One-step surfactant-free synthesis of KSc2F7 microcrystals: controllable phases, rich morphologies and multicolor down conversion luminescence properties

Article abstract of DOI:10.1039/c8ce00716k

Homogeneous KSc₂F₇ microcrystals with controlled and abundant morphologies were prepared by a one-step hydrothermal route without employing any templates or surfactants. Particularly, uniform and well-defined KSc₂F₇ crystals with rod, sheet, flower-like, thin rectangular, xylitol-like and thin plate morphologies were achieved by manipulating the different environments of crystal growth, such as increasing the concentration of F^-, changing pH and adding different surfactants. In addition, a phase transformation from monoclinic ScF₃ to orthorhombic KSc₂F₇ was observed at different molar ratios of F/Sc and pH values, respectively. A more interesting thing is that the KSc₂F₇ crystals doped with 5% Ln³⁺ (Ln = La, Ce, Sm, Eu, Gd, Tb and Lu) showed rich morphologies and various sizes. The obtained KSc₂F₇:Ln³⁺ products showed multicolor emission with red (Eu³⁺), green (Tb³⁺) and blue (Ce³⁺), respectively. And the best emission conditions for KSc₂F₇ crystals are F/Sc/K = 3.5:1:3, pH = 7 and without any surfactants. The synthetic KSc₂F₇:Eu³⁺/Tb³⁺/Ce³⁺ phosphors may potentially be used for multicolor illumination and displays.

Full text of DOI:10.1039/c8ce00716k

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One-step surfactant-free synthesis of KSc2F7
microcrystals: controllable phases, rich
morphologies and multicolor down conversion
luminescence properties†
Cite this: DOI: 10.1039/c8ce00716k
a
b
a
a
a
*
Jie Xiong, Jie Yu, Yufeng Zhang, Wenpeng Xia, Shanshan Hu,
Yanfei Zhangc and Jun Yang
a
*
Homogeneous KSc2F7 microcrystals with controlled and abundant morphologies were prepared by a one-
step hydrothermal route without employing any templates or surfactants. Particularly, uniform and well-
defined KSc2F7 crystals with rod, sheet, flower-like, thin rectangular, xylitol-like and thin plate morphologies
were achieved by manipulating the different environments of crystal growth, such as increasing the con-
centration of F−, changing pH and adding different surfactants. In addition, a phase transformation from
monoclinic ScF3 to orthorhombic KSc2F7 was observed at different molar ratios of F/Sc and pH values, re-
spectively. A more interesting thing is that the KSc2F7 crystals doped with 5% Ln3+ (Ln = La, Ce, Sm, Eu, Gd,
Tb and Lu) showed rich morphologies and various sizes. The obtained KSc2F7:Ln3+ products showed
multicolor emission with red (Eu3+), green (Tb3+) and blue (Ce3+), respectively. And the best emission con-
ditions for KSc2F7 crystals are F/Sc/K = 3.5 : 1 : 3, pH = 7 and without any surfactants. The synthetic KSc2F7:
Eu3+/Tb3+/Ce3+ phosphors may potentially be used for multicolor illumination and displays.
