Phase transitions in the hexagonal tungsten bronze RbNbW2O9

Abstract

The hexagonal tungsten bronze RbNbW2O9 is shown, by variable-temperature powder neutron diffraction and symmetry-mode analysis, to display a significantly different phase transition sequence compared to the related CsNbW2O9 composition. At ambient temperature, RbNbW2O9 adopts the polar orthorhombic space group Cmc21. Upon heating, the thermal evolution of the crystal structure proceeds via two transitions. These correspond to sequential loss of two distinct octahedral tilting modes, leading to space group P63mc at around 655K, and space group P6mm near 700 K. The polar distortion is retained up to the highest temperature studied here. The differences in structural behaviour between the proper ferroelectric RbNbW2O9 and the improper ferroelectric CsNbW2O9 emphasises the need for careful crystallographic analyses of materials of this type.