Cristobalite-Related Phases in the KAlO2–KAlSiO4 System

Author: Husheer S.L.G.   Thompson J.G.   Melnitchenko A.  

Publisher: Academic Press

ISSN: 0022-4596

Source: Journal of Solid State Chemistry, Vol.147, Iss.2, 1999-11, pp. : 624-630

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Abstract

A study of the K-rich end of the potassium aluminate–silica system (K1-xAl1-xSixO2 ; 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 ar=2√2ap and cr=2cp (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.