CRISTOBALITE-RELATED PHASES IN THE KAlO2-KAlSiO4 SYSTEM

摘要

A study of the K-rich end of the potassium aluminate-SiO2 system (K1-xAl1-xSixO2; 0 less than or equal to x less than or equal to 0.25) at temperatures up to 1500 C has confirmed the existence of a cubic cristobalite-type solid solution extending to x approx equal to 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) angstroms, Z=16). The RT structure has been modeled using chemically restrained refinement of Guinier-Hagg powder XRD data by the Rietveld method (reduced chi2=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 RT between x approx equal to 0.10 and 0.125. The XRD data for this phase can be indexed to a tetragonal superstructure with ar=2 square root of 2ap and cr=2cp (r=resultant, p=parent). Analysis of K-rich specimens by XRD and DSC required extreme care to avoid rapid degradation by atmospheric moisture. 25 refs.