Hydrothermal synthesis and characterization of Na3Y(PO 4)2 phosphate

Article abstract of DOI:10.1016/j.jallcom.2014.05.203

The Na₃Y(PO₄)₂ phosphate has been obtained by hydrothermal route and characterized by powder XRD, differential scanning calorimetry/thermogravimetric analysis (DSC/TGA), SEM and FTIR spectroscopy. The effect of synthesis c

Full text of DOI:10.1016/j.jallcom.2014.05.203

Accepted Manuscript
Hydrothermal synthesis and characterization of Na Y(PO ) phosphate
3
4 2
Aleksandra Matraszek, Irena Szczygieł, Bogdan Szczygieł
PII:
S0925-8388(14)01310-3
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.05.203
JALCOM 31392
Reference:
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Revised Date:
Accepted Date:
30 January 2014
23 May 2014
27 May 2014
Please cite this article as: A. Matraszek, I. Szczygieł, B. Szczygieł, Hydrothermal synthesis and characterization of
Na Y(PO ) phosphate, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.05.203
3
4 2
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Hydrothermal synthesis and characterization of Na Y(PO ) phosphate
3
4 2
*a
a
b
Aleksandra Matraszek , Irena Szczygieł , Bogdan Szczygieł
a) Department of Inorganic Chemistry, Faculty of Engineering and Economics,
Wrocław University of Economics, Komandorska 118/120, 53-345 Wrocław
b) Faculty of Chemistry, Wrocław University of Technology,
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
*
aleksandra.matraszek@ue.wroc.pl, Phone: +48 71 368 0321, Fax: +48 71 368 0292
Abstract
The Na Y(PO ) phosphate has been obtained by hydrothermal route and characterized by
3
4 2
powder XRD, differential scanning calorimetry/thermogravimetric analysis (DSC/TGA),
SEM and FTIR spectroscopy. The effect of synthesis conditions as well as the addition of
complexing agents and surfactants on phase purity and microstructure of synthesized powders
has been studied. The powders obtained by the hydrothermal technique exhibit uniform
spindle-shape. The Na Y(PO ) compound shows three polymorphic transitions and can be
3
4 2
crystallized in trigonal, monoclinic and/or orthorhombic structure.
Keywords: rare earth alloys and compounds; chemical synthesis; microstructure; phase
transitions; thermal analysis;
1
. Introduction
Double sodium rare earth phosphates of the formula Na RE(PO ) have been extensively
3
4 2
studied in terms of their structural and optical properties for the last decade [1-8]. These
phosphates, doped with optically active ions and excited with light in the UV range, show an
intense luminescence and can be applied as LED's in plasma display panels [9-11].
It has been reported that double phosphates of the Na RE(PO ) type can crystallize in five
3
4 2
3
+
different crystallographic systems depending on their synthesis method and the size of RE
ion. Low temperature forms have tetragonal (RE = Y, Ce, Gd) [12-14], orthorhombic (RE =
Y, La-Gd) [1, 12, 15-17], or two monoclinic (RE = Gd-Lu) [12, 18] structures. High
1

temperature modification of the phosphates shows a trigonal symmetry of ideal glaserite [17].
The highest luminescence efficiency was found for the doped compounds with the monoclinic
structure, also derived from glaserite [9, 10].
The recent review concerned with the glaserite type compounds describes in detail their
chemical and structural diversity [19]. This type of structure has been widely observed in
XA M(TO ) compounds, where X, A, M and T are four different cationic positions. The X and
2
4 2
A sites are occupied by alkali metals or alkali earth metals and also by Ag, Tl or Pb. The M
atom could be represented by one of 32 metals, inter alia rare earth elements. At the T position
transition metals (V, Cr, Fe, Mo, Ru, W, Re) or non-metallic elements (Si, P, S, Se) can be
found. There is a variety of space groups that represent the family of compounds, including
such as P3m, C2/m, P2/m, P1, C2/c . In X RE(PO ) glaserites, the primary structural unit
3
4 2
is PO tetrahedron, that shares its oxygen ions with REO octahedra [19, 20]. The reduction in
4
6
the from symmetry from trigonal to monoclinic is possible by rotation of PO tetrahedrons.
4
This is also accompanied by an increase of RE coordination number [20].
It has been reported that the X RE(PO ) phosphates are characterized by rich
3
4 2
polymorphism. Unfortunately, it is difficult to precisely characterize polymorphic
transformations in the phosphates by thermoanalytical methods. The observed number of
thermal effects as well as their onset temperatures often vary according to synthesis technique
and thermal history of the double rare earth phosphates [21-23].
Literature data on the polymorphism of the Na Y(PO ) compound is sparse. Two
3
4 2
structural modifications of the Na Y(PO ) compound have been reported: the orthorhombic
3
4 2
one with the space group Pca2 and the unit cell parameters a = 13.94 Å, b = 5.292 Å and c =
1
1
1
8.33 Å [1, 24] and the tetragonal one with unit cell parameters a = b = 12.918 Å and c =
0.613 Å [13].
2

