Cristobalite-Related Phases in the KAlO₂-KAlSiO₄ System

A study of the K-rich end of the potassium aluminate-silica system (K_1-xAl_1-xSi_xO₂; 0≤x≤0.25) at temperatures up to 1500°C has confirmed the existence of a cubic cristobalite-type solid solution extending to x≈0.235 at 1500°C. The end member, KAlO₂, transforms on cooling to an orthorhombic KGaO₂-type structure at 531°C (orthorhombic Pbca: a=5.4327(8), b=10.924(2), c=15.469(2) Å, Z=16). The room temperature structure has been modeled using chemically restrained refinement of Guinier-Hägg X-ray powder diffraction (XRD) data by the Rietveld method (reduced χ²=0.074, wRp=0.065, Rp=0.049). With progressive substitution of SiO₂ into KAlO₂ the cubic-to-orthorhombic phase transition temperature is lowered. A new cristobalite-related phase field has been identified as existing close to room temperature between x≈0.10 and 0.125. The XRD data for this phase can be indexed to a tetragonal superstructure with a_r=2√2a_p and c_r=2c_p (r=resultant, p=parent). Analysis of K-rich specimens by XRD and differential scanning calorimetry required extreme care to avoid rapid degradation by atmospheric moisture.