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PHYSICS, CHEMISTRY, MATHEMATICS Original article
Kozlova T.O., et al. Nanosystems: Phys. Chem. Math., 2023,14 (1), 112-119.
http://nanojournal.ifmo.ru DOI 10.17586/2220-8054-2023-14-1-112-119
Synthesis and thermal behavior of KCe2(PO4)3, a new full-member in the A1 M2V(PO4)3 family
TaisiyaO. Kozlova1", Darya N. Vasilyeva1'2'6, Daniil A. Kozlov1'3'c, MariiaA. Teplonogova13'd, Alexander E. Baranchikov1'6, NikolayP. Simonenko1f, Vladimir K. Ivanov1g
1Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Moscow, Russia 2National Research University Higher School of Economics, Moscow, Russia 3Lomonosov Moscow State University, Moscow, Russia
°[email protected],[email protected], [email protected], [email protected],[email protected],f [email protected],[email protected]
Corresponding author: T. O. Kozlova, [email protected]
PACS 65.40.-b; 61.50.-f; 61.66.Fn
Abstract Hydrothermal treatment of nanoscale amorphous ceric phosphate gel in KOH aqueous solutions was found to result in a new KCe2(PO4)3 phase. The refinement of the KCe2(PO4)3 structure showed that it was isostructural to recently reported (NH4)Ce2(PO4)3. For the KCe2(PO4)3 phase, the unit cell parameters (sp. gr. C2/c) were a = 17.3781(3) A, b = 6.7287(1) A, c = 7.9711(2) A, P = 102.351(1) °, V = 910.53(4) A3, Z = 4. The thermal decomposition of KCe2(PO4)3 at 800 °C resulted in the mixture of crystalline CePO4 and KPO3.
Keywords cerium, potassium, polyphosphate, channel, hydrothermal
Acknowledgements This work was supported by Russian Science Foundation (Grant no. 21-73-00294, https://rscf.ru/en/project/21-73-00294/) using the equipment of the JRC PMR IGIC RAS. The authors thank Dr. A. V. Gavrikovfor FT-IR spectroscopy studies.
For citation Kozlova T.O., Vasilyeva D.N., Kozlov D.A., Teplonogova M.A., Baranchikov A.E., Simonenko N.P., Ivanov V.K. Synthesis and thermal behavior of KCe2(PO4)3, a new full-member in the A1 (PO4)3 family. Nanosystems: Phys. Chem. Math., 2023,14 (1), 112-119.
1. Introduction
A wide variety of crystalline phosphates of tetravalent metals is mainly represented by double salts [1]. Consi-
derable structural diversity is characteristic of Ajm2v(PO4)3 compounds, which usually belong to the "NaZr2(PO4)3"
(NASICON) or "NaTh2(PO4)3" structural types [2]. The structure of these double phosphates includes a metal-phosphate
three-dimensional framework and is characterized by high thermal and chemical stability. Their structural features provide
the ability for ion exchange and prospects for their application as matrices for radioactive elements immobilization [3-6],
as well as for design of luminescent materials [7-9].
Interestingly, in comparison with actinide phosphates, only 13 crystal structures of ceric phosphates have been reliably
characterized until present [10] despite the rich coordination chemistry of Ce(IV) [11] and more than a century of research in ceric phosphates [12]. The lack of information on double cerium(IV) phosphates is primarily due to the high tendency of Ce(IV) to reduce to the trivalent state in the phosphate matrix and form cerium(III) phosphate, which is characterized by extremely high thermodynamic stability [13]. Note that for the compounds having A1 M2V(PO4)3 (where M = Ce1V) composition, only the NH4Ce2(PO4)3 phosphate isostructural to NH4Th2(PO4)3 was reliably characterised [14]. Taking in mind the proximity of the potassium and ammonium ionic radii (1.51 A [15] and 1.54 A [16], CN = 8, respectively), the existence of KCe2(PO4)3 being an isostructural analog of NH4Ce2(PO4)3 is anticipated. This assumption is also based on the recently reported pairs of isostructural compounds NH4Th2(PO4)3 [17] - KTh2(PO4)3 [18], and K2Ce(PO4)2 [19] -K2Th(PO4)2 [20].
