Polymorphism of Erbium Oxyfluoride: Selective Synthesis, Crystal Structure, and Phase-Dependent Upconversion Luminescence

Article abstract of DOI:10.1002/ejic.201700658

Phase-selective synthesis and structure switching behavior of a functional material are essential to enable comparative studies on the structure–property relationship. Here, we report a controllable fluorination route to phase-pure erbium oxyfluorides with orthorhombic (O-ErOF) and rhombohedral (R-ErOF) structures. This facile method adopts polytetrafluoroethylene (PTFE) as the fluoridizer, and the phase selectivity can be easily achieved at specific fluorination temperatures. The phase evolution and detailed crystal structures of erbium oxyfluoride were characterized by powder X-ray diffraction (PXRD) at various sintering temperatures and Rietveld refinements, respectively. An irreversible phase transition from O-ErOF to R-ErOF was observed under heating around 600 °C. The upconversion (UC) luminescence properties of R-ErOF and O-ErOF were studied comparatively by means of photoluminescence, P–I, and UC decay curves. Despite their similar components and crystal structure, R-ErOF exhibits stronger (more than 20 times) red UC emission than O-ErOF. The anomalous UC behavior of the two polymorphs of ErOF was associated with the energy transfer processes dependent on their crystal structure.

Full text of DOI:10.1002/ejic.201700658

A Journal of
Accepted Article
Title: Polymorphism of Erbium Oxyfluoride: Selective Synthesis,
Crystal Structure and Phase-Dependent Upconversion
Luminescence
Authors: Ting Wen, Ruixian Ding, Yannan Zhou, Yubing Si, Baocheng
Yang, and Yonggang Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700658
Link to VoR: http://dx.doi.org/10.1002/ejic.201700658

10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
Polymorphism of Erbium Oxyfluoride: Selective Synthesis,
Crystal Structure and Phase-Dependent Upconversion
Luminescence
Ting Wen, *[a,b] Ruixian Ding, [a] Yannan Zhou, [a,b] Yubing Si, [a,b] Baocheng Yang*[a,b] and Yonggang
Wang*[c]
Abstract: Phase-selective synthesis and structure switching
behavior of functional material are essential to enable
deciding the site symmetry, the strength of crystal-fields and
thus the f-f transition probabilities.2 It is also known that the
geometric relationship of the RE cations in the host matrices
affect the interplays between adjacent RE centers directly.3
Therefore crystal structure variation of the host materials is
considered as an effective way to regulate the optical
properties of RE-based UC phosphors, including the emission
efficiency and the emitting band selectivity. Take the
NaYF4:Yb/Er UC phosphor as an example, the green
a
comparative studies on the structure-property relationship. Here,
we report a controllable fluorination route to phase-pure erbium
oxyfluorides with orthorhombic (O-ErOF) and rhombohedral (R-
ErOF)
structures.
This
facile
method
adopts
polytetrafluoroethylene (PTFE) as the fluridizer and the phase
selectivity can be easily achieved at specific fluorination
temperatures. The phase evolution and detailed crystal structures
of erbium oxyfluoride were characterized by powder X-ray
diffraction at various sintering temperatures and Rietveld
refinements, respectively. An irreversible phase transition from O-
ErOF to R-ErOF was observed under heating around 600 oC. The
upconversion (UC) luminescence properties of R-ErOF and O-
emissions (525 nm, H11/2→4I15/2; 542 nm, S3/2→4I15/2) in
hexagonal NaYF4 host materials are about an order of
magnitude more intense than those in their cubic
counterparts.4 On the other hand, UC emission G/R
(green/red) ratios of the hexagonal and cubic phases
presents an opposite tendency.5 Crystallographic phase
control of NaYF4:Yb/Er nanoparticals by Gd3+ ions doping
lead to tunable UC emission colours (orange to green) and
the UC emission G/R ratio of the hexagonal phase is over
twice higher than that of the cubic-hexagonal-mixed phase.6
Lanthanide oxyfluorides are considered to be attractive
candidates for UC host materials owing to their combination
of both merits of fluorides and oxides counterparts, such as
low phonon energy, excellent mechanical strength, and good
chemical and thermal stability. Thus, RE3+ doped lanthanide
oxyfluorides have been frequently reported exhibiting intense
and multifarious UC photoluminescence properties.7
Significantly, lanthanide oxyfluorides crystallize in several
distinct polymorphisms, such as the cubic fluorite structure,
the tetragonal PbFCl-type, the rhombohedral SmSI-type, and
the orthorhombic Vernier-type with an incommensurate
structure.8 Almost all of these structures originate from fluorite
structure which has been proved to be the best UC host
lattice (such as CaF2 and NaREF4).9 The transmutation of
them depends on the stoichiometric ratio between O2- and F-
ions and the arrangement ordering of anions in the lattice.10
So, it is not surprising to see a given lanthanide oxyfluoride
adopting several different structures through similar synthetic
routes and also the reversible or irreversible transition
between these allotropes. For example, lanthanum
oxyfluoride (LaOF) crystallized in cubic, tetragonal and
trigonal structures through thermal decomposition of the
same precursor under different conditions.11 These structure-
selective or phase-switching systems are perfect candidates
for comparative studies on the structure-property relationship
of UC luminescence materials, and the exploration of new
systems is of timely urgency.
