger ones rather not. Larger LaVO4 nanocrystals have also much more defected surface in comparison
to those based on YVO4 and GdVO4 hosts.
The applied method of synthesis allowed a sufficiently good crystallization of products, which is
important for efficient luminescence. The degree of crystallization is visible in Figures 3b, d and f in which
crystal planes can be clearly identified. Also, XRD patterns with well separated and intense peaks confirm
this fact. Due to the good crystallinity of the as-obtained products, additional annealing in the oven was not
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required. It is known that high-temperature annealing usually results in the growth of nanocrystals and their
sintering. Annealing results in formation of nanocrystal agglomerates, which makes their subsequent post-
processing, e.g. surface functionalization or formation of colloids and suspensions, very difficult. Moreover,
it results in limited potential applications of the sintered nanopowders, as sintering excludes the use of the
materials in bioimaging and medicine, for which small sizes and modifiable surfaces are crucial.
3.2. Spectroscopic properties
Luminescent properties of REVO4:Eu3+ materials were analyzed by measuring excitation and emission
spectra presented in Figs. 4, 5 and 6 with the excitation spectra on the left and the emission spectra on the
right. Broad and intense bands appearing in the excitation spectra are related to the overlapped charge
3-
transfer (CT) bands: one from O2- to Eu3+ CT, and the other from O2- to V5+ CT (within VO4 group). Less
intense and sharp peaks at around 395 nm are the excitation bands from f-f transitions in Eu3+ ions. The
difference in the characteristics of these bands is caused by the mechanism responsible by excitation after
absorption of UV photons. Charge transfer transitions are allowed by the Laporte selection rules, whereas the
f-f transitions of Eu3+ ions are forbidden, therefore resulting in their low intensities. They can be observed
when Eu3+ ions are embedded in the medium in which the transitions are partly allowed by vibronic coupling
or via mixing of higher configurations into 4f wave functions by the crystal field effects [63]. Also, as 4f
electrons are efficiently shielded, the 4f-4f transitions are manifested by narrow, atomic-like spectral lines.
These wide and intense excitation bands can be divided into O2- to Eu3+ CT, visible at lower
wavelengths, and O2- to V5+ CT, at higher ones [64-67]. In fact, there are two kinds of transitions occurring
3-
in the VO4 group: the symmetry allowed one 1A1 → 1T2 (t1 → 2e) and the symmetry forbidden one 1A1 → 1T1
(t1 → 2e) [68-70]. The first one occurs at around 300 nm and is associated with an intense peak at lower
wavelengths, whereas the maximum of the band corresponding to the second transition should appear at
around 340 nm. However, the intensity of the second band is usually very low in comparison to that of the
first type transition. These three discussed transition bands are visible in the spectra measured for higher-
doped samples (see Fig. 4-6, left). The appearance of the symmetry-forbidden O2--V5+ CT band can be
attributed to a decrease in symmetry of the vanadate groups, which in practice means an increase in the
crystal lattice distortions at higher concentrations of the dopant. The maxima of CT bands are similar for the
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REVO4 host materials used and no significant differences resulting from the type of host compound can be
distinguished. However, some effects connected with increased concentration of Eu3+ ions can be observed.
It seems that the maxima of O2--V5+ CT shift towards lower energy side with increasing amount of Eu3+ ions
in the material. This observation applies to all three host compounds. Other effects of increasing
concentration of Eu3+ ions were the changes in the O2--Eu3+ CT bands, i.e. their increasing intensity,
especially visible in Fig. 5 (left), and a shift towards higher wavelengths, which affect the shape and
characteristics of the recorded bands. The two kinds of excitation peaks start to split at higher concentrations
of Eu3+ ions. The observed changes can be related to decreasing size of nanocrystals with growing amount of
Eu3+ in structure, especially visible in the LaVO4 host material (decreasing size of nanocrystals is also visible
in the XRD patterns, in the Fig. 2).
