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Enikeeva M.O., et al. Nanosystems: Phys. Chem. Math., 2023,14 (6), 660-671.
http://nanojournal.ifmo.ru
Original article
DOI 10.17586/2220-8054-2023-14-6-660-671
Synthesis under hydrothermal conditions and structural transformations of nanocrys-tals in the LaPO4-YPO4-(H2O) system
MariaO. Enikeeva1'2 ", OlgaV. Proskurina1'2'6, EvgenyYu. Gerasimov3 c, Vladimir N. Nevedomskiy1d, Victor V. Gusarov1e
1 loffe Institute, 194021 St. Petersburg, Russia
2St. Petersburg State Institute of Technology, 190013 St. Petersburg, Russia
3Federal Research Center, Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia
[email protected], [email protected], [email protected], d [email protected], [email protected] e .ru
Corresponding author: M.O. Enikeeva, [email protected]
Abstract Structural transformations of nanocrystals in the LaPO4-YPO4-(H2O) oxide system were investigated under hydrothermal conditions at 230oC, depending on isothermal holding times (2 hours, 7 days and 28 days). It was shown that before hydrothermal processing, phases crystallize in the system with rhabdophane structures Lai-xYxPO4 • nH2O (0 < x < 0.80) and xenotime YPO4. It has been determined that increasing the duration of isothermal treatment under hydrothermal conditions leads to the transformation of the rhabdophane phase into phases with monazite and equilibrium xenotime structures, with an intermediate crystallization of the metastable monazite phase. It is noted that only after 28 days of hydrothermal treatment at 230oC, the system approaches the equilibrium composition of the phases with monazite structures La0.97Y0.03 PO4 (with crystal sizes of 18-50 nm) and xenotime YPO4 (with crystal sizes of 45-90 nm). In a single-phase sample with a monazite structure La0 75Y0. 25PO4, the average crystal size remains unchanged at around 20 nm after 2 hours, 7 days and 28 days of hydrothermal treatment at 230oC.
Keywords nanocrystals, monazite, xenotime, rhabdophane, structural transformations, yttrium phosphate, lanthanum phosphate.
Acknowledgements Powder XRD measurements, Rietveld refinement of crystal structures and EDX analysis have been carried out with using the equipment and software of the Engineering Department of the St. Petersburg State University of Technology (Technical University). TEM investigations carried out with using the equipment of the Joint Research Center 'Materials science and characterization in advanced technology' (Ioffe Institute, St. Petersburg) and HRTEM study was carried out using the facilities of the "National center of investigation of catalysts" at Boreskov lnstitute of Catalysis. The present work of M.O.E., O.V.P. and V.V.G. was supported by the Russian Science Foundation (project no. 21-13-00260).
For citation Enikeeva M.O., Proskurina O.V., Gerasimov E.Yu., Nevedomskiy V.N., Gusarov V.V. Synthesis under hydrothermal conditions and structural transformations of nanocrystals in the LaPO4-YPO4-(H2O) system. Nanosystems: Phys. Chem. Math., 2023,14 (6), 660-671.
1. Introduction
Materials based on nanocrystalline rare-earth phosphates are of significant interest due to their wide range of applications in optics [1-4], catalysis [2,5,6], medicine [7-9], fabrication of gas sensors [10] and other fields [11-13]. Special attention is given to the binary- and multi-component systems based on rare-earth orthophosphates, as they are used as luminescent materials [14,15] and matrices for immobilizing toxic and radioactive waste [16-18].