Thus, this work was aimed at the synthesis of KCe2(PO4)3 double cerium(IV)-potassium phosphate. The synthesis
strategy was based on the hydrothermal treatment of nanoscale ceric phosphate gels. Their chemical composition can easily be adjusted by electrolyte switching and their fibrous 20 nm) structure makes them highly reactive and prone to crystallisation in aqueous media under relatively mild conditions. This strategy was successfully implemented recently to
synthesize new crystalline cerium(IV) phosphates, including NH4Ce2(PO4)3 [14,21,22].
© Kozlova T.O., Vasilyeva D.N., Kozlov D.A., Teplonogova M.A., Baranchikov A.E., Simonenko N.P., Ivanov V.K., 2023
2. Experimental Section
The following materials were used as received, without further purification: Ce(NO3)3 • 6H2O (pure grade, Lanhit Russia), potassium hydroxide (pure grade, Sigma Aldrich), phosphoric acid (85 wt.% aq, p = 1.689 g/cm3, extra-pure grade, Komponent-Reaktiv Russia), aqueous ammonia (25 wt.%, extra-pure grade, Khimmed Russia), isopropanol (extrapure grade, Khimmed Russia), distilled water.
First, ceric phosphate solution was synthesized according to the procedure reported earlier [21]. Briefly, nanocrys-talline (4-5 nm) cerium dioxide (0.100 g) obtained by precipitation from Ce(NO3)3 • 6H2O aqueous solution [23] was dissolved in concentrated phosphoric acid (5 ml) at 80 °C. The calculated molar ratio of Ce:P in the solution was 1:126. To the cooled solution, 35 ml of 1 M potassium hydroxide aqueous solution was added under vigorous stirring. The resulting gel-like precipitate 40 mL) was placed in 100 ml Teflon autoclave and subjected to hydrothermal treatment at 180 °C for 24 h. After cooling the autoclave, a precipitate was repeatedly washed using distilled water and dried at 60 °C in air.
Powder X-ray diffraction (PXRD) patterns were acquired using a D/MAX 2500 PC (Rigaku, Japan) powder diffrac-tometer with a rotating anode in the reflection geometry (Bragg-Brentano) with Cu Ka1j2 radiation and a graphite monochromator. PXRD patterns were collected in the 5 - 100 °26 range with a 0.01 ° step. The identification of the diffraction peaks was carried out using the ICDD database (PDF2, release 2020). PXRD pattern refinement was performed using Rietveld method using the MAUD software [24]. Structure refinement and quantitative phase analysis were carried out using structures of monazite (sp. gr. P 121/n1, a = 6.788 A, b = 7.0163 A, c = 6.465 A, ft = 103.43°,
V = 299.486 A3, Z = 4) [25] and KPO3 (sp. gr. P 121/a1, a = 14.02 A, b = 4.54 A, c = 10.28 A, ft = 101.5°
V = 641.194 A3, Z = 8) [26] taken from Crystallography Open Database [27].
Scanning electron microscopy (SEM) images were obtained using an Amber GMH (Tescan, Czech Republic) microscope operated at an accelerating voltage of 5 kV using a secondary electron (Everhart-Thornley) and backscattered electron (Low Energy BSE) detectors. Energy-dispersive X-ray spectroscopy (EDS) was performed using an Ultim Max (Oxford Instruments, UK) detector at an accelerating voltage of 20 kV.
The Fourier transform infrared (FT-IR) spectra of the samples were recorded using a Bruker ALPHA spectrometer in the range of 400 - 4000 cm-1 in attenuated total reflectance mode.
