Monoclinic-orthorhombic phase transition in the K₂MgSi₅O₁₂ leucite analog

The high-temperature cell parameters of K₂MgSi₅O₁₂ have been measured by X-ray powder diffraction between room temperature and 842 K. A first-order ferroelastic phase transition between the room-temperature P2₁/c structure and a high-temperature Pbca structure occurs at 622 K. The spontaneous strain behaves as the order parameter for the transition, following a simple Landau model. Simple lattice dynamics simulation of the magnesiosilicate framework reveals that acoustic mode softening drives the elastic instability. The first-order character of the transition is attributed to a large excess volume, which arises from collapse of the framework around the alkali site. At higher temperatures the orthorhombic cell edges converge toward a metrically cubic structure, but this transformation requires Mg-Si disorder, which was not observed on the time scale of these experiments.