Cristobalite-Related Phases in the KAlO2–KAlSiO4 System

• Published in: Journal of solid state chemistry Vol. 147; no. 2; pp. 624 - 630
• Main Authors: Husheer, Shamus L.G., Thompson, John G., Melnitchenko, Alexandra
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.11.1999; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; cristobalite-related superstructure; cristobalite-type; Exact sciences and technology; Inorganic compounds; KGaO2-type; Physics; potassium aluminate; potassium aluminosilicate; Rietveld refinement; solid solution, XRD; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; synthesis; Water, oxides, hydroxides, peroxides; synthesis; solid solution, XRD; cristobalite-related superstructure; Rietveld refinement; cristobalite-type; potassium aluminate; potassium aluminosilicate; KGaO2-type; Inorganic compounds; Phase studies; Phase transformations; Modulated materials; Aluminosilicates; XRD; Experimental study; Ion substitution; Solid solutions; Silicon oxides; Superstructure; Potassium oxides; Chemical composition; Aluminium oxides; Crystal structure
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1006/jssc.1999.8426

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.