Luminescence spectra collected for the Eu3+-doped REVO4 materials are presented in the right hand part
of Fig. 4-6. Red emission was observed under UV excitation. The presented spectra were similar as the
structure of REVO4 host materials is almost the same. The scheme of Eu3+ ions energy levels with observed
transitions is presented in the Supplementary materials (Fig. S1). The main emission band was the same for
5
all samples, and it was related to the D0-7F2 transition (peak centered around 620 nm). The other emission
5
7
7
7
bands were related to transitions from the D0 excited state of Eu3+ to F0 (588 nm), F1 (595 nm), F3 (652
nm) and 7F4 (at around 700 nm). At a lower wavelength (539 nm) also the transition from the higher excited
state of Eu3+ ion can be observed: 5D0-7F1. The observation of transitions from the 5D1 excited state suggests
that the multiphonon quenching of Eu3+ ions in their 5D1 excited state and cross-relaxation between Eu3+ ions
5
are low. The lowest intensity was observed for the D0-7F0 emission band, which is strictly forbidden
according to the Judd-Ofelt theory [71, 72]. However, this transition can be observed because of the J-
mixing effects [63]. These effects are much more efficient in the YVO4:Eu3+ materials. In the remaining two
groups of products, the 5D0-7F0 transition is less visible.
5
The D0-7F1 transition is usually the most intense in the luminescence spectra of materials with a
centrosymmetric crystal structure [29]. In tetragonal REVO4 materials, the Eu3+ ions occupy sites of
D2d symmetry without an inversion center and of relatively high symmetry [26]. At the same time, the
5
hypersensitive, electric dipole D0-7F2 transition band is much more intense than the band corresponding to
5
the magnetic D0-7F1 transition (and the most intense of the emission bands), which usually indicates a low
12

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5
local symmetry. According to our theoretical predictions, the D0-7F2 transition band intensity must be very
low – assuming the database geometry. This issue will be discussed below in more details. The intensities of
5
5
5
the remaining two transition bands: D0-7F3 and D0-7F4 were low, which is quite typical. The D0-7F3
transition is forbidden by the Judd-Ofelt theory, and its presence is a manifestation of J-mixing effects. The
5
5
electric dipole D0-7F4 transition band shows intensity similar to that of the D0-7F1 transition band which is
typical of tetragonal REVO4 materials [29, 37].
Changes in the ratio of 5D0-7F1 and 5D0-7F2 transition bands can give information about alterations in the
Eu3+ coordination environment. In the prepared samples, this ratio was similar for all concentrations of the
Eu3+ ions used in particular host compounds. It confirms good incorporation of the dopant ions into the
REVO4 structure. No influence on the Eu3+ local environment was observed with increasing amount of these
ions in the material.
The main difference between the presented emission spectra is a slightly different character of the
emission peaks in the spectra of LaVO4 host compound (Fig. 5, right). In comparison to the LaVO4:Eu3+
materials, smaller nanocrystals of the YVO4 and GdVO4-based materials influenced the coordination of the
Eu3+ ions surrounding. The local environment of Eu3+ ions is much more distorted in smaller nanocrystals, in
particular for the Eu3+ ions placed on the surface of nanocrystals than for ions inside them and results in an
increased nephelauxetic effect. In larger, LaVO4:Eu3+ nanocrystals this effect was less prominent. Therefore,
the peaks observed in Fig. 5, on the right, are narrower, and their splitting is more pronounced.
Fig. 7 presents the effect of dopant ion concentration on the emission intensity for the three host
materials. The samples YVO4:x%Eu3+ and GdVO4:x%Eu3+ show the most intense luminescence. The
increase in Eu3+ ions concentration changed the emission intensity in a different way for each series of
nanophosphors. The luminescence intensity of the YVO4:x%Eu3+ nanocrystals is growing until 2% of Eu3+
ions concentration than a second maximum occurs at the Eu3+ ions concentration of 20%. For the samples of
LaVO4 :x%Eu3+ an increase in the luminescence emission is observed up to 20% of the dopant concentration.
The maximum of the GdVO4:x%Eu3+ emission is found for 15% Eu3+ concentration. It must be noted that
some of the LaVO4:Eu3+ samples were contaminated with another phase, probably monoclinic LaVO4. Low
phase purity of the LaVO4:Eu3+ samples was most likely responsible for their significantly lower emission
13

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intensity. On the other hand, considering the similarities of the spectra in Fig. 4-6, the Eu3+ ions in tetragonal
LaVO4 phase are responsible for the observed emission.
