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 firs t-order ferroelastic phase transition between the room-temperature P2( 1)/c structure and a high-temperature Pbca structure occurs at 622 K. The spontaneous strain behaves as the order parameter for the transiti on, following a simple Landau model. Simple lattice dynamics simulatio n of the magnesiosilicate framework reveals that acoustic mode softeni ng drives the elastic instability. The first-order character of the tr ansition is attributed to a large excess volume, which arises from col lapse of the framework around the alkali site. At higher temperatures the orthorhombic cell edges converge toward a metrically cubic structu re, but this transformation requires Mg-Si disorder, which was not obs erved on the time scale of these experiments.