Variable Temperature X-ray Crystal Structure Analysis of a Type I Langbeinite: Rb₂Cd₂(SO₄)₃

Abstract

The structure of a type I langbeinite, Rb 2 Cd 2 (SO 4 ) 3 , displays three different phases, cubic with a ) 10.378(5) Å (space group P2 1 3) 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 P2 1 ), and orthorhombic at 85 K with a ) 10.319(2), b ) 10.321(2), c ) 10.320(2) Å (space group P2 1 2 1 2 1 ), respectively. Precise single-crystal analyses of these phases indicate that Rb 2 Cd 2 (SO 4 ) 3 distorts initially from cubic to monoclinic upon cooling followed by a significant reorientation of the SO 4 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.

Bimetallic Sulfates AxMy(SO4)z.xH2O (A = Li, Na and M = 3d Metal): New Electrode Materials for Li- and Na-Ion Batteries

“…Another approach, well valued by chemists, consists in drawing their inspiration from already known compounds and operating either by homology or by structural modifications in order to convert them into attractive hosts for alkaline ions. With more than 4000 species identified on Earth, minerals turn out to be a valuable data bank of known phases (2,3). Moreover understanding the conditions of their formation may suggest a starting point for their artificial synthesis, which could then be generalized to their structural derivates.…”