3.3. Force constant calculation
Although the described approach to force constant calculation is universal (that is, it must work for any
number of bonds and any coordination geometry), in the case of high local symmetry, there might be a
problem to solve the equations. The problem is manifested as zero determinant of the coefficient matrix
when symmetric bonds are present. By symmetric bonds we mean such that are characterized by the same
length, force constant and the same set of projections onto the other bonds. Each of such bonds is represented
by a separate equation in the set, but it does not bring any new information, which results in the degeneracy
of the set. In our previous paper, we have solved this issue via reduction of the set down to several equations
in such a way that each one represented a unique bond. The method presented here offers a significant
improvement: the reduction of the set is no longer required. In order to calculate the matrix elements, the
central ion is displaced along each bond by a certain distance ꢂ, which numerically reduces the degeneracy
in the matrix. In other words, the bonds that were the same become not exactly the same as the central ion is
displaced. Consequently, the determinant of the matrix becomes non-zero (although remains quite small) and
the set becomes solvable. In the range of tested values of ꢂ from 0.00001 to 0.1 Å, the determinant changed
from about 5×10-56 to about 5×10-12 and had the sign opposite to that of ꢂ. It is interesting to note that the
negative or positive ꢂ of the same absolute value resulted in slightly different force constants. Thus, we
calculated k as the mean of k+ and k-, where k+ and k- were the solutions with positive and negative ꢂ,
respectively. Although the cost of this improved methodology was the introduction of a fit parameter,
namely ꢂ, the resulting solutions (with precision up to 3 significant digits) did not depend on ꢂ in the range
0.0001Å <= ꢂ <= 0.1Å. In other words, the introduction of the parameter did not create any significant
uncertainties.
The raw effective force constants obtained from DFT were contaminated by computational artifacts,
which means that although the oxygens of the coordination surrounding were crystallographically equivalent
3-
and belonged to the same chemical unit, VO4 , the calculated kef differed slightly. The atoms of the
surrounding can be divided into two groups (each of four atoms) according to the bond length: Group 1
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contains the atoms O2, O3, O4 and O5, with the Eu-O distance 2.583Å (DFT-optimized value); Group 2
contains atoms O6, O7, O8 and O9, with the Eu-O distance 2.520-2.521 Å (DFT-optimized value); central
atom label is Eu1; refer to Table 1 for details. It is reasonable to assume that bonds of the same length must
be characterized by the same force constants. As one can see from Table 1, the values of kef within the groups
are indeed closer to each other than between the groups, and the group mean values are significantly
different. However, inequalities within the groups are clear.
In the previous approach, we have used the group mean values of kef. In this approach, we tested both
options, either group mean or raw values. If the raw values were used, the obtained k were characterized by a
greater coefficient of variation (in Group 1) than the kef (within the group), namely 11.8% against 2%. Next,
the values of k+ and k- were extremely high at low ꢂ (up to 70000 eV/Å2 in some cases), while the values of k
= (k+ + k-)/2 were reasonable. Consequently, the group mean values of kef were used. It is interesting that the
values of k and ks were characterized by the same mean. We would like to emphasize here, that the need of
using group mean instead of raw kef values is not a drawback of the approach to force constant calculation,
but the drawback of the DFT calculation. One would expect the same values of kef for bonds of the same
length between atoms of the same type.
In Table 1, the raw effective force constants are displayed as kef, the corresponding force constants of
individual bonds are k, the symmetrized effective force constants (which are basically group means of kef) are
ksef, and the corresponding individual force constants are ks; the ks values were used in the calculations
discussed below.
3.4. Intensity Parameters
The Judd-Ofelt theory is a traditional theoretical framework used to characterize 4f-4f transitions of Ln3+
ions in a crystal field [55]. For a given ion and its chemical surrounding, intensities of the electric dipole
transitions are represented by only three parameters ꢀλ, λ = 2,4,6. In particular, integrated coefficient of
spontaneous emission of a transition between two manifolds J and J' via electric dipole mechanism is given
by [55]:
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4e2ω3
3hc3
n
(
n2 + 2 2
)
1
2
ED
JJ
(λ )
′
(9)
A
=
Ωλ J U
J
′
∑
9
2J +1
λ
Here, e is an elementary charge, ω is the transition angular frequency, ħ is Plank constant over 2π, c is the
speed of light, n is a refraction index of the material and the term in brackets are doubly reduced matrix
elements of Racah unit tensor operator U of rank λ. The intensity parameters can be obtained as a sum of Bλtp
crystal filed parameters (forced electric dipole and dynamic coupling parts), which depend on composition
and geometry of the central ion chemical surrounding [55]. One of the approaches to the calculation of those
parameters is based on analysis of polarizability of the electronic density of the central ion chemical bonds to
the ligand atoms (in such a way covalence effects are included) and polarizability of ligand atom core
orbitals [38].