The undefined polymorph of Na Y(PO ) has been prepared by a prolonged process
3
4 2
carried out in solid state at 900°C [25]. The authors have found that, upon heating of the
compound, a strong thermal effect on DTA curve at 850°C was observed. The effect was
assigned to βα-Na Y(PO ) polymorphic transition. In the authors’ opinion, the high
3
4 2
temperature form of Na Y(PO ) () can be supercooled to room temperature.
3
4 2
The optical characterization of Na Y(PO ) with the orthorhombic structure has been
3
4 2
presented in a series of papers [5-8]. The dielectric properties of the same phase have been
discussed in [26].
In our running study on the structural characterization of Na RE(PO ) :Yb phosphates
3
4 2
(RE=La, Gd, Y) [27], we found that Pechini method allows to get a monoclinic modification
of Na Y(PO ) phosphate with the space group C2/c and the unit cell parameters: a = 15.77 Å,
3
4 2
b = 19.18 Å, c = 6.96 Å, =92.52°. The XRD pattern of the phosphate was similar to that of
Na Gd(PO ) [18]. The Na Y(PO ) compound shows two structural transformations at 834
3
4 2
3
4 2
and 926°. Annealing of the monocline Na Y(PO ) phase at high temperature resulted in
3
4 2
structural transition and a change of its structure to orthorhombic one (unit cell parameters: a
13.94 Å, b = 5.292 Å and c = 18.33 Å), similar to that of Na Nd(PO ) [16]. Our study
=
3
4 2
dedicated to the Na RE(PO ) :Yb phosphates (RE= La, Gd or Y) will be published in the near
3
4 2
future.
In most cases rare earth double and ternary phosphates are synthesized by sintering of
substrates at high temperature (ceramic method) or crystallization of the compounds from
liquid (flux method) [5-8, 26]. The latter method is only applicable to the preparation of
compounds stable at high temperature. Only a few works present the results of research on
techniques alternative to the above ones. Na Ce(PO ) [28] and K Ce(PO ) ·H O [29]
3
4 2
2
4 2
2
compounds were precipitated from solutions under hydrothermal heating for 5-7 days at a
temperature of 250°C and 140°C, respectively. The results of our group's research on the low-
3

temperature syntheses of rare earth double and ternary phosphates have been shown in few
studies, in which the compounds obtained by Pechini [21, 22, 30] and hydrothermal [31]
methods were characterized in terms of morphology and properties of the obtained phases.
A number of useful properties of the studied compounds, including optical, are strongly
dependent on their structure and microstructure type. The aim of this work is to develop
optimal conditions for the synthesis of Na Y(PO ) (chemical composition of solutions,
3
4 2
temperature and time of the process) by hydrothermal method as well as determination of the
effect of the applied heat treatment on the quality of the obtained powders. Detailed
characterization of the synthesized powders was done by ICP-OES, X-ray diffraction (XRD),
thermal analysis (DSC/TGA), FT-IR spectroscopy and scanning electron microscopy (SEM).
The study of the thermal behavior of the Na Y(PO ) compound has been presented here in
3
4 2
order to complete the sparse literature data on its polymorphism and structural forms. This
should facilitate an interpretation of other physicochemical properties of the Na Y(PO )
3
4 2
double orthophosphate.
2
2
. Experimental
.1 Synthesis
The Na Y(PO ) phosphate was precipitated from sols under hydrothermal conditions. The
3
4 2
sols were prepared by digestion of yttrium nitrate and NH H PO (analytically pure, POCh
4
2
4
3
+
Gliwice, Poland) in distilled water. The concentration of Y and NH H PO in sols was equal
4
2
4
-3
and amounted to 0.050 mol·dm . Yttrium nitrate was obtained by digestion of 5.00 g of Y O
2
3
(99.99% POCh, Gliwice, Poland) in ~30 ml of aqueous nitric acid solution (~48%) and drying
of the clear solution over solid NaOH until crystals appeared. The sols were also modified by
addition of complexing agents: citric acid monohydrate (CA, analytically pure, Chempur,
Poland) and ethylenediaminetetraacetic acid (EDTA, >99.0% Fluka, Switzerland) or
surfactants: sodium dodecyl sulfate (SDS, >98.5%, Sigma-Aldrich, Germany), cetrimonium
4

