Crystal structures and phase transition behaviour in the 5d transition metal oxides AReO4 (A = Ag, Na, K, Rb, Cs and Tl)

Abstract

The structures of the six perrhenates (AReO₄ A = Ag, Na, K, Rb, Cs and Tl) at room temperature have been established using powder neutron diffraction methods. These demonstrate the rigid nature of the ReO₄ tetrahedra, with the Re–O distances decreasing very slightly and the O–Re–O bond angles approaching the regular tetrahedron value of 109.5° as the size of the A-type cation increases. Variable temperature synchrotron X-ray diffraction measurements show that RbReO₄ undergoes a I4₁/a to I4₁/amd transition near 650 K that is associated with a change in the orientation of the ReO₄⁻ tetrahedra about the scheelite b-axis associated with a Γ₃⁺ mode. CsReO₄ has an orthorhombic pseudo scheelite structure at room temperature with rotation of the ReO₄ tetrahedra about the c-axis described by mode M₄⁺ and this undergoes a first order orthorhombic to tetragonal (Pnma to I4₁/a) transition near 450 K with a transition to the I4₁/amd structure occurring above this. TlReO₄ is a rare example of a crystalline material displaying a re-entrant phase transition; 14₁/a to P2₁/c to 14₁/a. The monoclinic structure can be described as a scheelite superstructure that contains an ordering of tetrahedral rotations around the c-axis and along the b-axis with the irrep Γ₃⁺ and M₄⁺ both present. This behaviour is different to that described recently for the analogous Tc oxide TlTcO₄, which highlights the differences in the chemistry of these two systems.