A novel extension to the classical Judd-Oflet theory, where the overlap region is explicitly accounted for
as a surface contact between the lanthanide and the ligand ions [52] has been already applied to the
LaVO4:Eu3+ crystals [55]. Theoretical calculations of the Judd−Ofelt intensity parameters require a series of
input data [55]. Namely, the required data include the energy difference of the f-d configurations of the
lanthanide ion, overlap integrals between 4f valence orbitals of the central ion and the valence shells of the
ligated ions, the coordination geometry of the emission center, the overlap polarizabilities αOP [52], ligand
core effective polarizabilities α′, [52] and charge factor g parameters. Most of the required input data can be
obtained from literature, while the methodology [52] to calculate αOP and g was presented above. In this
paper, we introduce another approach to the calculations of bond force constants which are required to
calculate g [52]:
ꢆꢂ
(10)
ꢎ
ꢌꢂ = ꢍꢂ
2ꢀꢏꢂ
Here, i is an index of a ligand atom, R is bond length, k is the force constant and ꢂε is the first excitation
energy associated with the chemical bond (within the diatomic-like partitioning approximation). In this
study, we used the same software (CASTEP) and the same method to optimize the cell parameters of the
samples and to obtain the force constants. Consequently, the geometries from DFT calculations were used in
the intensity parameter calculations The details of ꢀλ, αOP and g values calculations are extensively discussed
in our previous paper [38].
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The ligand core effective polarizabilities (α′) presented in Table 2 were fitted to minimize the difference
between the theoretical and experimental ꢀλ. Although the Ln–O distances are slightly different, the oxygen
3-
ions are crystallographically equivalent and belong to the same chemical unit VO4 . The coordination
polyhedron symmetry is very close to D2d, which is quite high. If all of the coordination oxygen atoms were
considered equivalent, with the same α′ values, the value of the ꢀ2 parameter would be very low (∼0.05ꢁ10-20
cm2). On the contrary, the experimental ꢀ2 is two orders of magnitude higher. Therefore, in order to make it
possible to reproduce the experimental values of ꢀ2 and ꢀ4, it is necessary to break the symmetry of oxygen
atoms in the coordination polyhedron. This can be made in two ways. On the one hand, it is possible to
consider that there are more than one set of different coordinated oxygens, on the basis of the Ln–O slightly
different distances. If so, the fitting procedure is realized using more than one set of α′. The fitting results for
this case are shown in Table 2 for the studied systems. It can be noted that, even for very small distance
differences, the α′ are unexpectedly different, given that all oxygen atoms are equivalent and belong to VO4
unit. As it can be seen in Table 2, the highest α′ coefficient of variation (between oxygen groups) is obtained
for YVO4:Eu3+ (51.3%), while the difference in the Eu–O distances is only about 0.06 Å. The second largest
variation between the oxygen groups α′ is observed for the LaVO4:Eu3+ system, followed by GdVO4:Eu3+. It
is also observed that the α′ mean values follow the opposite trend, and increase in the following order:
YVO4< LaVO4< GdVO4.
The αOP values follow the expected variation with distance and are in the same range as reported
elsewhere for the compound of this kind [38]. The charge factors, which were obtained from the solid-state
force constants calculations, are in the accepted range and increase in the order YVO4< LaVO4< GdVO4.