bromide (CTAB, >98%, Sigma, USA) or triblock PEG-PPG-PEG copolymer (Pluronic P-123,
Aldrich, Germany). The surfactants were used in concentrations above their critical micelle
concentration (CMC).
3+
+
After one-day ageing, the sols were mixed with Na PO ·12H O in the Y :Na molar
3
4
2
ratio 1:45. The hydrothermal reactions were performed in a Magnum II autoclave (Ertec,
Poland) with heating of the sols by microwaves of the maximal power of 600 W. The
syntheses were controlled by temperature measurement performed at the bottom part of a
Teflon reactor. After 1 h of heating the obtained fine precipitates were centrifuged, washed
several times with distilled water and dried at 120°C. The Table 1 shows the conditions of the
hydrothermal processes, i.e. the concentration of complexing agents and/or surfactants in sols,
heating temperature, pressure.
The Na Y(PO ) compound has also been obtained by solid state reactions of NaNO
3
3
4 2
(
99.8%, PPH POCh Gliwice, Poland), Y O and NH H PO (analytically pure, POCh
2 3 4 2 4
Gliwice, Poland) at high temperature. The substrates were mixed in stoichiometric molar
ratio, ground in an agate mortar, pelletized and sintered at 400°C (2 h), 900°C (10 h), and
finally at 1100°C (15 h). So obtained compound was further annealed at different
temperatures for 10 h.
2
.2. Characterization methods
Phase analysis of powdered samples was made using XRD technique and a Siemens
D5000 diffractometer equipped with a Cu X-ray tube. The measurements were performed in
θ range of 5–90° with a 0.02° step and at least 6 s per step. The indexing of powder patterns
2
was made using the computer program DICVOL [32]. The lattice constants of the
Na Y(PO ) unit cell were refined using Checkcell software [33].
3
4 2
The quantitative composition of sample obtained by hydrothermal process has been
determined by inductively coupled plasma – optical emission spectrometry (ICP-OES, Varian
5

Vista-MPX). For the experiment, 0.0300 g of powder was digested in 5 ml of concentrated
HCl (analytically pure, 35-38%, Chempur, Poland) and diluted with re-distilled water to
3
0
.25 dm . The emission for each element was determined three times.
TM
The SETSYS apparatus (TG-DSC 1500; Setaram) was used for the DSC/TGA studies.
The powdered samples (~40 mg) were placed in a Pt crucible and heated to 1200°C at a
–1
heating rate of 10°C·min using Ar as the purge gas. The temperature and sensitivity
calibration factor for the thermal experiments were obtained at the melting points of NaCl
(
801 °C), Ca P O (1353 °C), K SO (1070°C) and for the phase transition temperature of the
2 2 7 2 4
latter compound (584°C).
-1
The IR spectra at room temperature were measured in the spectral range of 400–4000 cm
using a FT-IR Biorad 575C spectrometer. The samples were blended in KBr pellets.
The SEM images of carbon coated powders were registered using JSM 5800LV
microscope (Jeol, Japan).
3
3
. Results and discussion
.1 Characterization of the powders obtained by hydrothermal technique
The Figures 1-3 show the registered XRD patterns of the powders S1-S7 dried or sintered
at 1100°C. It can be seen the well-crystallized powders (S2-S7) were obtained in the
hydrothermal processes carried out at 240°C. Lowering of the synthesis temperature to 180°C
(powder S1) resulted in the precipitation of an amorphous powder (Fig. 1A) . The XRD
patterns of dried S2-S7 samples were similar, indicating the same crystal structure of
precipitated powders. The hydrothermally obtained Na RE(PO ) :Yb (RE= Y, Gd)
3
4 2
compounds have the structure of ideal glaserite with the trigonal unit cell and the space group
P3m1 (Z=1) [34]. In the present study, the indexing of the diffraction pattern of the yttrium
compound resulted in the following unit cell values: a=b=5.308(9) Å, c=7.089(0) Å, =120°,
3
cell volume V= 172.96 Å . The results of indexing procedure are shown in the Table 2. The
6