Depending on the synthesis conditions, rare-earth phosphates can be obtained in the form of phases with a monazite
structure (LREPO4, LRE=La^Gd, space group (SG): P2i/n) [19-21], xenotime structure (HREPO4, HRE=Tb^Lu, Y,
Sc; SG: I4x/amd) [19,21], rhabdophane structure (REPO4 • nH2O, RE=La^Gd, Y, n=0-1; SG: C2 or P6222) [22,23] and churchite or weinschenkite structure (HREPO4 • 2H2O, HRE=Tb^Lu, Y; SG: C2/c) [24-26]. Hydrated phases with rhabdophane and churchite structures are often considered as precursors for the formation of anhydrous phases with monazite and xenotime structures. It was found that the loss of structurally bound water with increasing temperature leads to the transition of the rhabdophane structure to monazite and the churchite structure to xenotime for individual compounds of the REPO4 • nH2 O type [27,28]. An exception is yttrium orthophosphate, which crystallizes under certain conditions into a phase with a rhabdophane structure that can transform into a phase with a xenotime structure [29]. However, it is also possible to form anhydrous phases with monazite and xenotime structures without the use of high temperatures [30,31].
© Enikeeva M.O., Proskurina O.V., Gerasimov E.Yu., Nevedomskiy V.N., Gusarov V.V., 2023
For the synthesis of nanocrystalline materials based on rare-earth orthophosphates, soft chemistry methods are commonly used, such as precipitation [28, 32-34], sol-gel [3, 35-38], hydrothermal and hydrothermal-microwave synthesis [1,4,38-41]. At times, precipitates obtained by soft chemistry methods are annealed at temperatures > 500oC to obtain samples with monazite and xenotime structures [18,20,43].
Studies of structural transformations in the LaPO4-YPO4-(H2O) system were investigated in works [34,36,44-49]. During heat treatment at temperatures of 1000-1600° C, phases with monazite and xenotime structures are formed in the system. There is a wide homogeneity range of solid-solution with a monazite structure (up to 42 mol. % YPO4 at T = 1600°C), while the solubility of LaPO4 in the xenotime phase is extremely low (less than a few percent at T = 1600°C). Phases with rhabdophane, monazite and xenotime structures were obtained in the LaPO4-YPO4-H2O system using soft chemistry synthesis methods. However, research in the low-temperature range remains poorly systematic and has little focus on phase transformations in the system with varying synthesis durations. This information can be valuable for improving the understanding of phase stability in the LaPO4-YPO4-H2O system and predicting the behavior of functional materials based on it.
Taking into account the above considerations, this study aims to investigate the structural transformations of nanocrys-tals in the oxide system LaPO4-YPO4-(H2O) under hydrothermal conditions, focusing on the influence of the duration of isothermal holding times.
2. Experimental
The following chemical reagents were used for the synthesis of nanopowders with the composition La1-xYxPO4 • nH2O (x =0-1): lanthanum (III) nitrate hexahydrate La(NO3)3 • 6H2O (puriss.), yttrium (III) nitrate hexahydrate Y(NO3)3 • 6H2O (puriss.) and ammonium dihydrogen phosphate NH4H2PO4 (puriss.).
The precipitation was carried out from solutions of the reagents in distilled water by adding the NH4H2PO4 solution to the rare-earth nitrate solution with continuous stirring at T = 25 ± 5°C.The resulting white suspension with pH=1 was constantly stirred for 15 minutes.
For hydrothermal synthesis, the suspension obtained by precipitation was transferred into a Teflon-lined autoclave (filling factor 0.7) and hydrothermally treated at T = 230°C and P ^10 MPa with isothermal holding times: 2 hours, 7 days and 28 days.
Samples obtained by both precipitation and hydrothermal treatment were washed with distilled water until reaching a pH of 7, centrifuged (at 10,000 rpm), dried at T = 75° C for 24 hours and subsequently ground using an agate mortar.
The elemental composition of the samples was determined using energy-dispersive X-ray spectroscopy (EDS) with a Tescan Vega 3 SBH scanning electron microscope (Tescan, Czech Republic) equipped with an Oxford Instruments INCA x-act microanalysis system (UK). The measurements were conducted in the energy range up to 20 keV in five different areas and the obtained values were averaged.