2
4
ErOF
were
studied
comparatively
by
means
of
photoluminescence, P–I and UC decay curves. Despite their
similar component and crystal structure, R-ErOF exhibits stronger
(more than 20 times) red UC emission than O-ErOF. The
anomalous UC behavior of the two polymorphisms of ErOF was
associated with the energy transfer processes dependent on their
crystal structure features.
Introduction
Trivalent rare-earth ions (RE3+)-doped upconversion (UC)
luminescent materials that can convert near-infrared (NIR)
photons into visible emissions exhibits applied potentialities in
the fields such as colour displays, photovoltaics, solid-state
lasers and biological imaging owing to their low
autofluorescence background, sharp emission bandwidths
and superior photostabilities.1 Since UC luminescence is a
multi-photon processes among the neighbouring RE3+
dopants (sensitizer and activator) spatially distributed in a
host matrix, the host structure plays a significant role in
[a]
Ting Wen, Ruixian Ding, Yannan Zhou, Yubing Si, Baocheng Yang
Institute of Nanostructured Functional Materials, Huanghe Science
and Technology College, Zhengzhou, Henan, 450006, China.
E-mail: tingwen@infm.hhstu.edu.cn (T.W.);
baochengyang@infm.hhstu.edu.cn (B.Y.)
[b]
[c]
Ting Wen, Yannan Zhou, Yubing Si, Baocheng Yang
Henan Provincial Key Laboratory of Nano-composite materials and
Applications, Zhengzhou, Henan 450006, China.
Yonggang Wang
HPSynC, Geophysical Laboratory, Carnegie Institution of
Washington, Argonne, IL 60439, USA.
E-mail: yyggwang@gmail.com (Y.W.)
In our previous works, we have studied the crystal
structures and UC properties of a series of Vernier-phase
lanthanide oxyfluorides as the host for RE3+ doping, such as
Y6O5F8, Lu5O4F7 and Yb5O4F7.12 Despite the similarity of their
chemical formula and structure prototypes, they do exhibit
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
distinctive UC luminescence performances, e.g. multi-colour
tunability and single-band UC outputs. Particularly,
monoclinic ScOF was demonstrated to be a unique host for
RE3+ UC generation, owing to the record-short Sc-Sc
distance and the specific coordination environment of the
doping sites with low-symmetry.13 Nevertheless, none of
these systems exhibits polymorphism or is phase-switchable,
thus cannot be adopted for a structure-dependent study. Here,
we report the fabrication of phase-pure erbium oxyfluoride
with orthorhombic (O-ErOF) and rhombohedral (R-ErOF)
structures using polytetrafluoroethylene (PTFE) as fluridizer.
The phase-temperature relationship of erbium oxyfluoride
was characterized by powder X-ray diffraction at different
reaction temperatures. An irreversible phase transition from
O-ErOF to R-ErOF was disclosed by thermal analysis. Then,
by means of photoluminescence spectroscopy, P–I curve and
UC PL decay curves, we studied the UC luminescence
characters of R-ErOF and O-ErOF and associated the UC
properties with their crystal structures.
recorded on
excitation laser.
a Raman spectrometer with a 532.1 nm
Results and Discussion
Selective phase formation and the crystal structures
Previously, we demonstrated that novel UC host,
a
monoclinic scandium oxyfluoride, could be synthesized via a
low-temperature fluorination route adopting PTFE as the
fluridizer.14 It is highly expected that the facile method can be
ported to the syntheses of other lanthanide oxyfluorides.