trigonal symmetry of the obtained double phosphate seems to be surprising, because this type
of structure has only been reported for high temperature polymorphs of the Na RE(PO )
3
4 2
orthophosphates [17]. However, it has often been observed that solvothermal synthesis can
lead to precipitation of metastable phases and also high temperature forms of compounds.
The chemical composition of the obtained phosphate was accurately determined by the
ICP-OES technique. The chemical analysis results of the sample S2 shown in the Table 3
indicate that the obtained compound has a chemical composition corresponding to the
Na Y(PO ) orthophosphate.
3
4 2
The SEM images of the powders precipitated from sols S2-S7 are shown in the Fig. 2. The
powder S2 is characterized by the presence of elongated spindle-shaped particles of about
3
x0.5 m in size of (Fig. 2A). As it follows from the SEM images, the addition of any type of
surfactant has no influence on the shape or size of Na Y(PO ) particles (Figs 2 D-F). None of
3
4 2
the surfactants modified the morphology of the precipitated phase, so the Na Y(PO )
3
4 2
crystallization process seems to be faster than micelles formation in sols.
However, we found significant differences in the microstructure of the powders obtained
in the presence of complexing agents (Figs 2B and C). The particles crystallized from the
solution containing CA (Fig. 2B) are less regular in shape, which may be caused by some
difficulties during phosphate crystallization. The effect of EDTA addition is even stronger:
the observed particles are larger and have an octagonal cross section. A significant difference
in the morphology of the S2 and S4 powders suggests the presence of phases differing in their
chemical composition
Annealing of the powders at 1100°C caused an increase in their crystallinity, which
facilitated the identification of the phase composition of the powders (Table 4). The main
crystalline phase in heat treated S2-S7 powders was Na Y(PO ) phase with monoclinic
3
4 2
structure (see section 3.2) (Figs. 3 B-C and 4). However, in powders S3 and S4 obtained from
7

sols with complexing agents (CA or EDTA), secondary phase in form of xenotime (YPO4)
was present (Fig. 4 A and B). YPO was also the main crystalline phase in S1 sintered at
4
1
100°C (Fig. 3 A). Since the reflections of xenotime were found in the XRD pattern of the
initially poorly crystallized sample S1 and also in the diffractogramms of the samples
obtained in the presence of CA or EDTA (powders S3 and S4), it can be assumed that both
complexing agents inhibit the crystallization process of Na Y(PO ) . Thus, the attempt to
3
4 2
apply CA and EDTA as the chemicals limiting the growth of Na Y(PO ) crystallites is not
3
4 2
well founded, because they mainly reduce crystallization efficiency of the double
orthophosphate, while worsening the phase purity.
3
.2 Polymorphism of the Na Y(PO ) compound
3 4 2
The DSC heating and cooling curves of Na Y(PO ) obtained by hydrothermal method
3
4 2
(precipitated from sol S2) and in solid phase, are shown in Fig. 5. The DSC curve of the
hydrothermally synthesized sample shows three endothermic effects at the following onset
temperatures: 840, 926 and 965°C. These effects are related to the presence of structural
changes in the compound, as evidenced by the differences in the XRD patterns (Fig. 6) and
the FTIR spectra (Fig. 7). The change of the crystal structure in powders heated at 900°C and
1
300°C was confirmed by XRD patterns of quenched samples.
On the DSC heating curve of the sample obtained by quenching from 1300°C (Fig. 5B) an
additional weak exothermal effect at ~ 535°C is visible. The presence of exothermic effects is
characteristic for metastable phases, mostly high-temperature structural polymorphs
overcooled to normal conditions [35-37]. The metastable phases convert on heating to a
thermodynamically stable polymorphs, which is accompanied by the heat release. The
achievement of an equilibrium state of powders could be then realized by a heat treating at a
temperature slightly higher than the onset temperature of the registered exothermic effect.
8

