Complex Polymorphism and Polytypism of Potassium Metaarsenate, KAsO3

The dehydration of KH2AsO4 to KAsO3 and the complex temperature-dependent polymorphism of KAsO3 were investigated by high-temperature X-ray powder diffraction. Four of the five KAsO3 polymorphs were structurally characterized by single crystal or powder X-ray diffraction and are described with attention to polytypism and structurally related phases. On heating, KH2AsO4 subsequently transforms into γ- (150–200 °C), β- (370–410 °C) and finally α-KAsO3 (500–510 °C) before melting at 640–650 °C. The dehydration is accompanied by formation of minor amounts of δ- and β-KAsO3. On cooling, α-KAsO3 subsequently transforms into δ-KAsO3 (320–300 °C) and γ-KAsO3 (starting at 270 °C). The α → β transformation was only observed on prolonged heating at 470 °C. The β-KAsO3 phase reversibly transforms into the metastable β′-KAsO3 below 250 °C. β-KAsO3 can be stabilized at lower temperatures by partial substitution of Rb for K. α- and γ-KAsO3 are long-chain polyarsenates with repetition period 2. The crystal structures can be derived from H-MPO3 (M = K, Rb) and the pyroxene orthoenstatite, respectively. The β- and β′-KAsO3 phases feature cyclotriarsenate units with approximately 3m (β-KAsO3) and 2 (β′-KAsO3) symmetry. Both β- and β′-KAsO3, are polytypic order–disorder structures composed of nonpolar layers of one kind. The major polytypes are of a maximum degree of order. Alternative stacking sequences are observed by twinning.