Notably, in the case of erbium, it is found that one can not
only fabricate phase-pure O-ErOF and R-ErOF using
commercial erbium oxide as the raw material, but also realize
phase selectivity by controlling the fluorination temperature
and atmosphere in partially sealed alumina crucible.
Experimental Section
Material syntheses
Powder samples of O-ErOF and R-ErOF were synthesized
via
a
low-temperature fluorination route by adopting
lanthanide oxides as raw materials and PTFE as the
fluridizer.14 In a typical procedure, 0.383 g Er2O3 (0.001 mol;
>99.5%) and 0.1 g PTFE powder (0.002 mol CF2; >99%)
were weighted and grounded together with ethanol for
several minutes. The resulting fine powder was placed in an
alumina crucible and sealed by an alumina cover. The
sample was slowly heated to target temperature (300-1000
oC) with a heating rate of 5 oC/min. After holding at the
reacting temperature for 5 hours, the furnace was allowed to
cool down to room temperature naturally.
Characterization
Powder X-ray diffraction (PXRD) data of all the samples were
collected at room temperature (25 oC) on a Bruker D8
Advance diffractometer using a germanium monochromatic
(Cu Kα). The data in the 2θ range of 5-130o were collected in
a step of 0.02o with the remaining time 1 s per step under the
tube conditions 40 kV and 40 mV. The least-squares
refinements of the cell parameters and atomic positions of the
samples were performed adopting the known structures of
YOF in the ICSD database as the initial models.15
The UC luminescence spectra were recorded on a modified
Hitachi F-4500 spectrophotometer with a tunable 10 W 980
nm laser diode (Lasever Inc.) as the excitation source. The
UC PL decay curves were measured using a FLS980
spectrophotometer equipped with a 150 W Xe lamp as the
excitation source. All of the measurements were performed at
room temperature. Thermogravimetric analysis (TG) and
differential thermal analysis (DTA) measurements were
conducted from room temperature up to 1000 oC using a
Figure 1. Powder X-ray diffraction patterns of as-synthesized powder
samples with different sintering temperature in the range of 300-1000 oC.
The green triangles indicate the diffraction peaks of Er2O3.
o
thermobalance TA SDT Q600, with heating rate of 10 C/min
in air or N2 atmospheres, respectively. Raman spectra were
Fig. 1 shows the PXRD patterns of the products obtained
with fixed Er2O3/PTFE ratio (Er/F=1:2) and reaction period (4
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10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
h), but at different fluorination temperatures varying from 300
to 1000 oC. No reaction was observed below or at 300 oC
since the diffraction peaks could be well assigned to the
structure of rhombohedral YOF (ICSD#14282)15 as the initial
model (as shown in Fig. 2b). The refinement plots are shown
in Fig. 2. Detailed atomic positions and other structural
parameters for O- and R-ErOF are listed in Table S1, S2,
respectively.
o
starting cubic Er2O3. At 400 C, new peaks appeared in the
products as a mixed phase and pure O-ErOF could be
o
o
obtained in the temperature range of 500-700 C. At 900 C,
another phase (R-ErOF) was obtained as a phase-pure
product, while O- and R- ErOF coexisted around 800 oC.
When the temperature reached as high as 1000 oC, cubic
Er2O3 reappeared due to the volatilization of PTFE and partial
decomposition of R-ErOF. The details of the PXRD pattern in
the range of 25–35 o showing the phase evolution of O- and
R-ErOF polymorphisms are provided in Fig. S1.
The crystal structures of O- and R-ErOF are shown in Fig.
3. O-ErOF crystallizes in an elongated orthorhombic unit cell
derived from fluorite-type structure, which has been proven
as one of the best hosts for lanthanide UC generation. There
are three crystallographic Er positions offered by the assorted
array of O2- and F- ions surrounding the 8-fold coordinated
Er3+ ions in the lattice, and they are arranged alternately
along the b-axis. The partial ordering (8d site) and disorderly
distribution of O/F ligands lead to the 5-fold elongation of the
fluorite structure (i.e., n in a × nb × c). Despite their normal
BVS values around +3 (Table S1), the three Er ions show
distinct coordination environments, either in the number of
O/F ligands or largely diverged bond lengths: Er1 in a C1 site
coordinated with 4O and 4F, Er2 in a C1 site with 4F, 2O and
2O/F, and Er3 in a Cs site with 4O and 4O/F, respectively (Fig.