Another way to break the symmetry in the REVO4:Eu3+ systems is by introducing random displacements
¯
in oxygen positions and then obtaining the final ꢐꢑas an average. Different configurations can be obtained
ꢒ
ꢖ
from the experimental or equilibrium geometry, where the positions ꢃꢁ = ꢓꢂ, ꢔꢂ, ꢕꢂ of each atom in the first
ꢂ
coordination sphere are allowed to vary randomly, within a range of certain maximal displacement χ applied
¯
ꢒ
ꢖ
to each ꢓꢂ, ꢔꢂ, ꢕꢂ coordinate, i.e. |ꢂxi|, |ꢂyi|, |ꢂzi| <= χ. In this way, the average intensity parameters ꢐꢑare
defined as follows:
17

ACꢜCEPTED MANUSCRIPT
ꢒ
ꢖ
ꢐꢑ ꢃꢁ , ꢃꢁꢘ, ꢃꢁꢙ,..., ꢃꢁꢚ ꢂ
ꢗ
¯
(11)
ꢐꢑ = ꢊ
ꢛ
ꢂ
¯
Whereꢐꢑare the intensity parameters calculated for each configuration i with slightly different positions of
ꢒ
ꢖ
the N coordinated atoms ꢃꢁ , ꢃꢁꢘ, ꢃꢁꢙ,..., ꢃꢁꢚ . The summation runs over Q configurations in which each x, y, z of
ꢗ
the ꢇꢃꢁ ...ꢝꢈ coordinates differs from its equilibrium value by a random value within ±χ range. Increasing χ
ꢗ
means larger displacements of the atom positions from the initial (experimental or calculated) geometry,
¯
meaning more symmetry reduction in the ꢐꢑ average calculation.
¯
¯
¯
As it can be seen in Figs. 8-10, both ꢐꢘ and ꢐꢞ increase with increasing χ, with ꢐꢘ being the most
sensitive to the increase in χ. Besides, it is important to highlight that the optimal value of displacement
parameter χ decreases in the order YVO4:Eu3+> GdVO4:Eu3+> LaVO4:Eu3+, with the highest value for
YVO4:Eu3+. Such a trend is consistent with the other data shown in Fig. 2-6. The XRD peak width decreases,
while the crystallinity of the products and nanoparticle size increase: 7-8 Å for YVO4:Eu3+, 9-10 Å for
GdVO4:Eu3+ and 31-32 Å for LaVO4:Eu3+. The value of the emission peak width, in LaVO4:Eu3+ it is the
smallest, which can also be attributed to higher crystallinity. These results, together with Table 3 data,
emphasize that it is important to obtain the intensity parameters as an average for the materials of this type.
In other words, strong deviations from the database geometry (crystal distortions) must be introduced to the
geometry in order to match theoretical and experimental intensity parameters. This result suggests that the
real nanocrystals must be significantly distorted. Surprisingly, such distortions would be manifested in XRD
in no other way than the noise [38]. Considering the real materials presented here, one can notice both
relatively wide peaks in XRD and quite smeared emission peaks, which supports the hypothesis of distorted
crystals. However, the strongest argument is the following. Irrespective of the parameters in the model, bare
coordination polyhedron from the database geometry corresponds to very low ꢀ2; such is the nature of
5
interaction of the 4f electrons of Eu3+ with the crystal field in the described materials. No D0-7F2 emission
5
should be expected – but it is still there, with the D0-7F2 peak being the most intense. Consequently, the
original symmetry must be distorted and reduced, in such a way that a low-symmetry coordination geometry
is obtained. Random displacements of the atoms, that mimic a distorted crystal, seem to be a reasonable
5
explanation. Noteworthy, the same property of the D0-7F2 transition is manifested in a single crystal
REVO4:Eu3+ [69, 73, 74], even at very low temperature of 15 K [73], which means that crystal distortions are
18

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an intrinsic property of the REVO4 crystal lattice, unrelated to the nanoscale size of the particles and
temperature.