Received 3rd May 2018,
Accepted 1st June 2018
DOI: 10.1039/c8ce00716k
rsc.li/crystengcomm
decrease in nonradiative relaxation compared with other ma-
terials.10 In particular, fluoride lattices are excellent because
1. Introduction
On account of their sharp emission lines, long lifetimes and
superior light fastness, rare earth ion doped-inorganic micro/
nanomaterials are earning widespread respect for biological
labeling, medical imaging and multicolor displays.1–4 Due to
their unique electronic configuration, rare earth ions have
abundant energy levels which can be selected and designed to
absorb or emit specified lights.5 Rare earth luminescent mate-
rials with stable physical and chemical properties can with-
stand high power electron beams, high energy radiation and
strong ultraviolet radiation. This is why rare earth ions are
valued in the field of optics. In addition, the host material is
an important affecting factor for the luminescence efficiency
of rare earth ions. Inorganic materials based on rare earth
(RE) ions have attracted much attention, including oxides,
fluorides, molybdates and vanadates.6–9 In particular, inor-
ganic rare earth fluoride materials have more outstanding su-
periority in optical applications originating from their low
phonon energy and high transparency which could lead to a
they have very low vibrational energy and quenching of the
emission state of rare earth ions is minimal.11 Over the past
decade, multifarious RE3+ doped-fluoride micro/nanocrystals
have been obtained in different ways. The most common way
is the decomposition of metal trifluoroacetate precursors in
the presence of oleic acid and octadecene.12 Besides, high
temperature coprecipitation and solvothermal technology are
also commonly used methods.13 Although good control over
the phase, morphology and composition of fluoride nano-
crystals (NCs) can be realized in these methods, they may also
have problems such as complex experimental conditions,
cumbersome procedures and high reaction temperatures.13–15
In addition, surfactants are most often employed to control
crystal growth rates to achieve materials with different
shapes; however at the same time, it is hard to remove the re-
sidual surfactants entirely by rinsing or post-calcination. For
phosphors, they would further act as luminous quenching
centers and thus reduce the phosphors' luminescence proper-
ties; what's more, some are even toxic.16–19 Therefore, from
the point of view of simplicity and energy saving, there is still
an urgent need to develop a simple and mild solution phase
method to produce pure fluoride micro/nanocrystals.
a School of Chemistry and Chemical Engineering, Southwest University, Chongqing
400715, China. E-mail: hushan3@swu.edu.cn, jyang@swu.edu.cn
b Academic Affairs Office, Southwest University, Chongqing 400715, China
c AECC Harbin Dongan Engine Co Ltd, Harbin 150066, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ce00716k
Recently, ScF3 and KSc2F7 microcrystals were prepared by
a simple hydrothermal method. Here, we found that different
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F/Sc ratios, pH values and surfactants can produce crystals
with various phases, forms and sizes. Thus, hydrothermal
synthesis is more appropriate for the preparation of highly
crystalline fluoride micro/nanomaterials under relatively gen-
tle conditions. In addition, the exploration of rare earth fluo-
rides (RE = Y3+/Ln3+) has been a hot topic.20–28 Nonetheless,
Sc3+-based fluorides containing ternary systems as phosphor
matrices have largely been overlooked.29–39 In the periodic ta-
ble, scandium (Sc) is not only the first element of group IIIB,
but also the first transition metal. Sc3+-based fluorides such
as NaxScF3+x, M2NaScF6 (M = K, Rb, Cs), NaScF4, Na3ScF6,
KSc2F7, K2NaScF6 and Sr2ScF7 have been prepared and
reported.39,40 These Sc3+-based fluorides are mostly used as
upconversion (UC) photoluminescent bodies for lanthanide
activators. Whether it is for up or down conversion light is
very important. To the best of our knowledge, Ln3+-doped
fluoride crystals that can be used for down-conversion lumi-
nescent materials are of little concern. In our work, we real-
ized the hydrothermal synthesis of KSc2F7:Ce3+/Tb3+/Eu3+
microcrystals and researched the luminescence properties of
Ce3+ (blue), Tb3+ (green) and Eu3+ (red) in the KSc2F7 host,
respectively.
ammonium fluoride (NH4F), ammonium hydroxide (NH3
·H2O), and hydrochloric acid (HCl) were purchased from
Chuandong Chemical Reagents Company (China) and used
without further purification. ScCl3, TbCl3 and EuCl3 were pre-
pared by dissolving Sc2O3, Tb4O7 and Eu2O3 in hot dilute HCl
solution and then removing excess HCl, respectively.
