Cristobalite-related phases in the KAlO2-KAlSiO4 system

A study of the K-rich end of the potassium aluminate-silica system (K1-xAl1 -xSixO2; 0 less than or equal to x less than or equal to 0.25) at temperatu res up to 1500 degrees C has confirmed the existence of a cubic cristobalit e-type solid solution extending to x approximate to 0.235 at 1500 degrees C . The end member, KAlO2, transforms on cooling to an orthorhombic KGaO2-typ e structure at 531 degrees C (orthorhombic Pbca: a = 5.4327(8), b = 10.924( 2), c = 15.469(2) Angstrom, Z = 16). The room temperature structure has bee n modeled using chemically restrained refinement of Guinier-Hagg X-ray powd er diffraction (XRD) data by the Rietveld method (reduced chi(2) = 0.074, w Rp = 0.065, Rp 0.049). With progressive substitution of SiO2 into KAlO2 the cubic-to-orthorhombic phase transition temperature is lowered. A nem crist obalite-related phase field has been identified as existing close to room t emperature between x approximate to 0.10 and 0.125. The XRD data for this p hase can be indexed to a tetragonal superstructure with a(r) = 2 root 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 avoi d rapid degradation by atmospheric moisture. (C) 1999 Academic Press.