Complex Polymorphism and Polytypism of Potassium Metaarsenate, KAsO₃

The dehydration of KH 2 AsO 4 to KAsO 3 and the complex temperature-dependent polymorphism of KAsO 3 were investigated by high-temperature X-ray powder diffraction. Four of the five KAsO 3 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, KH 2 AsO 4 subsequently transforms into γ- (150–200 °C), β- (370–410 °C) and finally α-KAsO 3 (500–510 °C) before melting at 640–650 °C. The dehydration is accompanied by formation of minor amounts of δ- and β-KAsO 3 . On cooling, α-KAsO 3 subsequently transforms into δ-KAsO 3 (320–300 °C) and γ-KAsO 3 (starting at 270 °C). The α → β transformation was only observed on prolonged heating at 470 °C. The β-KAsO 3 phase reversibly transforms into the metastable β′-KAsO 3 below 250 °C. β-KAsO 3 can be stabilized at lower temperatures by partial substitution of Rb for K. α- and γ-KAsO 3 are long-chain polyarsenates with repetition period 2. The crystal structures can be derived from H - MPO 3 (M = K, Rb) and the pyroxene orthoenstatite, respectively. The β- and β′-KAsO 3 phases feature cyclotriarsenate units with approximately 3 m (β-KAsO 3 ) and 2 (β′-KAsO 3 ) symmetry. Both β- and β′-KAsO 3, 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.