Synthesis,structure formation,and thermal expansion of A+M2+MgE4+(PO4)3 |
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| Authors: | V. I. Pet’kov A. A. Alekseev E. A. Asabina E. Yu. Borovikova A. M. Koval’skii |
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| Affiliation: | 1.Lobachevsky State University of Nizhny Novgorod,Nizhny Novgorod,Russia;2.Lomonosov Moscow State University,Moscow,Russia;3.National University of Science and Technology MISiS,Moscow,Russia |
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| Abstract: | The compounds AMMgE(PO4)3 (A = Na, K, Rb, Cs; M = Sr, Pb, Ba; E = Ti, Zr) were synthesized by the sol–gel procedure followed by heat treatment and studied by X-ray diffraction, differential thermal and electron microprobe analysis, and IR spectroscopy. The phosphates crystallize in the kosnarite (KZr2(PO4)3, space group (Rbar 3)) and langbeinite (K2Mg2(SO4)3, space group P213) structural types. The structure of KPbMgTi(PO4)3 was refined by full-profile analysis (space group P213, Z = 4, a = 9.8540(3) Å, V = 956.83(4) Å3). The structure is formed by a framework of vertex-sharing MgO6 and TiO6 octahedra and PO4 tetrahedra. The K and Pb atoms fully occupy the extra-framework cavities and are coordinated to nine oxygen atoms. A variable-temperature X-ray diffraction study of KPbMgTi(PO4)3 showed that the compound expands isotropically and refer to medium-expansion class (linear thermal expansion coefficients α a = α b = α c = 8 × 10–6°C–1). The number of stretching and bending modes of the PO4 tetrahedron observed in the IR spectra is in agreement with that predicted by the factor group analysis of vibrations for space groups (Rbar 3) and P213. A structural transition from the cubic langbeinite to the rhombohedral kosnarite was found for CsSrMgZr(PO4)3. In the morphotropic series of ASrMgZr(PO4)3 (A = Na, K, Rb, Cs) the kosnarite–langbeinite transition occurs upon the Na → K replacement. The effect of the sizes and electronegativities of cations combined in AMMgE(PO4)3 on the change of the structural type was analyzed. |
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