3. Results and discussion
According to powder X-ray diffraction data, hydrothermal treatment of ceric phosphate gel resulted in the formation of a crystalline product with a diffraction pattern similar to NH4Ce2(PO4)3 phase (monoclinic, sp. gr. C2/c, a = 17.4719(4) A, b = 6.76928(14) A, c = 7.99286(14) A, ft = 102.873(1)°, V = 921.57(4) A3, Z = 4) [14]. According to EDS analysis, the average K:Ce:P atomic ratio was close to 1:2:3, which corresponds to the nominal composition of KCe2(PO4)3. PXRD pattern of the obtained sample showed that it is contained an admixture of CePO4 with monazite structure (PDF2 [00-032-199]), so further refinement of the structure was carried out taking into account the two-phase composition of the powder. Crystal structure refinement by the Rietveld method was performed using the MAUD software. Experimental and calculated PXRD patterns are shown in Fig. 1. Despite the fact that the measured diffraction pattern corresponded well to the calculated one, the refinement was characterized by rather large R-factor values (Rp = 0.087, Rwp = 0.13), which may be caused by unaccounted structural deviations or the presence of water molecules in the channels of cerium(IV)-potassium phosphate structure. The KCe2(PO4)3 phase, similarly to its ammonium analogue, crystallizes in a monoclinic crystal system (sp. gr. C2/c, a = 17.3781(3) A, b = 6.7287(1) A, C = 7.9711(2) A, ft = 102.351(1)°, V = 910.53(4) A3, Z = 4).
The structure of the resulting cerium(IV)-potassium phosphate is shown in Fig. 2. The coordination number of cerium in this structure was 9, Ce is surrounded by oxygen atoms of phosphate groups, while the potassium cations are arranged in channels along the c axis. Note that the CePO4 content in the initial sample determined by quantitative phase analysis is rather small and amounts to 3.1 ± 0.1 wt. %. The formation of cerium(III) phosphate upon hydrothermal treatment of ceric phosphate gels is rather unusual. This fact along with the similar report [21] can contribute to one of the most disputable topics of the chemistry of nanocrystalline ceria, namely its oxygen non-stoichiometry [28-30]. The possibility of the partial reduction of Ce(IV) during hydrothermal synthesis of crystalline cerium(IV) phosphates, as well as monazite structure formation under hydrothermal conditions, have been also discussed recently [31-34] but the exact mechanisms were not analyzed in detail and require further studies.
The newly synthesized double ceric phosphate KCe2(PO4)3 complements the family of isostructural compounds having the composition of A1 (PO4)3 (A1 =Li, Na, K, NH4, M1V = Th, U) [17,18,35-37]. Note that the synthesis of the "KCe2(PO4)3" with a monazite structure was recently announced [38]. However, this announcement is doubtful due to the absence of the structural data. Moreover, at the temperatures (above 600 °C) used to synthesize this compound [38], cerium(IV) is extremely prone to reduction to trivalent state [34,39,40]. Similarly, the previously reported synthesis of BaCe(PO4)2 [38] was further argued [41].
Fig. 1. Rietveld plot for the powder obtained upon hydrothermal treatment of ceric phosphate gel (observed, calculated and difference PXRD profiles along with Bragg peak positions)
Fig. 2. Crystal structure of KCe2(PO4)3. Yellow spheres denote Ce atoms, K atoms are shown in violet, O atoms are presented as red spheres, P atoms are located in PO4 tetrahedra (shown in gray)
The comparison of thermal behavior of isostructural NH4Ce2 (PO4)3 and KCe2 (PO4)3 compounds is quite interesting since the formation of single-phase cerium(III) orthophosphate with monazite structure during NH4Ce2 (PO4)3 thermolysis was previously observed [14]. On the other hand, the thermolysis of K2Ce(PO4)2 cerium(IV)-potassium phosphate above 850 °C resulted in the formation of K3Ce(PO4)2 cerium(III)-potassium phosphate as well as of CePO4 [19].
