High-temperature behavior of synthetic analogues of scottyite BaCu₂Si₂O₇ and colinowensite BaCuSi₂O₆

Shapenkov S. V., Zolotarev A. A. jr., Zhitova E. S.., Krivovichev S. V., Krzhizhanovskaya M. G.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 2. P. 125-134

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Language: Russian

Abstract

Analogues of two minerals: scottyite BaCu₂Si₂O₇ and colinowensite BaCu₂Si₂O₆, have been synthesized by hydrothermal method. According to the single crystal X-ray diffraction, synthetic analogue of scottyite is orthorhombic, space group Pnma, a = 6.8805(15), b = 13.190(3), c = 6.8903(17) Å; analogue of colinowensite is tetragonal, I4₁/acd, a = 9.9811(2), c = 22.3369(6) Å. The high-temperature (HT) X-ray powder diffraction study has shown that both phases are stable within the range of temperature from 26 to 900 °C. Calculated thermal expansion coefficients for scottyite are α_а = 910 6, α_b = 510^-6, α_с =810^-6 °С^-1 at T = 200 °C; α_а = 1010^-6, α_b = 610^-6, α_с = 1210^-6 °С at T = 400 °C; α_а = 1010^-6, α_b = 810^-6, α_с = 1610^-6 °С^-1 at T = 600 °C, and the main HT deformation of the structure is associated with straightening of chains, built by copper squares, along the c axis. Calculated thermal expansion coefficients for colinowensite are α_а = 1010^-6 °С^-1, α_с = -510^-6 °С^-1 at T = 200 °C; α_а = 910^-6 °С^-1, α_с = 510^-6 °С^-1 at T = 400 °C; α_а = 910^-6 °С^-1, α_с = 1510^-6 °С^-1 at T = 600 °C. The thermal behavior of colinowensite is characterized by contraction of c parameter in the 25—400 °C range of temperature, while in the region 400—900 °C the phase becomes expanded. This high-temperature behavior is interpreted as a stretching of barium polyhedral chains.

Key words: scottyite, colinowensite, crystal structure, X-ray diffraction, thermal expansion.