3b). The diversity and low symmetry of the doping sites are
considered in favour of f-f transitions. While for R-ErOF, the
crystal structure is stoichiometric and highly ordered. The
unique Er site is surrounded by 4O and 4F with trigonal
symmetry along the c-axis. Compared with O-ErOF, such a
highly-ordered host lattice is desirable for lanthanide UC with
single-band outputs.
Figure 2. Rietveld refinement based on the powder X-ray diffraction data
of:(a) O-ErOF annealed at 700 oC and (b) R-ErOF obtained at 900 oC.
The O-ErOF adopts the so-called “Vernier”-type structure,
which is normally observed in lanthanide oxyfluorides for
small lanthanides (Y, Lu, Yb) with general formula REnOn-
16
1Fn+2
.
For erbium, only a n=3 member (Er3O2F5) is available
Figure 3. Crystal structures of O- and R-ErOF. (a) The orthorhombic
structure of O-ErOF viewed along a and b axes. (b) The assorted array of
O2- and F- ions offers three different types of Er3+ cation sites. Note that the
O2- and F- anions located at 8d sites shown as green balls are of
disordered distribution; (c) The rhombohedral structure of R-ErOF
presented as (110) and (001) sections.
in the ICSD database, however, its XRD pattern does not
match the as-obtained O-ErOF sample well (Fig. S2). Instead,
a more elongated n=5 orthorhombic unit cell can meet the as-
obtained sample much better. LeBail fitting on the PXRD
pattern of O-ErOF using orthorhombic space group Abm2
gives cell parameters: a = 5.4027(6) Å, b = 5 × 5.4907(3) Å
and c = 5.5092(8) Å. A high n value Vernier-phase is
reasonable for Er considering its similar ionic radius with Y
and Lu. The formula Er5O4F7 was also roughly supported by
the elemental measurement result. While, for the R-ErOF,
LeBail fitting on the PXRD pattern yields a very good result
with space group R-3mH, a = b = 3.7846(1) Å, c = 18.8347(2)
Å. Rietveld refinement on the PXRD pattern of O-ErOF was
performed using Y5O4F7 (ICSD#68949)17 as the initial
structure model. For brevity, the O/F distribution and disorder
were treated by following those in Y5O4F7. Rietveld
refinement on R-ErOF was also performed using the known
TG and DSC analysis
Besides the selective formation of O- and R-ErOF, it is also
expected that one phase of ErOF can transfer to the other
upon heating or cooling. TG and DSC analyses provide the
thermal stability of O-ErOF and possible phase transition
between O- and R-ErOF. As shown in Fig. 4, a sharp
o
endothermic peak appears at 595.5 C in the DSC curve for
O-ErOF when subjected to high temperature in both air and
nitrogen atmospheres. There is no obvious weight loss along
with the endothermic process, which suggests a possible
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10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
structural phase transition to R-ErOF. Subsequently, the
phase transition from O-ErOF to R-ErOF was confirmed by
ex-situ sintering experiments, by which mass R-ErOF
powders could be achieved from O-ErOF under heating
above 600 oC for 3 h. The evolution of the PXRD patterns
presenting the phase transition is provided in Fig. S3. An
interesting question arising here is: why O-ErOF can form
particle size much (Fig. S5). Thus, the greatly enhanced UC
PL emission from R-ErOF than that from O-ErOF is an
intrinsic result due to their distinct crystal structure features.
o
around 600 C (Fig. 1) as well as transfer to R-ErOF at the
same temperature? If one can realize that fluorination
process is under a kinetically-controlled atmosphere, the
different results under same temperature are not difficult to
understand.
Figure 5. The UC emission spectra of the powder samples under 980 nm
excitation (~3 W cm-2). (a) The UC emission spectra of O-ErOF and R-
ErOFF; the inset shows the detail of the UC emission spectrum of O-ErOF
in the range of 450-750 nm; (b) The UC emission spectra of O-ErOF and
R-ErOFPT
.
To gain further insight into the UC mechanisms of O- and
R-ErOF, the UC intensities of green (4S3/2→4I15/2) and red
(4F9/2→4I15/2) emissions were measured as a function of the
pump power density P (P–I curve) upon excitation at 980 nm.
The P–I measurements of O-ErOF and R-ErOF were plotted
as double logarithmic scale curves in Fig. 6a and 6b,
respectively. The slope values were determined to be 1.68 for
Figure 4. TG and DSC analysis of O-ErOF powder.