X-ray diffraction (XRD) analysis of the samples was performed based on the powder diffraction patterns obtained using a Rigaku SmartLab 3 X-ray diffractometer (Cu Ka- radiation) in the angular range of 26 = 10-90° with a step size of 0.01° and scan speeds of 0.2°/min and 0.8°/min. Phase analysis of the samples was carried out using the ICSD PDF-2 and CSD databases. Quantitative phase analysis was performed using the SmartLab Studio SmartLab Studio II v4.4.241.0 (Rigaku Corporation, Japan). The average crystallite sizes were determined using non-overlapping reflections for each phase: the 200 reflection for the monazite structure phase, the 101 reflection for the xenotime structure phase and the 111 reflection for the rhabdophane structure phase. The average crystallite size of xenotime phase in the samples obtained by precipitation was determined using the Scherrer formula [50] based on 101 and 200 reflections. The composition of the variable composition phase with a monazite structure was determined using Retgers law [51] and literature data [20,46].
The investigation of the crystal structure and microstructure of the samples was carried out using high-resolution transmission electron microscopy (HRTEM) on an electron microscope ThemisZ (Thermo Fisher Scientific, USA), which provided a maximum lattice resolution of 0.07 nm and on an electron microscope JEM- 2100F (JEOL Ltd., Akishima, Tokyo, Japan) with an accelerating voltage of 200 kV. During the operation of the ThemisZ microscope, images were recorded using a Ceta 16 CCD sensor (Thermo Fisher Scientific, USA). The microscope ThemisZ was also equipped with an EDX Super-X spectrometer (Thermo Fisher Scientific, Bleiswijk, The Netherlands) with a semiconductor Si detector providing an energy resolution of 128 eV.
To construct particle size distribution histograms, particle images were analyzed by the ImageJ software. There were at least 40 particles in the samples. The distribution histogram was processed using the lognormal distribution law.
3. Results and discussions
According to the elemental analysis data, the molar ratio of RE:P is close to 1:1 in all samples, which corresponds to the stoichiometry of rare-earth phosphate. For the designation of the samples, an experimentally determined molar ratio of yttrium and lanthanum is used, expressed as Yn%, where n%= n(Y)/(n(Y) + n(La))100% Yn% - represents the molar percentage of yttrium relative to the sum of rare-earth elements, n(Y) is the molar fraction of yttrium in the sample and n(La) is the molar fraction of lanthanum in the sample.
Figure 1 shows the X-ray diffraction patterns of the samples obtained by precipitation. Nanocrystalline samples with compositions of 0%-Y73% exhibit a rhabdophane structure. In the Y81% sample, reflections of the rhabdophane structure are present, as well as broadened reflections corresponding to the xenotime structure, which are also observed in the Y93%-Y100% samples.
Fig. 1. The X-ray diffraction patterns of the samples obtained by precipitation
Figure 2(a) present the dependency of the unit cell volume per formula unit on the composition of the system for the rhabdophane and xenotime structure phases in the samples obtained by precipitation method. In the system, a solid solution with a rhabdophane structure Lai_xYxPO4 • nH2O (x < 0.81) is formed. In the Y81% sample, the obtained values of the V/z parameter for the rhabdophane structure have larger values compared to those for the Y73% sample, which is due to the crystallization of the YPO4 xenotime-structured phase in this sample as well. The presence of an amorphous phase containing lanthanum cannot be excluded in the Y93%-Y98% samples.
The dependence of the weighted average crystallite size with rhabdophane structure on the YPO4 content in the system is shown in Fig. 2(b). For La1_xYxPO4 • nH2O (x < 0.5), the formation of nanocrystals with an average size of 5-10 nm. Further increase in the molar fraction of yttrium orthophosphate in the system leads to an increase in the sizes of rhabdophane-structured crystals several times. In the Y81% sample, the value D = 45 ± 30 nm. The average size of xenotime-structured phase crystals in Y93%-Y100% samples, determined by the Scherrer formula, does not exceed 5 nm.