2.2. Synthesis
In a typical synthesis for the preparation of the KSc2F7 host, 3
mmol KCl was added to 30 ml deionized water to form a
clear solution by stirring. Then, 1 mmol ScCl3 solution was
introduced into the former solution. And then, 3.5 mmol
NH4F was added to the above solution and then the mixture
was fully stirred for another 30 min. The pH of the precursor
dispersion was adjusted to a certain value by adding HCl (0.1
mol L−1) or NH3·H2O (0.1 mol L−1). Finally, the obtained
homogenous white solution was transferred into a 50 ml
stainless steel Teflon-lined autoclave, sealed and kept at 200
°C for 24 h. After that, the autoclave was moved out from the
furnace and cooled down to room temperature naturally. The
products were washed with water and ethanol several times
At present, Ln3+-doping is regarded as a novel route to
control the growth process of micro/nanomaterials and mod-
ify their phase, size, morphology, dimensionality and physi-
cal/chemical properties.41–48 Therefore, Ln3+-doping must se-
lect appropriate host materials. In general, doping with ions
of similar radii and the same valence state contributes to the
induction of the host matrix. Studies on the incorporation of
plasma ions into UC/DC fluoride materials have been
discussed over the past decade.41,43,48 Here, we synthesized
novel Ln3+-doped KSc2F7 (Ln = La, Ce, Sm, Eu, Gd, Tb and
Lu) crystals without any surfactants by a hydrothermal
method. The doping has a great effect not only on the growth
process, but also on the crystal phase, morphology and lumi-
nescence properties of the final crystals. In addition, this
phenomenon can be used for other lanthanide doped fluo-
ride materials, which are widely used in light-emitting bio-
markers and volume dimensions.23,49
In this paper, we prepared KSc2F7 crystals by a one-step
hydrothermal method. We found that the phase, shape and
size of the final products were affected by the F/Sc ratio, the
different pH values, the different kinds of additives and the
doping of different Ln3+ ions. By adjusting the above factors,
we obtained well-formed KSc2F7 crystals with fine morphol-
ogy. Moreover, we obtained strong red, green and blue light
by separately doping Eu3+, Tb3+ and Ce3+. The rich down-
conversion luminescence properties have some potential ap-
plications in multicolor illumination and displays.50
2. Experimental section
2.1. Chemicals
The raw materials including scandium oxide (Sc2O3, 99.99%),
europium oxide (Eu2O3, 99.99%), terbium oxide (Tb4O7,
99.99%), cerium nitrate (CeIJNO3)3), potassium chloride (KCl),
Fig. 1 XRD and SEM results of the as-synthesized KSc2F7 host under
the conditions F/Sc/K = 3.5 : 1 : 3, pH = 7 and without any surfactants:
XRD pattern (a) and SEM image (b), respectively.
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the (200) peak gradually increases with increasing F/Sc ratio
and the (001) peak decreases abruptly at F/Sc = 7 due to the
different crystal growth orientations caused by the high con-
centration of F−.51
Fig. 3 shows various morphologies of the KSc2F7 host with
different F/Sc ratios. When F/Sc = 3, cubic ScF3 is obtained as
displayed in Fig. 3a. When F/Sc = 3.5, relatively uniform and
well-defined KSc2F7 microrods are formed (Fig. 3b). When
the F/Sc ratio is increased to 4, the originally uniform micro-
rods gradually become a mixture of rods and pieces, as
shown in Fig. 3c. When the F/Sc ratio is increased to 5, all
samples are sheets as shown in Fig. 3d. As the F/Sc ratio in-
creases to 6, the sheets begin to self-assemble to overlapping
aggregates (Fig. 3e). Finally when F/Sc = 7, the product has
some flower-like structures (Fig. 3f). Here, we compared the
SEM results of the samples during the synthesis process and
washing process (Figure S1†), and found that their morphol-
ogies were the same. Since the sample was first synthesized
and then washed, we concluded that the self-assembly takes
place during the synthesis process. The atomic ratio of F/Sc
in the KSc2F7 molecule is 3.5, and KSc2F7 cannot be formed
when F/Sc < 3.5. Thus, ScF3 is formed when F/Sc = 3 (Fig. 2
and 3a). With the increase of F− concentration, the samples
gradually change from the rod-like structure to the sheet-like
structure when F/Sc ≥ 4. When the F− concentration is too
high, the samples begin to agglomerate to form a flower-like
structure. It can be seen that the F− concentration plays a key
role in the phase, morphology and size. The F− concentration
effect on crystal growth can be explained by the
Fig. 2 XRD patterns of the KSc2F7 host materials with different F/Sc
ratios.
to remove other remnants, and then dried at 70 °C for 24 h.
