Phase equilibria in the ErPO4–K3PO4 system

Journal of Thermal Analysis and Calorimetry, Jan 2013

The phase equilibria occurring in the ErPO4–K3PO4 system were investigated by the thermal analysis, FTIR, and X-ray powder diffraction methods. On the basis of obtained results, the related phase diagram is proposed. This system includes one intermediate compound, K3Er(PO4)2; the double phosphate melts incongruently at 1355 °C and occurs in two polymorphic forms; transformation β/α-K3Er(PO4)2 proceeds at 420 °C. The eutectic occurs at the composition of 58.5 wt% K3PO4, 41.5 wt% ErPO4 at 1317 °C.

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Phase equilibria in the ErPO4–K3PO4 system

D. Piotrowska 0 T. Znamierowska 0 I. Szczygie 0 0 D. Piotrowska (&) T. Znamierowska I. Szczygie Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wrocaw University of Economics , Komandorska 118/120, 53-345 Wrocaw, Poland The phase equilibria occurring in the ErPO4K3PO4 system were investigated by the thermal analysis, FTIR, and X-ray powder diffraction methods. On the basis of obtained results, the related phase diagram is proposed. This system includes one intermediate compound, K3Er(PO4)2; the double phosphate melts incongruently at 1355 C and occurs in two polymorphic forms; transformation b/a-K3Er(PO4)2 proceeds at 420 C. The eutectic occurs at the composition of 58.5 wt% K3PO4, 41.5 wt% ErPO4 at 1317 C. - Many papers about double phosphates of the general formula MI3Ln(PO4)2 (where MI denotes an alkali metal and Ln is a rare earth element or Y or Sc) have been published. The information is mainly focused on the preparation methods, crystalline structure, and application possibilities of those compounds. According to the data, lanthanide-alkali metal double phosphates are of technological importance for applications in optics and electronics [110]. In view of relevant information from the literature, double phosphates of the formula MI3Ln(PO4)2 should occur in the systems of LnPO4MI3PO4 (where Ln denotes a rare earth element or yttrium, and MI does an alkali metal). According to our research group results, such compounds occur in the Ln2O3MI2OP2O5 oxide systems on the LnPO4MI3PO4 subsystems, where Ln = La, Ce, Nd, Y and MI = Na, K, Rb [1118]. It should be noted that, in the system YPO4 Na3PO4, two intermediate compounds of Na3Y(PO4)2 and Na3Y2(PO4)3 occur; both compounds melt congruently. Also, in the system YPO4Rb3PO4, two intermediate compounds occur; namely Rb3Y(PO4)2 which melts congruently, and the Rb3Y2(PO4)3 which is unstable and decomposes in the temperature range between 1300 and 1330 C. In each of the other investigated systems, a single I intermediate of M3Ln(PO4)2 occurs; it melts incongruently. I Double phosphates M3Ln(PO4)2 are usually obtained in a solid-state reactions by sintering an equimolar mixture of MI3PO4 and LnPO4. In the present paper, the results of investigation of the ErPO4K3PO4 subsystem are presented. The related phase diagram has not been reported so far. It is known from the literature that K3Er(PO4)2 exists as well. According to Refs. [19, 20], the compound appears in two polymorphic modifications. The high-temperature one crystallizes in the hexagonal system (S.G. P 3, glaserite-type) and the low-temperature one does in the monoclinic system (S.G. P21/m, a = 7.371(1), b = 5.595(1), c = 9.318(1) A , and b = 90.90(1) ). A polymorphic transition in K3Er(PO4)2 exhibits at 436.4 C [19]. The parent orthophosphates ErPO4 and K3PO4 are known for congruent melting at 1896 20 C [2123] and 1620 20 C [24], respectively. Polymorphism of both orthophosphates was investigated by many authors (see, e.g., [21, 2431]). Erbium orthophosphate, ErPO4, is related to REPO4 group with xenotime structure, isostructural to zircon (ZrSiO4). Orthophosphate ErPO4 crystallizes in the tetragonal system (S.G. I41/amd, a = 6.8614(5), c = 6.0082(9) A , Z = 4) [21]. The compound exists in one polymorphic form. According to the literature on K3PO4, polymorphism has revealed significant disagreements. This problem will be described in the Results section. The following commercial materials: Er2O3 (Aldrich), and NH4H2PO4, (NH4)2HPO4, K2CO3, K3PO4 3H2O (POCh) all analytically pure were used to prepare the test samples from the ErPO4K3PO4 system. The erbium orthophosphate ErPO4 was synthesized from Er2O3 and NH4H2PO4 by the method described in [32]. Potassium orthophosphate K3PO4 was obtained from K3PO4 3H2O by dehydration at 900 C for 1 h. Phase equilibria in the ErPO4K3PO4 system were investigated by thermoanalytical methods and X-ray powder diffraction at room temperature. Samples for investigations were presynthesized by the reaction in the solid phase. The substrates were weighed out in fixed amounts, thoroughly mixed (in weighing bottle), rubbed in an agate mortar, and then sintered. The sintering temperature and time were determined experimentally. The DSC/TG analysis during heating was carried out using a calorimeter SETSYSTM (TGDSC 1500; SETARAM) up to 1300 C (heating rate: 10 K min-1, argon atmosphere, platinum crucibles; mass of samples 1530 mg). The DTA/TG-heating was performed by means of a derivatograph type 3427 (MOM, Hungary) within temperature range of 201400 C with a heating rate 5 C min-1. Platinum crucibles and an air atmosphere were used; mass of samples was 400600 mg. The standard substance was Al2O3. The temperatures were measured by a Pt/PtRh10 thermocouple standardized for the melting points of NaCl (801 C), K2SO4 (1070 C), Ca2P2O7 (1353 C), and the transition points of K2SO4 (583 C). The high-temperature experiments (above 1400 C) were conducted (...truncated)


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D. Piotrowska, T. Znamierowska, I. Szczygieł. Phase equilibria in the ErPO4–K3PO4 system, Journal of Thermal Analysis and Calorimetry, 2013, pp. 121-126, Volume 113, Issue 1, DOI: 10.1007/s10973-012-2883-4