The Fig. 5 also shows the cooling curve of Na Y(PO ) powder with visible two
3
4 2
exothermal effects. The effect at 797°C is characterized by a considerable temperature
hysteresis of about 40°C, compared to the effect recorded during the heating of the same
sample (at 840°C). The presence of the thermal hysteresis on the cooling curves is a
characteristic feature of a reconstructive conversion [38]. These transformations are
associated with a significant structural change, requiring inter alia, breaking of some bonds
and building of new ones [39]. The transformation process can be kinetically hindered by
slow diffusion in solid state. In addition, a large volume change of an elementary cell of
double RE orthophosphates has also been reported to influence the transformation kinetics in
glaserites [23]. Sometimes the hysteresis is so large that the thermal effects on the cooling
curves become apparent after a significant modification of the DTA measurement conditions
[
36].
On the DSC cooling curve of the Na Y(PO ) compound (Fig. 5B), no thermal effect at
3
4 2
9
64°C is visible. Worthy of note, on the heating curve of the same phosphate obtained by the
Pechini method, no transitions have been observed at this temperature [27]. In our previous
works [21, 22] we reported thermal properties of the Na Ce(PO ) and K Nd(PO ) glaserite-
3
4 2
3
4 2
type phosphates. The applied synthesis route of compounds significantly influenced
transformation mechanism of the compounds. On the DSC curves of fine grained samples
obtained by Pechini method one thermal effect was only recorded while for the compounds
synthesized by sintering at high temperature the same polymorphic transformation was
accompanied by two thermal effects. It was also striking that the total enthalpy change
accompanying the transformation was constant and independent of the conditions of the
powder synthesis. The likely reason of the difference in the structural transformation process
was a different degree of material crystallinity, which is low for the powders synthesized by
Pechini method (nanocrystalline samples). In other phosphates a strong influence of
9

preparation techniques and the size of single crystals on structural changes has also been
found [23, 40].
The XRD patterns of Na Y(PO ) subjected to various heat treatments are shown in Fig. 6.
3
4 2
The XRD pattern of Na Y(PO ) heated at 500°C (Figure 6B) is characterized by an increased
3
4 2
number of reflections as well as by their broadening. There is an evident change of the
Na Y(PO ) trigonal structure, which is assisted by lowering of the crystallinity degree of the
3
4 2
powder. It can be caused by recrystallization of the hydrothermally obtained metastable
phase. It should be emphasized that the compounds obtained under elevated pressure are not
always thermodynamically stable and during subsequent thermal processing may undergo
structural changes. Similarity of XRD patterns obtained at 500 and 700°C (Figs. 6B and C)
indicates the presence of the same structural modification. The differences between the
registered diffractogramms (the spectra at 700°C are of better resolution) appear to be
associated with a higher crystallization degree of the sample only. The recorded diffraction
pattern of Na Y(PO ) (Fig 6C) corresponds to the diffractogramm of the monoclinic form
3
4 2
orthophosphate synthesized by Pechini technique [27].
Quenching of Na Y(PO ) from 900°C and 1300°C to room temperature resulted in
3
4 2
consolidation of subsequent polymorphs of the compound. The phase obtained by quenching
from 1300°C has an orthorhombic structure, isomorphous to that of Na Nd(PO ) [16].
3
4 2
Unfortunately, the attempt to index the pattern of the sample quenched from 900°C did not
bring a reasonable solution, hence the structure of the phase remains unknown.
From the literature data [4, 41-43] follows, that the RE phosphates have an IR absorption
-1
bands in the 1170-1000 cm range attributed to the asymmetric stretching of PO groups,
4
-1
while the symmetric stretching of the same groups manifests in the 1000-900 cm range.
Asymmetric and symmetric deformation modes of PO can be observed in higher frequency
4
-1
-1
ranges, typically in 650-520 cm and 490-400 cm .
1
0

