Investigation of structural states in the series MGaSiO 4, MAlGeO 4, MGaGeO 4 ( M = Na, K)

Abstract

MGaSiO 4, MAlGeO 4, and MGaGeO 4 phases ( M = Na, K) have been synthesized using flux, hydrothermal, and melt growth techniques and characterized by TEM and single crystal and powder X-ray diffraction. The K compounds crystallize with a (2√3 A, C) hexagonal unit cell which is a superstructure of the ( A, C) hexagonal kalsilite (KAlSiO 4) cell. The room-temperature polymorphs of the Na compounds crystallize with a (√3 A, 3 A, C, γ ⋍ 90°) monoclinic cell and are isostructural with beryllonite (NaBePO 4). TEM data suggest that they transform to a kalsilite-like (√3 A, C) hexagonal cell at high temperature.