Variable temperature X-ray crystal structure analysis of a type I langbeinite: Rb₂Cd₂(SO₄)₃

Abstract

The structure of a type I langbeinite, Rb₂Cd₂(SO₄)₃, displays three different phases, cubic with a = 10.378(5) Å (space group P2₁3) at room temperature, monoclinic at 120 K with a = 10.328(3), b = 10.322(3), c = 10.325(3) Å, β= 89.975(1)° (space group P2₁), and orthorhombic at 85 K with a = 10.319(2), b = 10.321(2), c = 10.320(2) Å (space group P2₁2₁2₁), respectively. Precise single-crystal analyses of these phases indicate that Rb₂Cd₂(SO₄)₃ distorts initially from cubic to monoclinic upon cooling followed by a significant reorientation of the SO₄ tetrahedra, resulting in an orthorhombic symmetry upon further cooling. The three structures have been established unequivocally using the same crystal. There is no indication of the formation of an intermediate triclinic phase or any lattice disorder as conjectured in several earlier reports on compounds belonging to the type I langbeinite. The bond valence sum analyses of the coordination around the Rb sites indicate asymmetry in the bond strengths which could be the driving force of the ferroelectric behavior in these materials.