The FTIR spectra of the obtained Na Y(PO ) phosphate are shown in Fig. 7. The number
3
4 2
of FTIR bands of the Na Y(PO ) polymorphs changes with the increased temperature of
3
4 2
sintering. In the spectrum of the trigonal form of the double sodium yttrium phosphate, a few
bands are visible only. The low number of absorption bands is a characteristic feature of the
phosphates with a high degree of site symmetry of the PO polyhedron (C ) and has also
4
3v
been observed in other types of hexagonal phosphates [41-44]. It is evidenced from the FTIR
spectra that the number of IR bands is larger for powders heated at higher temperatures
compared with the trigonal Na Y(PO ) form. Indeed, the comparison of vibrational spectra of
3
4 2
the trigonal Na RE(VO ) with the PO point symmetry of C [44] and that of orthorhombic
3
4 2
4
3v
Na RE(PO ) (C ) [4, 41] indicates that lowering of the PO site symmetry results in splitting
3
4 2
2v
4
of IR spectra. This should also be valid for the monocline form of the orthophosphate, in
which the PO unit has C symmetry. The main experimental IR absorption bands of the
4
1
powders heated at different temperatures as well as their assignment to the internal modes are
summarized in the Table 5.
Based on the significant differences in the XRD and FTIR spectra, the existence of at least
four structural polymorphs of the Na Y(PO ) orthophosphate can be confirmed: the
3
4 2
metastable phase obtained directly by crystallization under hydrothermal conditions
trigonal), monocline one existing at temperatures below 840°C, the phase with an undefined
yet structure in the temperature range of 840-926°C and the phase formed above 926°C
orthorhombic).
(
(
4
. Conclusions
The Na Y(PO ) phosphate could be obtained by hydrothermal method in form of
3
4 2
monodisperse spindle-shaped particles of a few microns size. The addition of surfactants had
no effect on particle morphology, whereas the use of ethylenediaminetetraacetic or citric acid
inhibited Na Y(PO ) crystallization and resulted in the formation of multi-phase powders.
3
4 2
1
1

The proposed hydrothermal technique is a perfect tool for the obtaining of doped
3
+
Na Y(PO ) phases. The active RE ions can be introduced by dissolving of an appropriate
3
4 2
nitrate in the starting sol, which ensures a homogenous distribution of the doping ion in the
Na Y(PO ) matrix.
3
4 2
The Na Y(PO ) compound is characterized by rich polymorphism. The resulting powder
3
4 2
has an ideal structure of glaserite, which changes into other polymorphs under heating. At
least four structural forms of the double orthophosphate can be obtained by choosing a
suitable synthesis technique. Because the crystal structure of a compound influences its main
physicochemical properties and particular luminescence of doped phases, special attention
should be paid to the applied heat treatment of the Na Y(PO ) phosphate. The rich
3
4 2
polymorphism of Na Y(PO ) also complicates the analysis of the other properties of the
3
4 2
compound and explains the difficulties in interpreting the results of some other studies.
Acknowledgements
The study was financially supported by National Science Centre under grant N N209 767240.
1
2

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1
7

Figures Captions
Fig. 1. The XRD patterns of the samples S1 and S2 dried at 120°C.
Fig. 2. The SEM images of dried precipitates from the sols S2–S7 obtained by hydrothermal
heating at 240°C for 1 h; A) sol S2, B) sol S3, C) sol S4, D) sol S5, E) sol S6, and F)
sol S7.
Fig. 3. The XRD patterns of the samples S1 (A) and S2 (B) sintered at 1100°C and cooled
slowly to room temperature. The diffractogram of the monoclinic Na Y(PO ) phase
3
4 2
obtained in solid state at 1100°C is shown for comparison (C).
Fig. 4. The XRD patterns of the samples S3 (A), S4 (B), S5 (C), S6 (D), and S7 (E) sintered at
100°C and cooled slowly to room temperature. The main phase is the monoclinic
form of Na Y(PO ) .
1
3
4 2
Fig. 5. The heating and cooling DSC curves of Na Y(PO ) : (A) obtained by a hydrothermal
3
4 2
technique (sample S2), and (B) the phosphate synthesized by ceramic method and
quenched from 1300°C.
Fig. 6. The XRD patterns of Na Y(PO ) phosphate obtained by hydrothermal method (A and
3
4 2
B) or ceramic method (C - E) quenched from different temperatures, where A is
trigonal, B and C – monoclinic, D – undefined and E – orthorhombic form of
Na Y(PO ) .
3
4 2
Fig. 7. The FTIR spectra of Na Y(PO ) phosphate obtained by hydrothermal method (A and
3
4 2
B) or ceramic method (C - E) quenched from different temperatures.
1
8