Figure 3(a,c,e) present microphotographs of Y23%, Y73% and Y100% nanoparticles obtained by precipitation method. Microphotographs for the LaPO4 sample obtained by precipitation method are presented in the work [32]. Based on the analysis of the images, histograms of particle size distribution according to their cross-sectional area (thickness) were constructed, as shown in the Appendix (Supporting Materials Fig. S1(a,b,c)). The Y23% nanoparticles with rhabdophane structure have a rod-like shape (Fig. 2(a)) with a narrow distribution of particle thicknesses (Fig. S1(a)). The thickness of the nanorods varies in the range of 5-10 nm.
In the Y73% sample (Fig. 3(c)), the particles have an prolate form with a significantly lower aspect ratio of geometric parameters compared to the LaPO4 sample [32]. The particle size distribution for the Y73% sample with rhabdophane structure, shown in Fig. S1(b), is considerably wider than that of the Y23% sample. The particle thickness varies in the
Fig. 2. a) The dependence of unit cell per formula unit of structure on the yttrium content; b) The mean volume weighted crystallite size (D) of the rhabdophane-structured nanocrystals Lai_x YxPO4 • nH2O (x < 0.81), obtained by precipitation method, on the composition of the system
range of 10 to 50 nm. The average crystal sizes for the Y23% and Y73% samples (Fig. 2(b)) with rhabdophane structure correlate quite well with the average particle thicknesses (Fig. S1(a,b)) of these samples. This was also shown in the work [45] for samples with the composition LaPO4:YPO4=1:3.
In the Y100% sample (Fig. 3(e)), there are nanocrystals with interplanar distances d « 3.45 A and d « 2.56 A corresponding to the (200) and (112) planes of the xenotime structure. The cross-sectional size of the nanoparticles in the sample does not exceed 10 nm (distribution shown in Fig. S1(c)) and correlates with the crystal size of the xenotime-structured phase, as determined by the Scherrer formula 5 nm).
The microdiffraction patterns of the Y23%, Y73% and Y100% samples are shown in Fig. 3(b,d,f). In the Y23% and Y73% samples, only rhabdophane structure reflections are observed, while in the Y100% sample, only xenotime-structured phase reflections are presented.
Figure 4(a) shows a microphotograph of particles from the Y73% sample with rhabdophane structure, synthesized by precipitation method. The results of elemental mapping are shown in Fig. 4(b). Despite the Y73% sample with rhabdophane structure being single-phase, the distribution of elements La(La-series) and Y(Ka-series) in each particle is not homogeneous. Energy-dispersive analysis of an individual particle showed that the Y:La ratio at the edges of the particle is close to 4:1, while in the central part of the particle, it is approximately 3:2. The total amount of yttrium in all particles corresponds to the composition determined by energy-dispersive X-ray analysis using SEM, i.e., close to 73 mol.% YPO4. Apparently, the compositional inhomogeneity of the particles in the Y73% sample can be attributed to the difference in the solubility product of lanthanum and yttrium phosphates in an acidic media. This leads to the formation of nanocrystals with a rhabdophane structure containing a relatively higher amount of La than indicated by the stoichiometry of the precipitated phase. As a result, the mother liquid becomes depleted in La3+ ions and an yttrium-enriched rhabdophane phase crystallizes on the periphery of the particles. Fig. S2 shows the image of an individual particle from the Y73% sample with complete elemental mapping, where the presence of lanthanum can also be observed predominantly in the central part of the particle.
In the LaPO4-YPO4-(H2O) system, after hydrothermal treatment (HT) at 230° C with different isothermal holding times (2 hours, 7 days and 28 days), phases with monazite, rhabdophane and xenotime structures crystallize. The diffraction patterns of the samples are presented in the Supporting Materials in Fig. S3, S4 and S5.