According to the thermal analysis, the thermolysis of KCe2(PO4)3 (with an admixture of ca. 3.1 ± 0.1 wt. % CePO4) proceeds in two main stages (Fig. 3). The first stage begins at ~220 °C and ends at 540 °C and is apparently associated with the release of water (probably presented in the channels of the KCe2 (PO4)3 structure) as well as oxygen [10,42]. The second stage begins at about 600 ° C and corresponds to the release of oxygen due to the reduction of cerium(IV) to cerium(III).
To clarify the processes occurring at each stage of thermal decomposition of KCe2 (PO4)3 and to estimate the composition of the thermolysis products, the synthesized KCe2(PO4)3 (with 3.1 ± 0.1 wt. % CePO4 admixture) was annealed at either 580 °C or 800 °C in a muffle furnace for 2 h in air (the heating rate was 5 °C/min).
Quantitative phase analysis of PXRD data showed that the heating at 580 ° C did not change dramatically the phase composition of the powder. Like the bare sample, it contained KCe2(PO4)3 as the major component and an admixture of CePO4 (23 ± 5 wt. %). The reduction of cerium(IV) to cerium(III) at high temperatures is characteristic of tetravalent cerium phosphates and has been repeatedly observed recently [14,19,43,44].
According to PXRD, the thermal treatment of KCe2 (PO4)3 at 800 ° C resulted in its complete decomposition and the formation of CePO4 and potassium polyphosphate KPO3 (PDF2 35-819). According to quantitative phase analysis, the content of KPO3 in the product was 21.0 ± 1.2 wt. % (34.6 ± 2.0 mol. %), which agreed well with the anticipated molar ratio CePO4:KPO3 =2:1. Thus, the thermolysis of KCe2(PO4)3 in air can be described by the following reaction scheme:
KCe2(PO4)3 ^ 2CePO4 + KPO3 + 102 t
(1)
95 —i—»—i—»—i—■—i—■—i—■—i—■—i—*—i—■—i—■—
100 200 300 400 500 600 700 800 900 1000 t, °C
Fig. 3. Thermal decomposition curve of KCe2(PO4)3 (with 3.1 ± 0.1 wt. % CePO4 admixture) in air
In accordance with eqaution (1), the total weight loss for the KCe2(PO4)3 phase should be ca. 2.5 wt. %, whereas the experimentally measured weight loss was much greater 4 wt. %). Such a difference can be due to the presence of water molecules in the structure of KCe2(PO4)3 which were not accounted during the structural analysis. These H2O molecules can be trapped in the KCe2(PO4)3 structural channels and release from the phase at temperatures up to 540 °C. Our estimates of the water content resulted in the chemical composition of KCe2(PO4)3 • 0.4H2O. Most probable, the water content can vary in a certain range and the exact chemical composition of KCe2 (PO4)3 • xH2 O needs further refinement.
Fig. 4. Rietveld plots for the samples obtained after the thermal treatment of KCe2(PO4)3 (with 3.1 ± 0.1 wt. % CePO4 admixture) sample at 580 0C (a) or 800 0C (b) (observed, calculated and difference PXRD profiles along with Bragg peaks positions for CePO4 and KPO3)
Figure 5 shows the IR spectra of the bare KCe2(PO4)3 sample (with 3.1 ± 0.1 wt. % CePO4 admixture) and the samples obtained after its thermal treatment at 580 °C or 800 °C. The IR spectra for the samples before and after the thermal treatment at 580 ° C are almost identical and coincide well with the IR spectrum of the NH4 Ce2 (PO4 )3 phase [14]. In the regions of 1100 - 900 cm-1 and 650 - 440 cm-1 the characteristic absorption bands are observed, which are related to the stretching and bending vibrations of phosphate anions, respectively [33,45].
In the infrared spectrum of the powder obtained at 800 °C, extra absorption bands are presented at 1280 cm-1, 865 cm-1, 760 cm-1, 680 cm-1, and 490 cm-1 which correspond to polyphosphate moieties [46-49]. Thus, IR data agree well with the XRD results and confirm the formation of potassium polyphosphate KPO3 upon the thermolysis of cerium(IV)-potassium phosphate KCe2 (PO4)3.
