Structural changes upon the temperature dependent C2/c → P2₁/c phase transition in LiMe³⁺Si₂O₆ clinopyroxenes, Me = Cr, Ga, Fe, V, Sc and In

Abstract

Within the Li-clinopyroxene series LiMe³⁺Si₂O₆, a temperature induced C2/c → P2₁/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al³⁺ and the pure In³⁺ compound down to 80 K. Within the LiSc_1–xIn_xSi₂O₆ solid solution se ries, the transition temperature rapidly decreases with increasing In³⁺ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi₂O₆, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q² (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc³⁺ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi₂O₆, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P2₁/c, decrease of the coordination number of Li⁺ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi₂O₆ is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P2₁/c phase transition.