The phase quantities and the dependence of the unit cell per formula unit of structure on the yttrium content after hydrothermal treatment are shown in Fig. 5(a-f). HT of samples for 2 hours (Fig. 5(a,b)) leads to the formation of a solid solution with a monazite structure La1_xYxPO4 (x < 0.38), the formation of a solid solution with a rhabdophane structure and the formation of a phase with a xenotime structure. The single-phase solid solution with a monazite structure has a composition of 0 < x < 0.25. Partial phase transformation the rhabdophane structure to the monazite structure occurs in the samples 0.25 < x < 0.70, which is consistent with the data on the LaPO4:YPO4=1:1 sample obtained under hydrothermal-microwave conditions [49]. In samples Y71%-Y80%, one phase with the rhabdophane structure is formed, similar to before HT (Fig. 2(a)). For samples with a composition of 0.85 < x < 0.92 after 2 hours of HT, the initial precipitate, which contains not only amorphous phase but also nanocrystalline xenotime, transforms into phases with
Fig. 3. TEM images and microdiffraction patterns samples synthesized by precipitation method: a,b) Y23%; c,d) Y73% and e,f) Y100% (YPO4)
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Fig. 4. a) TEM image and b) element mapping particles from the Y73% sample, synthesized by precipitation method
structures of monazite, rhabdophane and xenotime. The monazite phase is depleted in YPO4 component compared to the nominal composition of the samples and has the composition La1-xYxPO4 (x < 0.28). The fraction of monazite phase in samples containing 0.85 < x < 0.92 is approximately 10%. The composition of the xenotime phase can be considered close to YPO4 within the experimental error. The composition of the rhabdophane phase La1-xYxPO4 • nH2O is difficult to determine accurately due to the narrow concentration range of a single-phase region after hydrothermal treatment. The fraction of xenotime phase in samples with a composition of 0.85 < x < 0.92 increases along with the fraction of yttrium phosphate in the system. In samples Y97% and Y100%, only the xenotime phase is observed in the diffraction patterns.
Figure 5(c,d) shows the phase ratios and changes in the unit-cell volume normalized to the number of formula units for the monazite and xenotime phases in samples after 7 days of HT. The samples have completely transformed from the rhabdophane structure to phases with monazite and xenotime structures. For samples with a composition of x < 0.25, the formation of the solid solution with a monazite structure is observed, similar to that after 2 hours of HT. Samples Y53%, Y74% and Y84% contain two phases with different compositions but with a monazite structure, as well as a phase with a xenotime structure. The diffraction patterns of the samples, presented in Fig. S4, show reflections of two monazite phases with the different dissolved yttrium, as well as reflections of the xenotime structure. For the Y53% sample, the profiles of the reflections of the two monazite phases heavily overlap, indicating their similar composition and a relatively large error in determining the parameters of the unit cell. In the Y74% and Y84% samples, the fraction of the monazite phase enriched in yttrium, with a composition of 0.28 < x < 0.37, does not exceed 30% and decreases with an increase in the amount of yttrium in the system. The fraction of the second monazite phase, with a composition of 0.09 < x < 0.12, does not exceed 8%.
The phase ratios and changes in the volume of the unit cell, normalized to one formula unit, for the monazite and xenotime phases in samples after 28 days of HT, are shown in Fig. 5(e,f). Samples Y54%, Y74% and Y84% contain a xenotime phase and a single phase with a variable composition monazite structure. The phase ratio in the entire two-phase region has a linear character, indicating a nearly equilibrium phase ratio in the LaPO4-YPO4-(H2O) system. It is worth noting that the composition of the monazite phase La1-xYxPO4 (x « 0.03) in the Y84% sample can be considered as the closest to the equilibrium at T = 230°C, P ~ 10 MPa (hydrothermal conditions). At compositions closer to this ratio of LaPO4:YPO4, phase transformations occur more slowly and in samples after 28 days HT, solid solutions with a monazite structure La1-xYxPO4 exist with a composition of 0.07 < x < 0.17.
