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

Recent optical and differential scanning calorimetry measurements indicate phase transitions in leonite-type compounds at low temperatures. The crysta l structures of these phases, i.e., leonite, K2Mg(SO4)(2). 4H(2)O,"Mn-leoni te", K2Mn(SO4)(2). 4H(2)O, and mereiterite, K2Fe(SO4)(2). 4H(2)O, have been determined at low temperatures. The leonite structure (space group C2/m at room temperature) is composed of sulfate tetrahedra and MeO6 octahedra whi ch 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 da ta at ambient and low temperatures indicate that the dynamic disorder in le onite and "Mn-leonite" is "frozen" in two steps and thus results in two new , ordered structures at low temperatures. In mereiterite only one transitio n from the dynamically disordered to the ordered structure is observed. The two low-temperature crystal structures of leonite have been refined to R = 0.0236 at 170 K (space group I2/a, Z = 8, a = 11.780(2) Angstrom, b = 9 .486(2) Angstrom, c = 19.730(4) Angstrom, beta = 95.23(3)degrees, V = 2195. 6 Angstrom (3)), and to R = 0.0230 at 100 K (space group P2(1)/a, Z = 4, a = 11.778(1) Angstrom, b = 9.469(1) Angstrom, c = 9.851(2) Angstrom, beta = 95.26(1)degrees, V = 1094.01 Angstrom (3)). The two low-temperature crystal structures of "Mn-leonite" have been refined to R = 0.0272 at 185 K (space group I2/a, Z = 8, a = 12.035(2) Angstrom, b = 9.549(2) Angstrom, c = 19.8 39(4) Angstrom, P = 94.99(3)degrees, V= 2271.3 Angstrom (3)), and to R = 0. 0237 at 110 K (space group P2(1)/a, Z = 4, a = 12.031(1) Angstrom, b = 9.53 1(1) Angstrom, c = 9.902(1) Angstrom, beta = 95.02(1)degrees, V = 1131.08 A ngstrom (3)). The low-temperature crystal structure of mereiterite has been refined to R = 0.0219 at 185 K (space group P2(1)/a, Z = 4, a = 11.834(2) Angstrom, b = 9.502(1) Angstrom, c = 9.913(2) Angstrom, beta = 94.87(1)degr ees, V = 1110.66 Angstrom (3)). The different behavior of mereiterite (i.e., stability range, sequence of t ransitions) in comparison to the Ma, and Mn endmembers may be explained by more distorted MeO6 octahedra and by strongly different hydrogen bond lengt hs around the disordered sulfate groups.