The results given in Table 3 were obtained assuming the values of α′ to be intermediate between the
values in Table 2 and those obtained by breaking the symmetry by means of χ. Analysis of the data in Tables
2 and 3 reveals that both cases are extreme situations of symmetry breaking, namely by freezing the ligand
atoms positions while allowing its α′ to vary in groups or by constraining the ligand atoms α′ to be the same,
while allowing changes in their positions. In the third approach, variations in both α′ and atomic positions
were applied in the fitting procedure; simultaneous symmetry reduction by means of changes in the effective
polarizability α′ and the increase in χ displacements was analyzed. The calculations were performed using an
external algorithm, a part of the JOYSpectra program, that performs a scan procedure modifying the values
¯
of α' for each value of χ and calculating the ꢐꢑvalues for each point. As an example, for the compound
YVO4:Eu3+, the calculation was carried out with χ = 0.01 Å changing α′ by a fixed step value for the two
ligand atomic groups (α′1 and α′2) in order to move away from the average value (~1.87 Å3 in this case). In
this way, for χ = 0.01 Å the values of ꢐꢘ,ꢞ were calculated with α′1 = 1.89 Å3 and α′2 = 1.85 Å3, then with α′1
¯
= 1.91 Å3 and α′2= 1.83 Å3 and so on. The results of these calculations are presented in Figs. 11-13, where
the surfaces were obtained by distance weighted least squares method adjustments for the 1000 points with
the smallest errors. Figs. 11-13 show the behavior of the relative error f(error) (Eq. 12) between the theoretical
¯
ꢐꢑ and experimental ꢀλ with respect to the α′1 – α′2 difference (ꢂα') and the displacement χ.
¯
ꢐꢑ,theo − ꢐꢑ,exp
ꢟ
ꢒ
= ꢊ ꢠ
ꢖ
ꢠ
(12)
error
ꢐꢑ,exp
ꢑꢡꢘ,ꢞ
¯
In equation 10 the value of ꢐꢑ,theois the average of 20000 random configurations and ꢐꢑ,exp are the
experimental intensity parameters.
Despite the fact that the minimal error points in Figs. 11-13 correspond to quite a high ꢂα', it is very
clear from these figures that the assumption of larger distortions results in significantly lower and more
realistic ꢂα' of about 0.1, while the applied distortions still remain acceptable. The oxygen ions in the two
groups are crystallographically identical and only differ in the RE-O difference; the ratios of the distances
are 0.97:1 in YVO4, 0.98:1 in LaVO4 and 0.99:1 in GdVO4; consequently, it is reasonable to expect the ratio
19

ACCEPTED MANUSCRIPT
of α'1: α'2 to be of 0.99:1 – 0.9:1 rather than 2:1. Therefore, a more significant mechanism of symmetry
reduction is that involving random distortions.
The results of the calculations presented in Figs. 11-13 are summarized in Table 4, which shows that the
displacement
χ
increases in the following order: YVO4:Eu3+< GdVO4:Eu3+< LaVO4:Eu3+. It is interesting to
highlight that this order follows the trend in the ionic radii of the ions Y3+ (1.019 Å), Gd3+ (1.019 Å) and La3+
(1.16 Å). Although the ionic radii do not reflect directly the Eu–O distances obtained for the studied
materials (Table 2), it is expected that more compact crystal structures (with smaller ions) lead to smaller
dopant displacements.
¯
Tables 4 and 5 summarize the information on the highlighted points in Figs. 11-13, where ꢐꢑare very
close to the experimental ones.
4. Conclusions
Nanoparticle luminescence materials based on YVO4, LaVO4 and GdVO4 doped with Eu3+ ions were
synthesized under hydrothermal conditions assisted with microwave radiation. Properly chosen synthesis
conditions: pH, temperature, type of Ln3+ salts used and synthesis duration resulted in crystalline nanosized
phosphors that did not require further calcination. For this reason, the nanophosphors synthesized exhibit
promising morphological properties, such as good homogeneity and dispersity. It has been proved that
REVO4 type compounds are suitable host for the red emitting Eu3+ ions and maintains its crystal structure
even at high concentration of the dopant ions. All the samples presented very good physicochemical
properties and are most promising for further studies e.g. for biomedical applications, in which pure, small
and homogenous nanocrystals with intense visible emission are needed.
Theoretical calculations of Judd-Ofelt intensity parameters ꢀλ indicate inconsistencies between the
intense 5D0-7F2 emission of the materials and relatively high D2d local symmetry of RE sites in the materials.
It was shown that random displacements of atoms in the materials can reduce local symmetry and
5
7
¯
significantly increase the ꢀ2 parameter, corresponding to the D0- F2 emission peak. The theoretical ꢐꢑ were
calculated as means from thousands of different randomly distorted geometries. It is highly probable that
20

ACCEPTED MANUSCRIPT
such an approach approximates and describes distortions of atomic positions that occur in real materials.