With a similar method, KSc2F7 nano/microcrystals doped
with 5% La3+/Ce3+/Sm3+/Eu3+/Gd3+/Tb3+/Lu3+ were prepared.
2.3. Characterization
X-ray power diffraction (XRD) measurements were performed
on a Purkinje General Instrument MSALXD3 using Cu Kα ra-
diation (λ = 0.15406 nm). Morphology analysis was carried
out on
a field emission scanning electron microscope
(FESEM, Hitachi, S-4800). The PL excitation and emission
spectra were detected using an F-7000 spectrophotometer
(Hitachi, Japan) equipped with a 150 W xenon lamp as the
excitation source.
3. Results and discussion
3.1. Phase identification and morphology
XRD patterns of the samples that were synthesized under the
conditions F/Sc/K = 3.5 : 1 : 3, pH = 7, 200 °C for 24 h and
without any surfactants are shown in Fig. 1a. We can see that
all the peaks are accurately fitted with orthorhombic KSc2F7
(JCPDS No. 39-0784) without any other phases or impurities,
which explains the phase purity of the products. The mor-
phology of the products was observed by SEM. As demon-
strated in Fig. 1b, the KSc2F7 host consists of uniform micro-
rods of 10 μm in length and 0.4 μm in width.
3.1.1. Effect of F− ions on the morphology and phase. It
has been reported that the transitions of the phase and mor-
phology could be affected by the F− concentration.31 Here, we
changed the concentration of F− ions and kept other condi-
tions unchanged (pH = 7, without any surfactants). Fig. 2 dis-
plays the XRD results of the samples with different F/Sc ra-
tios. Clearly, adjusting the amount of F− does change the
crystallographic phase of the products. ScF3 appears when F/
Sc = 3, and the phases are always orthorhombic KSc2F7 when
altering the F/Sc ratio from 3.5 to 7. It is worth noting that
Fig. 3 SEM images of the KSc2F7 host materials with different F/Sc
ratios: (a) F/Sc = 3, (b) F/Sc = 3.5, (c) F/Sc = 4, (d) F/Sc = 5, (e) F/Sc = 6
and (f) F/Sc = 7, respectively.
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crystallization rate control model.52 The nano-/microcrystal
growth begins with the nucleation process between F− and
rare earth ions. The change in crystal morphology is mainly
caused by the nucleation process, and the following growth
process has a relatively slight influence. A low F− concentra-
tion slows down the nucleation rate and produces a relatively
small amount of nuclei, resulting in higher monomer
concentrations.
Thus, highly uniform microrods can be synthesized under
low F− concentration conditions. In contrast, high concentra-
tions of F− can increase the crystallization rate so that the
monomer concentration is reduced to a critical level to form
less regular sheet-like aggregates. Thus, different morphol-
ogies of the KSc2F7 crystals are obtained (Fig. 3).