Tables Captions
Table 1. Concentration of complexing agents and surfactants in the sols as well as the
temperature of hydrothermal processes.
Table 2. The refined and indexed XRD planes of the hydrothermally obtained Na Y(PO )
3
4 2
phosphate.
Table 3. The results of chemical analysis of the sample S2 (ICP-OES method).
Table 4. The phase composition of the powders S1-S7.
Table 5. Wavenumbers observed in the IR spectra of the Na Y(PO ) polymorphs obtained at
3
4 2
different temperatures.
1
9

Table 1. Concentration of complexing agents and surfactants in the sols as well as the
temperature of hydrothermal processes.
Sol
S1
-
S2
-
S3
5:1
-
S4
-
S5
-
S6
-
S7
-
CA:Y molar ratio
EDTA:Y
-
-
5:1
-
-
-
concentration of SDS
-3
-3
-
-
-
-
-
-
-
-
5.2·10
-
-
-
-
(mol·dm )
concentration of CTAB
-3
-3
1.1·10
(
mol·dm )
concentration of
Pluronic P–123
-
-
-
-
-
-
2.0
-3
(
g·dm )
heating temperature
°C)
maximal pressure (bar)
1
80
8
240
20
240
20
240
20
240
20
240
20
240
20
(
2
0

Table 2. The refined and indexed XRD planes of the hydrothermally obtained Na Y(PO )
3
4 2
phosphate.
h
k
l
2 (°)
2 (°)
(2)
-0.002
0.004
-0.002
0.002
-0.001
0.003
-0.002
-0.004
0.000
0.001
0.001
obs
calc
0
0
0
0
1
0
0
1
0
1
2
1
1
0
1
1
2
2
2
3
1
2
0
1
2
2
0
1
2
2
0
4
0
19.290
23.043
25.099
31.854
33.741
41.256
47.077
59.160
60.348
63.007
70.954
19.292
23.039
25.101
31.852
33.742
41.253
47.079
59.164
60.348
63.006
70.953
2
1

Table 3 The results of chemical analysis of the sample S2 (ICP-OES method).
Concentration
ppm)
Concentration
(mmol·dm )
(Element/Y)exp.
molar ratio
Element
-3
(
Na
Y
30±4
38±6
28±4
1.305±0.174
0.427±0.067
0.904±0.129
3.06
1.00
2.12
P
2
2

Table 4 The phase composition of the powders S1-S7.
Powders dried at 120°C
S1
amorphous
Na Y(PO ) (trigonal)
S2-S7
3
4 2
Powders sintered at 1100°C (slowly cooled)
S1
YPO4
S2, S5, S6, S7
S3, S4
Na Y(PO ) (monoclinic)
3 4 2
Na Y(PO ) (monoclinic), YPO
4
3
4 2
2
3

Table 5. Wavenumbers observed in the IR spectra of the Na Y(PO ) polymorphs obtained at
3
4 2
different temperatures.
Temperature
900°C
Assignment
1
20°C
700°C
1170
1300°C
1
1
094
066
 (PO )
as
4
1
1
1
1
1
107
071
065
048
019
1108
1093
1076
1049
1028
1017
1107
1095
1075
1050
1033
1013
1
011
1013
1
9
9
9
003
97
1000
997
974
961
948
 (PO )
s
4
999
989
76
61
9
8
6
50
70
08
956
20
950
9
734
734
613
614
604
594
586
611
594
 (PO )
as
4
6
5
5
02
96
90
5
5
83
64
581
76
568
5
578
565
564
548
5
5
57
44
544
4
4
73
58
 (PO )
s
4
4
4
43
26
442
428
419
442
419
4
19
418
2
4

Figure 1