Transformations of the stuffed cristobalites, K2MSiO4, M=Mg, Zn, Co, Cd, with temperature and composition

The effects of composition and of temperature on the orthorhombic, Pca21 to cubic, F4¯3m transition of the stuffed cristobalite structure are reported. A distorton index which measures the departure of the orthorhombic unit cell from a metrically cubic cell shows that at room temperature, distortion increases in the progression K2CdSiO4 <K2MgSiO4 <K2ZnSiO4≈K2CoSiO4. High temperature X-ray powder measurements document an apparently discontinuous transition to a structure of F4¯3m symmetry. Differential scanning calorimetry shows a sharp, reversible, first order transition to the high temperature phase at about 500–600 °C for these compounds. Measured transformation enthalpies in the range of 7 to 16 J/g correlate roughly with the distortion index. The transformation involves tetrahedral rotation to an orientationally disordered cubic structure which retains an ordered M2+/Si distribution.