3.1.2. Effect of pH on the morphology and phase. It is well
known that many factors can affect the phase and morphol-
ogy of crystals, so we explored the effect of pH on crystal
growth and kept other conditions unchanged (F/Sc = 3.5,
without any surfactants). The pH of the precursor dispersion
was adjusted by adding a certain acid (HCl, 0.1 mol L−1) or
ammonium hydroxide (NH3·H2O, 0.1 mol L−1). Fig. 4 shows
the XRD results of samples with different pH values. From
the XRD results, we can see that the crystal phase of the sam-
ples changes from the pure cubic phase ScF3 (pH = 3) to the
pure orthorhombic phase KSc2F7 (pH = 7/9) going through
the mixture of the two phases (pH = 5). This indicates that
KSc2F7 microcrystals are suitable for growth under neutral or
alkaline conditions. To examine precisely the effect of pH on
the morphology evolution of the KSc2F7 products, we
obtained the SEM images with more various pH values. The
corresponding SEM images are shown in Fig. 5. Obviously,
when the pH = 3 and 5, as shown in Fig. 5a and b, the crystal
morphologies are from uniform cubes (ScF3) to a mixture of
cubes and rods (ScF3 and KSc2F7), which is also in agreement
with the results from the XRD analysis in Fig. 4. In contrast
to Fig. 5c and d, it is not difficult to find that the morphology
Fig. 5 SEM images of KSc2F7 crystals prepared at different pH values:
pH = 3 (a), pH = 5 (b), pH = 7 (c) and pH = 9 (d), respectively.
of the KSc2F7 crystals changes from a rod to a xylitol-like
shape (rectangle) when the pH increases from 7 to 9.
The above experimental facts show that when the pH is
less than 7, the increased H+ will combine with F− to form
weak acid HF, resulting in the reduction of F− in solution to
prevent the formation of KSc2F7. When the pH is greater than
7, the increased OH− in the solution promotes the release of
F− and the formation of KSc2F7; and its morphology also has
a shift from rods to pieces, like the previous case with differ-
ent fluoride concentrations. When the pH is above/below the
critical values, the nucleation and growth behavior are con-
trolled by kinetics due to the presence of H+ or OH− ions.
Thus, KSc2F7 crystals possessing different morphologies can
be obtained by adjusting the pH of the reaction solution. In
general, the pH of the solution can effectively change the bal-
ance between the ion transport rates and chemical potential,
and then change the growth crystal orientation, as well as ul-
timately produce different phases or different forms of
products.53
3.1.3. Effect of surfactant on the morphology and phase.
Fig. 6 shows the morphological changes of hydrothermal
products by adding different surfactants and maintaining the
same other reaction conditions (F/Sc/K = 3.5 : 1 : 3, pH = 7).
The as-formed product without an additive is mainly com-
posed of microrods (Fig. 1b). In the presence of EDTA (1
mmol), coarse microrods with a bigger diameter can be
obtained (Fig. 6b). We further analyzed their XRD patterns
(Fig. 6a) and found no other phase diffraction peaks except
for orthogonal KSc2F7.
EDTA has four carboxyl groups (–COOH) and two lone pair
electrons on two nitrogen atoms that can serve as binding
sites. Therefore, EDTA has an excellent ability to chelate
metal ions and adsorb on the grain surface, which will effec-
tively adjust the growth rate of different crystal planes. When
EDTA was added to the ScCl3 solution, Sc3+ ions formed the
intermediate complex Sc3+–EDTA, which greatly reduced the
concentration of free Sc3+ in the solution and helped to
Fig. 4 XRD patterns of the KSc2F7 host materials (F/Sc = 3.5) with
different pH values.
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Fig. 7 XRD patterns of the KSc2F7:Ln3+ crystals: (a) pure KSc2F7, (b)
Fig. 6 XRD patterns (a) and SEM images of the obtained KSc2F7
crystals with different surfactants (b: EDTA; c: SDBS; d: urea).
KSc2F7:5% Lu3+, (c) KSc2F7:5% Tb3+, (d) KSc2F7:5% Gd3+, (e) KSc2F7:5%
Eu3+, (f) KSc2F7:5% Sm3+, (g) KSc2F7:5% Ce3+, (h) KSc2F7:5% La3+
.
control the nuclear growth. Thus, when F− is introduced into
the system, it will compete with EDTA for Sc3+ ions and form
KSc2F7 microrods with a larger diameter. From a thermody-
namic point of view, the initial crystalline phase structure
has a tendency to self-aggregate into larger microrods to min-
imize surface energy.54 When SDBS (0.5 mmol) or carbamide
(1 mmol) was employed as an organic additive, the obtained
products were oblong microblocks or micropieces, as illus-
trated in Fig. 6c and d, respectively. It can be found that
these results are similar to those at high pH = 9 (Fig. 5d).
