The crystal structures of the low-temperature phases of leonite-type compounds, K2Me(SO4)2·4H2O (Me2+= Mg, Mn, Fe)

Abstract

Recent optical and differential scanning calorimetry measurements indicate phase transitions in leonite-type compounds at low temperatures. The crystal structures of these phases, i.e., leonite, K2Mg(SO4)2⋅4H2O, “Mn-leonite”, K2Mn(SO4)2⋅4H2O, and mereiterite, K2Fe(SO4)2⋅4H2O, have been determined at low temperatures. The leonite structure (space group C2/m at room temperature) is composed of sulfate tetrahedra and MeO6 octahedra which are interconnected by K cations and hydrogen bonds of the H2O molecules. Previous structure investigations at room temperature have shown that one of the sulfate groups is disordered. Refinements of single-crystal X-ray data at ambient and low temperatures indicate that the dynamic disorder in leonite and “Mn-leonite” is “frozen” in two steps and thus results in two new, ordered structures at low temperatures. In mereiterite only one transition from the dynamically disordered to the ordered structure is observed.