The dependencies of the mean volume weighted crystallite size of phases in the samples after 2 hours of HT are shown in Fig. 6(a), after 7 days in Fig. 6(b) and after 28 days in Fig. 6(c). After 2 hours of HT, the crystallites with a monazite structure do not exceed 20 nm in samples with compositions of x < 0.60. The crystallites of the rhabdophane phase, which coexist with the monazite phase, have similar values of the mean crystallite size (around 20 nm). In the single-phase region under these HT conditions, the rhabdophane crystallites reach 80 nm. It should be noted that the dependence of the mean volume weighted crystallite size of the rhabdophane phase after HT-treated similar to samples, obtained by precipitation method and, tends to increase with an increase in the molar fraction of YPO4 in the system. The crystallites with a monazite structure in samples Y86%-Y92% exhibit significant variation in size, ranging from 20 to 100 nm. The xenotime phase crystallites have the mean volume weighted size of D « 60 nm with a fairly wide range of values.
Fig. 5. Phase ratio and change in the unit-cell volume normalized to the number of formula units for rhabdophane, monazite and xenotime structures on the yttrium content of samples after HT: a,b) 2 hours; c,d) 7 days; e,f) 28 days
Fig. 6. The dependence of the mean volume weighted crystallite size on the composition of the system of samples after HT: a,b) 2 hours; c,d) 7 days; e,f) 28 days
After 7 days of HT, the mean volume weighted crystallite size of the monazite phase in samples with x < 0.50 was found to be 20 ± 10 nm (Fig. 6(b)). In samples with a LaPO4 :YPO4 ratio close to 1:1, there is a tendency for the weighted average crystallite size of the xenotime phase to increase to 90±35 nm compared to the Y100% sample (D = 45±25 nm). The weighted average crystallite sizes of the monazite phase in the Y74% and Y84% samples are quite close, averaging 50 ± 20 nm. The determination of the average crystallite size of the monazite phase in the Y53% sample has a larger error due to the overlap of two reflections from monazite phases with different compositions. It is worth noting that the crystallite sizes may be smaller than 20 nm for both compositions of the monazite phase.
After 28 days under hydrothermal conditions, the mean volume weighted crystallite size of the monazite phase in the Y0% sample twofold increase compared to that after 2 hours of HT, reaching 38 ± 21 nm. The average crystallite sizes of the xenotime structure in samples with a LaPO4:YPO4 ratio of approximately 1:1 tend to increase, similar to the samples after 7 days of HT. The decrease value of the mean volume weighted crystallite sizes of the xenotime structure in the Y54% sample is likely caused by the transformation of the metastable phase with a monazite structure into phases with xenotime and monazite structures. As a result of the transformation process, new nanocrystals with a smaller size and a xenotime structure are formed, which affects the size distribution characteristics. The weighted average crystallite sizes of the monazite and xenotime structures in the Y74% and Y84% samples have a larger range of values compared to samples of the same composition after 7 days of HT. This fact is likely also caused by the transformation of the metastable phase with a monazite structure into more stable phases - a monazite phase with a different composition and a xenotime phase. It is worth noting that there was no increase in the mean volume weighted crystallite size of the monazite structure in the Y25% sample after 28 days of HT (D « 20 nm).
Figure 7 shows TEM image of nanoparticles and their distribution in the cross-sectional sizes of a sample with a composition of Lao.75Yo.25PO4 with monazite structure after 28 days of HT. The monazite nanoparticles have rod-like shapes with a relatively high aspect ratio. The distribution of particles according to their smaller dimensions ranges from 10 to 40 nm, with a weighted average thickness of about 18 nm. It should be noted that a significant systematic error was most likely introduced into the particle size distribution during the sample preparation. It is due to the fact that upon completion of the ultrasonic dispersion of a suspension of particles in alcohol, the particles begin to settle to the bottom and the settling rate of large heavy particles is obviously higher. Nevertheless, the mean volume weighted crystallite size determined from the X-ray diffraction line profile (D = 20 ± 8 nm) correlates well with the TEM data on the average particle thickness.
Fig. 7. TEM image of the Lao.75Y0.2sPO4 sample synthesized by the hydrothermal method at 230°C for 28 days and the histogram of particle size distribution (on inset)
Hydrothermal treatment at 230° C for 28 days leads to the formation of phases with compositions close to the equilibrium state of the system: a monazite-structured phase La1-xYxPO4 (x < 0.03) and a xenotime-structured phase YPO4.