Generally, SDBS is hydrolyzed to form dodecylbenzene-
sulfonate which will be bound to hydrogen ions in water, and
then the solution would be alkaline due to the presence of ex-
cess hydroxide ions. In addition, urea will slowly hydrolyze
into ammonium carbonate and the hydrolysis of carbonate
ions results in an alkaline solution; on the other hand, from
the organic point of view, nitrogen atoms on amides in urea
molecules can absorb hydrogen ions, which can also show
that the urea solution is alkaline. Thus, we can attribute the
effects of SDBS and urea on the KSc2F7 morphology to pH
values like the above analysis and discussion (Fig. 5d). To
prove this, we tested the pH before and after the hydrother-
mal reaction: pH = 7 and pH = 9, respectively. From the
above analysis, it can be concluded that the self-assembly of
the precursor microstructure of alkali metal rare earth fluo-
ride and the pH of the reaction solution are undoubtedly cru-
cial to the morphology.
creases, which means that the preferred orientation of the
crystals exists.55 An obvious shift towards small 2-theta values
can be observed (see the red dotted line in Fig. 7) because of
the doping of Ln3+ ions with larger ionic radii (r = 0.085–
0.106 nm) compared to that of Sc3+ (r = 0.0745 nm).56,57
The SEM images of undoped KSc2F7 microcrystals and
KSc2F7:5% Ln3+ (Ln = Lu, Tb, Gd, Eu, Sm, Ce and La) are shown
in Fig. 8. From the SEM results, we can see that the length/di-
ameter of the doped KSc2F7 microcrystals have changed obvi-
ously (see Table S1†). The undoped KSc2F7 microcrystals were
measured to be relatively uniform and monodisperse micro-
rods of 10 μm in length and 0.4 μm in width. When the dopant
was changed from Lu3+ to La3+, the ion radius increased from
0.085 nm (Lu3+) to 0.106 nm (La3+). Correspondingly, the shape
of the KSc2F7:Ln3+ microcrystals changed from thin short rect-
angles (doping Lu3+, Fig. 8b), to cubic columns (doping Tb3+,
Fig. 8c), to bulk (doping Gd3+, Fig. 8d), to long thin blocks
(doping Eu3+ and Sm3+, Fig. 8e and f), and finally to small
sheets (doping Ce3+ and La3+, Fig. 8g and h) with lengths of
8.9, 3.1, 2.2, 1.9, 1.7, 1.4 and 0.7 μm, respectively. As a result,
the length of the KSc2F7:Ln3+ crystals decreases as the Ln3+ ion
radius increases. However, the change in width is not very regu-
lar. In general, the morphology of the KSc2F7 crystals is rela-
tively uniform after the addition of Ln3+ ions. When the Sc3+
ions are replaced by the Ln3+ ions with larger radii, the electron
charge density on the surface of the KSc2F7 crystals will in-
crease. The change of electron charge density on the surface of
microcrystals can drastically slow the diffusion of negatively
charged F− ions to the surface because of an increase in charge
repulsion, which results in a tunable reduction of the KSc2F7
microcrystals.13,58–62
3.1.4. Effect of doped Ln3+ on the morphology. The influ-
ence of dopants on morphological evolution was studied.
