High-Temperature Heat Capacity of Pb 9 R(GeO 4 ) 3 (VO 4 ) 3 (R = La, Pr, Nd, Sm) Apatites
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VI INTERNATIONAL CONFERENCE ON THERMAL ANALYSIS AND CALORIMETRY IN RUSSIA
High-Temperature Heat Capacity of Pb9R(GeO4)3(VO4)3 (R = La, Pr, Nd, Sm) Apatites L. T. Denisovaa,*, E. O. Golubevaa, V. M. Denisova, N. V. Belousovaa, and L. G. Chumilinaa aSiberian
Federal University, Institute of Nonferrous Metals and Materials Science, Krasnoyarsk, 660025 Russia * е-mail: [email protected] Received April 15, 2020; revised April 22, 2020; accepted April 24, 2020
Abstract—The apatite-like Pb9Pr(GeO4)3(VO4)3 and Pb9Sm(GeO4)3(VO4)3 compounds were prepared by solid-phase synthesis using oxides as starting chemicals: PbO, Pr2O3 (Sm2O3), GeO2, and V2O5. The successive annealing was carried out at 773–1073 K in the air. The effect of rare-earth elements on the structure of the Pb9R(GeO4)3(VO4)3 (R = La, Pr, Nd, Sm) apatites and basic thermodynamic functions was investigated. The temperature dependence (350–1050 K) of the heat capacity of the Pr(Sm)-containing apatites has been determined by differential scanning calorimetry. It has been established that the Cp = f(T) curve for the Pb9Pr(GeO4)3(VO4)3 compound has an extremum associated with a polymorphic transformation in the region of 978 K. Keywords: apatite-like compounds, rare-earth elements–lead-containing apatites, solid-phase synthesis, structure, high temperature heat capacity DOI: 10.1134/S0036024420130099
A general problem of materials science is the development of modern compounds and the creation of multifunctional materials on their basis. Complex oxide compounds with an apatite structure can serve as such materials. An important feature of these oxides is the ability of their structural units to be replaced by other ions to form new compounds with the same structure, yet differing in composition. In this case, it becomes possible to influence the properties of apatites and, as a result, expand the range of their practical application. The general chemical formula of the apatite-like compounds can be represented in the form M10(AO4)6X2: M are cations (Na+, K+, Ca2+, Sr2+, Ba2+, Y3+, RE3+, Th4+, Mn2+, Cd2+, Tl+, Pb2+, Bi3+), A are cations (Si4+, Ge4+, P5+, V5+, As5+, S6+, Cr6+) surrounded by oxygen tetrahedra, and X are anions (F–, Cl–, Br–, I– OH–, O2–, S2– Se2–, N3–, CuO–) or a vacancy [1]. Studies of apatites with vacancies, especially those in which X positions are unoccupied, have shown that the studied systems are phosphate lead systems with the general formula Pb8M2(PO4)6, where M is a monovalent ion (Na, K) or lead vanadatogermanates Pb5(GeO4)(VO4)2, in which lead can be replaced by a rare-earth element. In contrast to the well-studied calcium hydroxyapatite Ca10(PO4)6(OH)2 [2–4], lead-containing apatites have not been sufficiently studied to date [5]. According to [6], the partial replacement of the lead in the Pb5(GeO4)(VO4)2 (or
Pb10(GeO4)2(VO4)4) compound allows one to obtain a new class of apatites with the general formula Pb10 ‒ xRx(GeO4)2 + x(VO4)4 – x (x = 0–3). For such compounds, there are fragmentary data on their crystal structure: Pb8La2(GeO
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