4
O-ErOF and 1.87 for R-ErOF for the S3/2→4I15/2 transition,
4
1.64 for O-ErOF and 1.87 for R-ErOF for the F9/2→4I15/2
transition, respectively. It is known that the dependence of the
UC emission intensity on the pump power density can be
expressed as: I ∝ Pn, where n is the number of pump
photons corresponding to the slope in the double-logarithmic
coordinate. Thus, the observed slopes above were indicative
of two-photon pumping processes for the population of both
UC PL properties of O- and R-ErOF
The as-prepared O- and R-ErOF with two distinct crystal
structures provide a rare platform to study phase-dependent
UC emission properties with lanthanide dopants. Besides the
O-ErOF sample fabricated from PTFE fluorination route, two
R-ErOF samples were adopted in the comparative studies:
one was obtained from PTFE fluorination method (denoted as
R- ErOFF), the other was obtained via phase transition from
4
4S3/2 and F9/2. The probable UC mechanism for O- and R-
ErOF is schematically illustrated in Fig. 6c with the energy
levels diagram of Er3+. The two-photon pumping process
o
O-ErOF at 700 C (denoted as R-ErOFPT). Fig. 5 shows the
comprises of
a
ground-state-absorption (GSA) and
UC luminescence spectra of O-ErOF compared with those of
R-ErOFF and R-ErOFPT. It is obvious that the UC emission
outputs of both R-ErOF and R-ErOFPT are much stronger
than that of O-ErOFF (200 and 30 times at 675 nm,
respectively). This 675 nm red emission peak is attributed to
4
subsequent excited-state-absorption (ESA) process. F7/2
state can be also populated by non-radiative energy transfer
process (ETU) from neighbouring Er3+ ion at the metastable
4
level of I11/2 (ETU1) at the same time. With the non-radiative
4
4
4
the f-f transition F9/2→4I15/2. Another miner green emission
relaxation to the S3/2 and F9/2 levels, it results in
corresponding green and red emissions. Another ETU
process commonly occurs in high-doping UC materials
band near 545 nm can be assigned to the f-f transition
4S3/2→4I15/2. Since particle size is another noticeable factor
besides host structure determining the UC emission intensity,
the impact from different sintering temperatures is evaluated
by SEM images (Fig. S4). The particle size of R-ErOFF is
much larger than that of O-ErOF. While for R-ErOFPT, phase
transition from O-ErOF at relative low temperature (700 oC,
the preparation temperature of O-ErOF) does not change the
4
should be indicated, that is, a photon at I11/2 level transits to
the 4I13/2 state via a multi-phonon relaxation process and then
4
4
is excited to F9/2 level through energy transfer from the I11/2
level of neighbouring Er3+ ion (ETU2). Besides, cross-
relaxation (CR) process should be included into the energy
migration process regarding the high Er3+ concentration.18
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10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
Figure 6. Log–log plots of the UC emission intensity versus the excitation power density under 980 nm excitation for: (a) O-ErOF and (b) R-ErOF samples. (c)
Schematic energy level diagrams of the f–f transitions processes corresponding to the UC bands. Decay curves of the UC luminescence of O-ErOF and R-ErOF
samples. UC decays of O-ErOF and R-ErOF from (d) 4S3/2→4I15/2 and (e) 4F9/2→4I15/2
.
Both ETU2 and CR are responsible for the strong red UC
emission of Er3+ in both O- and R-ErOF.
In lanthanide UC materials, the activator ion is always
controlled at relatively low concentration to avoid
a
UC PL decay behaviour is another useful evidence to
identify the distinct UC processes in O- and R-ErOF (Fig. 6d,
unfavourable cross-relaxation process among the RE ions. It
is worth noting that ErOF can be considered as a highly self-
doped host material, where cross-relaxation may lead to
serious quenching of the excited state.2 However, there are
still several reports of highly doped RE UC materials in which
concentration quenching is supposed that can be suppressed
geometrically at sub-lattice scale.19 The basic principle is that,
the energy transfer probability highly depends on the
migration distances between adjacent active ions located on
the sub-lattice.3 This may also be applied to explain the UC
intensity difference between O- and R-ErOF. The probability
of energy transfer between two adjacent Er3+ follows the
relationship: P = CEr−Er exp(−2R/L), where R is the distance
between the energy donor–acceptor pair, CEr−Er is the Er−Er
interaction constant, and L is the effective Bohr radius. The
closer is the Er-Er distance, the higher is the probability of
energy transfer between them.