KSc2F7:Ln3+ (Ln = Lu, Tb, Gd, Eu, Sm, Ce and La) microcrys-
tals were prepared without any surfactant at 200 °C, F/(Sc +
Ln)/K = 3.5 : 1 : 3 and pH = 7. Fig. 7 shows the XRD patterns
of the pure and KSc2F7:5% Ln3+ (Ln = Lu, Tb, Gd, Eu, Sm, Ce
and La) crystals, respectively. The samples have high crystal-
linity and all the peaks are in good agreement with the or-
thogonal structure of the KSc2F7 crystals (JCPDS 39-0784). It
is noteworthy that the relative intensities of the (200) diffrac-
tion peaks gradually become stronger as the Ln3+ radius in-
3.2. Luminescence properties
Since KSc2F7 crystals have many influencing factors (such as
different F/Sc ratios, pH values and various additives), and in
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Fig. 9 PL excitation and emission spectra of the KSc2F7:5% Eu3+
microcrystals.
7F0–5G2, F0–5L6 and F0–5D2 transitions, respectively. The ex-
citation into the 7F0–5L6 transition of Eu3+ at 394 nm yields
the emission spectrum of KSc2F7:Eu3+ crystals (right side of
Fig. 9), which consists of two emission lines with the stronger
one at 596 nm (5D0–7F1). It is known that the Eu3+ ion is a
good probe for the chemical environment of the lanthanide
ion. The emission at 596 nm (5D0–7F1) originates from the
electric dipole transition of the symmetry of the local envi-
ronment around the Eu3+ ion and the emission at 619 nm
(5D0–7F2) originates from the magnetic dipole transition that
is insensitive to local symmetry.65 The dominant emission
7
7
Fig. 8 SEM images of the products doped with different Ln3+ ions: (a)
pure KSc2F7, (b) KSc2F7:5% Lu3+, (c) KSc2F7:5% Tb3+, (d) KSc2F7:5%
Gd3+, (e) KSc2F7:5% Eu3+, (f) KSc2F7:5% Sm3+, (g) KSc2F7:5% Ce3+, (h)
5
centered at 596 nm corresponds to the D0–7F1 magnetic di-
KSc2F7:5% La3+
.
pole transitions in the NSPO matrix. Thus, the diverse crystal
structures will lead to the intensity discrepancy in these two
emissions.
order to know which kinds of reaction conditions are most
suitable for luminescence, we have investigated the same ac-
tivator Tb3+ luminescence under different conditions, such as
different F/Sc ratios, pH values and various additives (see Fig.
S2–S4† for details). Generally, KSc2F7:1% Tb3+ crystals with a
microrod shape show the highest luminescence intensity un-
der the conditions K/F/Sc = 3 : 3.5 : 1, pH = 7, and without any
surfactants. Then, we studied the luminescence properties of
KSc2F7:x% Ln3+ (Ln = Ce, Tb, Eu) samples under the best con-
ditions, which showed efficient down conversion lumines-
cence properties and can be used for multicolor lighting and
displays.
The Tb3+ ion, which is a significant activator, can be well
incorporated into many main lattices, leading to strong green
emission. Fig. 10 displays the photoluminescence excitation
and emission spectra of the KSc2F7:1% Tb3+ phosphors. Moni-
tored at 549 nm, PEL shows a strong broad band in the range
of 200–280 nm and several weak peaks in the range of 300–
500 nm, respectively. The band extends from the 4f8–4f75d1
transition of the Tb3+ ion, while the excitation peak is caused
by the spin and orbit forbidden 4f transitions. We know that
Tb3+ has a simple 4f configuration energy level structure.
