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

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 LiSc1– x In x Si2O6 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.