Within such an approach, the value of random displacement magnitude χ required to reproduce experimental
ꢀ2 decreases (i.e. assumed crystal distortions decrease) in the order YVO4:Eu3+, GdVO4:Eu3+, LaVO4:Eu3+. It
can be concluded from both emission spectra and XRD patters that materials crystallinity increases in the
same order, indicating a good agreement between theory and experiment.
An alternative way to reduce symmetry is to introduce different ligand effective polarizabilities (α′) for
the two kinds of oxygen ions present in the material. Applying of symmetry reduction, experimental and
theoretical ꢀλ can be matched. Finally, both approaches were applied simultaneously. It was shown that the
difference in α′ and the maximal distortion length χ were correlated: higher χ led to smaller ꢂα' and vice
5
versa. Consequently, both mechanisms can be responsible for the observed intense D0-7F2 emission;
however, as high ꢂα' are unlikely, random distortions seem to play a more important role.
Acknowledgments
Funding for this research was provided by the Polish National Science Centre in the form of grant No.
2016/21/B/ST5/00110. Andrii Shyichuk gratefully acknowledges the Polish National Science Centre for the
support provided within grant No. 2013/08/T/ST5/00490. All of the DFT calculations were performed using
the resources of Wroclaw Centre for Networking and Supercomputing, which have also provided access to
Materials Studio suite and CASTEP (computational grant number 300).
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Figure captions
Fig. 1. Finite displacement of Ln atom along its bond to i-th ligand atom by ꢂ distance.
Fig. 2. XRD patterns of REVO4 nanomaterials synthesized by the hydrothermal method: (a) YVO4:x%Eu3+,
(b) LaVO4:x%Eu3+, (c) GdVO4:x%Eu3+.
Fig. 3. TEM images and nanocrystals size distributions of prepared products: (a) YVO4, (b) YVO4:5%Eu3+,
(c) LaVO4, (d) LaVO4:5%Eu3+, (e) GdVO4, (f) GdVO4:5%Eu3+.
Fig. 4. Excitation (left) and emission (right) spectra of the YVO4:x%Eu3+ samples measured at 77 K.
Fig. 5. Excitation (left) and emission (right) spectra of the LaVO4:x%Eu3+ samples
measured at 77 K.
Fig. 6. Excitation (left) and emission (right) spectra of the GdVO4:x%Eu3+ samples
measured at 77 K.
Fig. 7. Comparison of the overall luminescence intensity of the REVO4:Eu3+ samples and influence of the
Eu3+ concentration on the emission intensity; lines are B-splines.
3+
¯
¯
Fig 8. Trends inꢐꢘand ꢐꢞ(a) as a function of the displacement parameter χ for YVO4:Eu system. The
theoretical values for the displacement χ = 0.30 Å matching the experimental data are detached.
Superposition of some configurations of coordination polyhedron is shown (b), where oxygen atoms (2−9)
are colored in red and transparent gray are random configurations.
3+
¯
¯
Fig. 9. Trends in ꢐꢘand ꢐꢞ(a) as a function of the displacement parameter χ for LaVO4:Eu system. The
theoretical values for the displacement χ = 0.22 Å matching the experimental data are detached.
Superposition of some configurations of coordination polyhedron is shown (b), where oxygen atoms (2−9)
are colored in red and transparent gray are random configurations.
3+
¯
¯
Fig. 10. Trends inꢐꢘand ꢐꢞ(a) as a function of the displacement parameter χ for GdVO4:Eu system. The
theoretical values for the displacement χ = 0.235 Å matching the experimental data are detached.
Superposition of some configurations of coordination polyhedron is shown (b), where oxygen atoms (2−9)
are colored in red and transparent gray are random configurations.
Fig. 11. Trends in f(error) for the YVO4:Eu3+ system; the highlighted point (0.065 Å; 1.32 Å3) represents the
best value of f(error) (0.0094) relative to the displacement χ and the polarizabilities differences ꢂα'.
Fig. 12. Trends in f(error) for the LaVO4:Eu3+ system; the highlighted point (0.125Å; 1.44 Å3) represents the
best value of f(error) (0.1185) relative to the displacement χ and the polarizabilities differences ꢂα'.
Fig. 13. Trends in f(error) for the GdVO4:Eu3+ system; the highlighted point (0.1 Å; 0.88 Å3) represents the best
value of f(error) (0.0190) relative to the displacement χ and the polarizabilities differences ꢂα'.
28