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, Pca 2(1) to cubic, F (4) over bar 3m transition of the stuffed cristobalit e structure are reported. A distorton index which measures the departu re of the orthorhombic unit cell from a metrically cubic cell shows th at at room temperature, distortion increases in the progression K2CdSi O4 <K2MgSiO4 <K2ZnSiO4 approximate to K2CoSiO4. High temperature X-ray powder measurements document an apparently discontinuous transition t o a structure of F (4) over bar 3m symmetry. Differential scanning cal orimetry shows a sharp, reversible, first order transition to the high temperature phase at about 500-600 degrees C for these compounds. Mea sured transformation enthalpies in the range of 7 to 16 J/g correlate roughly with the distortion index. The transformation involves tetrahe dral rotation to an orientationally disordered cubic structure which r etains an ordered M2+/Si distribution.