According to scanning electron microscopy, the KCe2 (PO4)3 phase is represented by 200 nm particles having the shape of truncated octahedrons. Thermal treatment at 580 °C does not significantly change the size of the particles, but after the treatment at 800 °C, large crystals up to several tens of micrometers in size are observed along with the relatively
Fig. 5. IR spectra for the samples obtained after the thermolysis of KCe2(PO4)3 (with 3.1 ± 0.1 wt. % CePO4 admixture) (a) at 580° C (b) or 800° C (c)
(C)
Fig. 6. SEM images for the samples obtained by thermolysis of the bare cerium(IV)-potassium phosphate (a) at 580 0C (b) or 800 0 C (c)
Fig. 7. SEM images for the sample obtained by thermolysis of the bare cerium(IV)-potassium phosphate at 800 °C: backscattered electron detection mode (left), secondary electrons detection mode (right)
Fig. 8. SEM image and the corresponding EDS distribution maps of P, Ce and K for the sample obtained by thermolysis of the bare cerium(IV)-potassium phosphate at 800 °C
small particles (Fig. 6). The images taken in the backscattered electron detection mode (Fig. 7) show that these large crystals have a lower average atomic weight than the smaller crystals. Thus, it can be assumed that small and large crystals correspond to CePO4 and KPO3 phases, respectively (Fig. 6,7).
Large size of KPO3 crystals is most probably related to their growth from melt which can form upon heating and thermolysis of KCe2(PO4)3. In the CePO4-KPO3-Ce(PO3)3 system [50], the CePO4-KPO3 quasi-binary section corresponds to an eutectic type system with an eutectic point of 790 °C (85 wt. % KPO3). Note, the eutectic point agrees the position of the endothermic effect in the differential scanning calorimetry data for the bare cerium(IV)-potassium phosphate (Fig. 3). Thus, upon its heating to 800 °C, the liquid KPO3-rich phase forms and, upon the subsequent cooling of the two-phase liquid-solid system, the crystallization of potassium polyphosphate occurs. Cerium(III) orthophosphate is anticipated to possess low solubility in the phosphate melt and the size of CePO4 crystals does not change significantly.
The EDS results corroborate the above considerations and indicate that the large aggregates dominantly contain potassium and phosphorus (Fig. 8).
4. Conclusions
In this paper, nanoscale amorphous ceric phosphate was used as a convenient starting material for the hydrothermal synthesis of crystalline ceric phosphates. We have demonstrated that the hydrothermal treatment of the gels obtained by mixing of the ceric phosphate solution and 1 M potassium hydroxide aqueous solution results in the formation of the previously unknown cerium(IV)-potassium phosphate, KCe2(PO4)3. KCe2(PO4)3 was found to be isostructural to NH4Ce2(PO4)3, and belongs to the A1 (PO4)3 (A1 =Li, Na, K, NH4, M1V = Th, U) family. Thermal analysis data indicated that double ceric phosphate KCe2 (PO4)3 decomposes through two stages with the formation of the mixture of CePO4 and KPO3 at 800 °C.
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Submitted 28 November 2022; revised 27 December 2022; accepted 28 December 2022
Information about the authors:
T. O. Kozlova - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; ORCID 0000-0002-9757-9148; [email protected]
D. N. Vasilyeva - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; National Research University Higher School of Economics, Moscow, Myasnit-skaya str., 20, 101000, Russia; [email protected]
D. A. Kozlov - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russia; ORCID 0000-0003-0620-8016; [email protected]
M. A. Teplonogova - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russia; ORCID 0000-0002-4820-8498; [email protected]
A. E. Baranchikov - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; ORCID 0000-0002-2378-7446; [email protected]
N. P. Simonenko - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; ORCID 0000-0002-4209-6034; [email protected]
V. K. Ivanov - Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky prospect, 31, Moscow, 119991, Russia; ORCID 0000-0003-2343-2140; [email protected]
Conflict of interest: the authors declare no conflict of interest.
