Spontaneous strain variations through the low-temperature displacive phase transition of LiGaSi2O6 clinopyroxene

The evolution of the lattice parameters and crystal structure of LiGaSi2O6 clinopyroxene has been studied by single-crystal X-ray diffraction from 100 to 1073 K. Complete single-crystal X-ray diffraction data have been collected at 22 temperatures. A displacive P2 1/c→ C2/c phase transition is observed at Tc = 286(10) K. The evolution of the spontaneous strain related to the phase transition with temperature has been studied in detail and compared to available data for pyroxenes with related compositions. Particular attention has been devoted to the ubiquitous presence of residual spontaneous strain after the phase transition by following the structure evolution in order to determine the related microscopic mechanism. The observed mean thermal expansion for the C2/c phase in the temperature interval 423-1073 K is α = 24.1(1) × 10-6 (K-1), which is similar to that of spodumene, and it is highly anisotropic. The ratio of axial thermal expansion coefficients is αa:αb:αc = 2.8:3.6:1, while in spodumene it is 1:2.9:1.3. This represents a clear difference in the mechanism of thermal expansion between the two structures which only differ in the substitution of Al by Ga at the M(1) site. The particular composition studied has fully extended chains of tetrahedra at T > Tc, although significant thermal expansion along c-axis is still observed in the C2/c phase. Extension along the c-axis is resolved therefore by dilatation of the (SiO4) tetrahedron, consequently increasing the thermal expansion along d(100) by 44 % compared to spodumene.