With increasing isothermal holding time of HT from 2 hours to 7 days and 28 days, a transformation of the rhabdophane-structured phase into the monazite-structured and xenotime-structured phases was observed in the system, along with a gradual redistribution of the YPO4 component between the monazite-structured and xenotime-structured phases. The La0.75Y0.25PO4 sample, crystallizing in the monazite-structured phase, does not change its phase composition or crystal size (D « 20 nm) with increasing duration of isothermal holding time under hydrothermal condition. Based on the obtained data, it can be presumed that the La0.75 Y0 25PO4 sample also crystallizes in a metastable monazite-structured phase, and that increasing the duration of HT will lead to transformations of this metastable phase into phases with more equilibrium compositions, such as the monazite-structured and xenotime-structured phases. On the other hand, analysis of literature data for the LaPO4-YPO4 system at T > 1000° C [46,48] has shown that only one phase with a monazite structure is observed for the La0.75Y0 25PO4 sample. It is likely that the stability of the monazite-structured phase La0.75Y0 25PO4 is related to the prehistory of the initial sample and homogenization of the La and Y atoms in the rhabdophane-structured phase. However, without a phase diagram for the LaPO4-YPO4-(H2O) system, extrapolating the literature data to low temperature ranges would still have significant uncertainty.
4. Conclusions
It has been shown that in the LaPO4-YPO4-(H2O) system, nanocrystalline samples of a solid solution with rhabdo-phane structure La1-xYxPO4 • nH2O (0 < x < 0.80) were obtained using a precipitation method. The average thickness of the nanocrystals with rhabdophane structure correlates well with the mean volume weighted crystallite size for the samples after precipitation, indicating their single-crystalline structure in cross-sectional size of the nanorods. After hydrothermal treatment at 230° C for 2 hours, phases with monazite, rhabdophane, and xenotime structures are formed in the system in various ratios. After hydrothermal synthesis for 7 days in the system, phase transformation from a rhabdophane-structured phase to nanocrystals of two different compositions with monazite structure and to a xenotime-structured phase occurs. The phase fraction of the metastable phase with higher yttrium solubility in monazite structure after 7 days of HT ranged from ^20 to ^35%. With an increase in the duration of isothermal holding time to 28 days, the metastable phase with higher yttrium solubility in monazite structure is completely transformed into phases with structures of YPO4 xenotime and La1-x YxPO4 monazite with compositions close to equilibrium.
Thus, it is shown that in the LaPO4-YPO4-H2O system at low temperatures, the phase with variable composition with a high yttrium content and a monazite structure, and the phase of indefinite composition containing a significant amount of lanthanum with a rhabdophane structure, are not in equilibrium. The stability of the monazite phase in a sample with a composition of La0.75Y0.25PO4 after 2 hours, 7 days and 28 days of HT at 230°C, which does not change its composition and remains in the form of nanocrystals with an average crystal size of about 20 nm, can be explained by the presence of this phase in a metastable state.
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Submitted 10 October 2023; revised 15 November 2023; accepted 16 November 2023
Information about the authors:
Maria O. Enikeeva - Ioffe Institute, 194021 St. Petersburg, Russia; St. Petersburg State Institute of Technology, 190013 St. Petersburg, Russia; ORCID 0000-0002-9633-4421; [email protected]
Olga V. Proskurina - Ioffe Institute, 194021 St. Petersburg, Russia; St. Petersburg State Institute of Technology, 190013 St. Petersburg, Russia; ORCID 0000-0002-2807-375X; [email protected]
Evgeny Yu. Gerasimov - Federal Research Center, Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia; ORCID 0000-0002-3230-3335; [email protected]
Vladimir N. Nevedomskiy - Ioffe Institute, 194021 St. Petersburg, Russia; ORCID 0000-0002-7661-9155; [email protected]
Victor V. Gusarov - Ioffe Institute, 194021 St. Petersburg, Russia; ORCID 0000-0003-4375-6388; [email protected] Conflict of interest: the authors declare no conflict of interest.
