An unprecedented structural phase transition in struvite-type compounds: dimorphism of KMgAsO4(H2O)6

Abstract

Abstract The crystal structure of struvite-type KMgAsO4(H2O)6 has been redetermined from single crystal X-ray diffraction data at room temperature. The previous structure model based on powder X-ray diffraction data was confirmed with higher precision and accuracy and with all hydrogen atoms located. KMgAsO4(H2O)6 undergoes a reversible phase transition of the continuous type at 263(2) K, changing the symmetry from orthorhombic to monoclinic. The corresponding Pnm21→P1121 symmetry reduction is of a translationengleiche type with index 2 and was monitored by temperature-dependent powder X-ray diffraction measurements. Such a phase transition is unprecedented for struvite-type compounds. The crystal structure of the monoclinic polymorph was determined from a two-domain crystal at 100 K. Except for the motion of one of the water molecules towards stronger hydrogen-bonding interactions, structural changes between the two polymorphs are small.