Phase Transition in K3Na(MoO4)2 and Determination of the Twinned Structures of K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 at Room Temperature

Abstract

The room-temperature phases of sodium potassium molybdates K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 are isostructural with the monoclinic low-temperature phases of KaNa(SeO4)2 and K3Na(CrO4)2, which are twinned distorted glaserite structures. In the molybdates there are two crystallographically independent potassiums and their environment slightly differs from those in K3Na(SeO4)2 and K3Na(CrO4)2. The excessive Na in K2.5Na1.5(MoO4)2 occupies the position of the more firmly bound potassium. A reversible phase transition at 513 K was discovered in KaNa(MoO4)2 by DSC (differential scanning calorimetry), but no such transition in K2.5Na1.5(MoO4)2 was detected. Both samples used in the diffractometer experiment were found to be composed of six domains being related by twinning operations of the point group 6. The twinning may be considered as a combination of a merohedral and a pseudo-merohedral twinning with two- and threefold rotations as twinning operations, respectively. However, a reversible domain switching, which is observable in the related ferroelastic crystals of KaNa(SeO4)2 and K3Na(CrO4)2, was not observed either in K3Na(MoO4)2 or in K2.5Na1.5(MoO4)2, either due to semitransparency of the samples or high ferroelastic distortion. This distortion is manifested by the values of the atomic displacement vectors which are about twice as large as those in the selenate or the chromate.