4
4
6e). All the PL decay curves from S3/2 and F9/2 levels of O-
and R-ErOF can be fitted well by using a single exponential
4
function. The lifetime of the S3/2 state is 13.4 μs for O-ErOF
4
and 20.2 μs for R-ErOF, and the lifetime of the F9/2 state is
20.8 μs for O-ErOF and 32.7 μs for R-ErOF, respectively. The
lifetimes of 4F9/2 are much longer than those of 4S3/2 in both O-
and R-ErOF, which indicates the significant contributions from
4
the ETU2 and CR processes to F9/2 level. Besides, R-ErOF
4
4
exhibits longer lifetimes in both S3/2 and F9/2 states
compared with those of O-ErOF, probably owing to the
distinct features of their crystal structures.
Fig. 7 shows the quasi-layered structures for O- and R-
ErOF and the arrangement of Er atoms in both phases. In O-
ErOF, the intra-layer Er-Er distance is mainly distributed in
3.56-3.73 Å and the inter-layer Er-Er distance varies from
4.13 to 4.16 Å. While, in R-ErOF, there are only two intra-
layer Er-Er distances of 3.58 and 3.78 Å, and an inter-layer
distance of 4.08 Å. The Er3+ ions in R-ErOF arrange in a
much more ordered manner and each Er3+ ion is compassed
regularly by three closest Er3+ ions. Contrastively, all the Er3+
ions in O-ErOF are surrounded randomly by four to six
closest Er3+ ions in a narrow range of 3.56-3.64 Å. More
energy migration pathways in O-ErOF may lead to more
cross-relaxation for energy loss. Moreover, the energy
migration to the crystal defects is also regarded as a possible
reason for the concentration quenching in high doping
Figure 7. Schematic representation of Er-Er distances in the layered sub-
lattice: (a) O-ErOF and (b) R-ErOF. (c) The compare of Er-Er distances
distribution between O-ErOF and R-ErOF.
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10.1002/ejic.201700658
European Journal of Inorganic Chemistry
FULL PAPER
crystalline materials.20 Fig. S6 shows the Raman
spectroscopy of O- and R-ErOF samples. Several sharp
Raman peaks in the 200-1000 cm-1 region are observed for
R- ErOF. Comparatively, for O-ErOF, the Raman bands are
broad and with low intensity, indicating the highly disordered
structure features and the existence of structure defects in
the host lattice.21 However, the detailed local structure study
need further characterizations such as pair-distribution-
function and extended X-ray absorption fine structure
spectrometer.
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In summary, we reported a facile fluorination route to
orthorhombic and rhombohedral erbium oxyfluorides. Both
phases can be selectively achieved with high purity by simply
controlling the sintering temperature. O-ErOF can also
transform to R-ErOF via a structural phase transition upon
o
heating around 600 C. R-ErOF shows stronger UC emission
than that of O-ErOF. Comprehensive and comparative
studies on the UC PL behaviours of O- and R-ErOF were
performed by using PL, P–I and UC decay curves. The
structure features of O- and R-ErOF at sub-lattice scale are
supposed responsible for the distinct UC PL outputs. Our
demonstration offers a new perspective to understand the
impact of crystal structure for UC PL properties.
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Acknowledgements
This work was supported by National Natural Science
Foundation of China 51602119, Natural Science Foundation
of Education Department of Henan 17A150014 and Key
Project of Science and Technology of Zhengzhou
153PKJGG139.
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Keywords: Selective synthesis, Up-conversion, Phase
transition, Oxyfluoride
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Phase-pure
orthorhombic
and
Key Topic* Phase-selective
rhombohedral erbium oxyfluorides (O-
ErOF and R-ErOF) were obtained
selectively. An irreversible phase
transition was observed from O-ErOF
to R-ErOF around 700 oC and the
anomalous upconversion behaviour of
synthesis
Ting Wen,* Ruixian Ding, Yannan
Zhou, Yubing Si, Baocheng Yang*
and Yonggang Wang*
Page No. – Page No.
the
two
polymorphisms
was
Polymorphism of Erbium
Oxyfluoride: Selective Synthesis,
Crystal Structure and Phase-
Dependent Upconversion
Luminescence
associated with their distinct structure
features.
*one or two words that highlight the emphasis of the paper or the field of the study
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