Malcherek, T., Bosenick, A., Cemic, L., Fechtelkord, M. and Guttzeit, A. (2004) Isomorphy of structural phase transitions in LiTaOSiO4, LiTaOGeO4 and titanite, CaTiOSiO4. Journal of Solid State Chemistry, 177 (10). pp. 3254-3262. DOI 10.1016/j.jssc.2004.05.043.

Abstract

Structural phase transitions in LiTaOGeO4 (LTGO) and LiTaOSiO4 (LTSO) have been observed using differential scanning calorimetry, X-ray diffraction and Li-7 MAS NMR spectroscopy. LTGO transforms from P2(1)/c to C2/c space group symmetry at T-c = 231 K, while the isomorphic transition occurs at T-c = 439 K in LTSO. An analogous phase transition is known to occur in the structurally related mineral titanite, CaTiOSiO4. Spontaneous strain accompanying this phase transition in LTSO is significantly stronger than in titanite. As in titanite non-vanishing strain components are observable for T-c < T < T-i, with a similar ratio T-i/T-c. Li-7 MAS NMR spectroscopy in combination with computation of the electric field gradient by first principle methods confirms that the tetrahedral Li coordination environment is retained during the phase transitions in LTGO and in LTSO. In LTSO substantial motional narrowing is observed, indicating increased mobility of the Li cation above 400 K. The narrowing of the spinning sidebands is significantly modified immediately above and below the critical temperature. (C) 2004 Elsevier Inc. All rights reserved.

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