When an electron is excited to a 5d shell, it produces two
4f75d1 excited states: a high spin state with a 5DJ configuration
and a low spin state with a 7FJ configuration. Obviously,
Fig. 9 depicts the room temperature excitation (monitored
at 596 nm) and emission (excited at 394 nm) spectra of the
5% Eu3+ doped KSc2F7 crystals. The resulting excitation spec-
trum (left side of Fig. 9) consists of several strong lines corre-
sponding to direct excitation of the europium state into a
higher excited state of europium f electrons. In fact, the posi-
tions of these lines are the same as the characteristic absorp-
tion bands of f–f transitions in trivalent europium.63,64 It is
shown that there are several excitation peaks at 318, 362,
5
according to Hund's rule, the energy of the DJ state will be
7
5
lower, while the transitions between FJ and DJ are spin for-
bidden. Thus, Tb3+ in a particular subject reveals two sets of
f–d transitions: the spin-allowed f–d transition with a stronger
and higher energy; and the spin-forbidden f–d transitions
with a weaker and lower energy, i.e., the peaks at 215 and 251
nm, respectively. Excited into the 4f8–4f75d band at 215 nm,
the prepared KSc2F7:1% Tb3+ samples have two emission
groups: one is the weaker blue emission with main peaks at
7
7
377, 394 and 465 nm corresponding to the F0–5H6, F0–5D4,
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the 4f05d1 excited state. As shown in Fig. 12, the emission
wavelength exhibits a small red shift as the Ce3+ content in-
creases. The red shift may be caused by the absorption pro-
cess or energy transfer between the unlike positions of Ce3+
ions. Since the overlap between the excitation and emission
spectra is small, the reabsorption process can be excluded.
The red shift can be partly ascribed to the changes in the
crystal field strength acting upon Ce3+; and the energy trans-
fer between the different sites of the Ce3+ ions is also respon-
sible for this red shift. With the increase of Ce3+ content, the
emission intensity of the phosphors first increased gradually
to a maximum and then decreased due to the concentration
quenching. The inset of Fig. 12 reveals the dependence of the
emission intensity on the concentration of Ce3+, and it can
be concluded that the optimum doping concentration of Ce3+
is 3%. Based on Dexter's theory of concentration quenching,
the relationship between the luminescence intensity (I) and
activator concentration (x) can be expressed in the form of a
modified equation as follows:69
Fig. 10 PL excitation and emission spectra of the KSc2F7:1% Tb3+
microcrystals.
5
418/439 nm due to the D3–7FJ (J = 5, 4) transitions; and the
other is the stronger green emission with main peaks at 491,
5
549, 591 and 621 nm, which can be ascribed to D4–7FJ (J = 6,
5, 4 and 3) transitions. In accordance with the magnetic di-
pole transition selection rule ΔJ = 1, Laporte's forbidden
transition 5D4–7F5 (549 nm) showed bright green lumines-
cence compared with the other emission transitions.66
where I is the integral intensity of the emission spectrum
from 300 to 500 nm, x is the doped rare earth ion concentra-
tion, K and β are the constants of the given host under the
same excitation conditions, and Q is the interaction type be-
tween rare earth ions. In order to discuss the mechanism of
concentration quenching, we obtained the slope of the con-
centration fraction −Q/3 by fitting log x (x = 0.01, 0.03 and
0.05) as the horizontal ordinate and log (I/x) as the vertical co-
ordinate curve. The various values of Q (8.65, 10.19 and
11.26) correspond to different mechanisms, including di-
pole–quadrupole, dipole–dipole or quadrupole–quadrupole
interaction. The log (I/x) ∼ log x curve for different Ce3+ con-
centrations (0.01, 0.03 and 0.05) is shown in Fig. 13.
Ce3+ ions were doped into the host as an activator, and
the luminescence properties have been studied. Fig. 11 shows
the excitation and emission spectra of the KSc2F7:3% Ce3+
phosphors. Monitored at 375 nm, the excitation spectrum of
the obtained Ce3+-doped KSc2F7 phosphors (Fig. 11) consists
of a strong peak at 297 nm and a broad band at 252 nm,
which corresponds to a transition from the Ce3+ ground state
2F5/2 to different components of the excited Ce3+ 5d state by
crystal splitting.67,68 The emission spectrum ranges from 300
to 450 nm with a maximum at 375 nm. The broad excitation
and emission spectra are caused by the spin and the allow-
able electron transitions between the 4f15d0 ground state and
Fig. 11 PL excitation and emission spectra of the KSc2F7:3% Ce3+
microcrystals.
Fig. 12 Emission spectra of KSc2F7:xCe3+
dependence of the emission intensity on the concentration of Ce3+
. The inset shows the
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