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TAL CHEMISTRY

Study of CalciumContaining Orthophosphates of NaZr2(PO4)3 Structural Type by HighTemperature XRay Diffraction A. I. Orlovaa, A. E. Kanunova, S. G. Samoilovb, A. Yu. Kazakovab, and G. N. Kazantsevb a

Nizhni Novgorod State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia email: [email protected] b Leipunsky Institute of Physics and Power Engineering, Obninsk, Kaluga oblast, 249033 Russia Received May 31, 2011

Abstract—Orthophosphates Ca0.5Ti2(PO4)3, Ca0.5Zr2(PO4)3, Ca0.75Zr2(SiO4)0.5(PO4)2.5, and CaMg0.5Zr1.5(PO4)3 (structural type NaZr2(PO4)3), having different occupancies of interframework posi tions by calcium, have been prepared by the sol–gel method with the subsequent thermal treatment of dried gels and investigated by IR spectroscopy and Xray diffraction. The analytical indexing of Xray diffraction patterns is performed within the sp. gr. R 3 . Hightemperature Xray diffraction was used to investigate the behavior of the orthophosphates upon heating: thermal expansion in the temperature range of 20–610°C (up to 500°C for Ca0.5Zr2(PO4)3). The coefficients of thermal expansion are calculated from the shift of diffrac tion peaks. The unitcell parameters of crystals at different temperatures are determined. The dependences of thermal expansion and its anisotropy on the occupancy of cation M positions by calcium are revealed. DOI: 10.1134/S1063774513020181

INTRODUCTION Phosphates, structural analogs of NaZr2(PO4)3 (NZP), attract the attention of researchers due to the variety of their properties and the unique possibility of changing and optimizing these properties by changing their composition. Ionic conductivity; luminescence; catalytic activity; and resistance to destructive factors, both natural and technogenic (high temperatures, thermal stresses, radiation, and chemical agents), are important properties of these compounds. The princi ples of the crystallochemical simulation of formula types of framework phosphates and other compounds with tetrahedral oxoanions, which were reported in [1–4], make it possible to substantiate the composi tion of compounds when developing artificial materi als with specified properties. One direction in modern materials science where this approach is justified for the family of isomorphic NZP phosphates is the development of crystalline (in particular, ceramic) compositions for biological appli cations. To this end, basic compounds should be cho sen proceeding from calciumcontaining phosphates. However, systematic data on the role of biogenic ele ments (calcium, magnesium, and silicon) in the for mation of framework compounds with NZP structure, including the information about thermal expansion, are rather limited. Phosphates with NZPlike structures are charac terized by low or ultralow thermal expansion, which is caused by expansion anisotropy. The parameter с

increases and the parameter а decreases with an increase in temperature for most of these com pounds [5]. In accordance with the thermal expansion model [4], the behavior of