The high pressure crystal structures of tin sulphate: a case study for maximal information recovery from 2D powder diffraction data

Our recently proposed method for automatic detection, calibration and evaluation of Debye-Scherrer ellipses using pattern recognition techniques and advanced signal filtering was applied to 2D powder diffraction data of tin sulphate in dependence on pressure. Three phase transitions towards higher pressure could be identified, and their respective crystal structures have been determined. The high pressure behaviour of the stereoactive lone pair of Sn(2+) was investigated. At ambient conditions, SnSO(4) crystallizes in a strongly distorted Barite structure type in space group Pnma (phase 1). In the pressure range between P = 0.15 and P = 0.2 GPa, it exhibits a displacive second order phase transition into a structure with space group P112(1)/a (phase II at P = 0.2 GPa: a = 8.7022(9) angstrom, b = 5.3393(5) angstrom, c = 7.0511(6) angstrom, gamma = 89.90(1)degrees). A second displacive phase transition occurs between P = 4.40 and P = 5.07 GPa into another structure with space group P (1) over bar (phase III at P = 13.5 GPa: a = 8.067(3) angstrom, b = 5.141(2) angstrom, c = 6.609(2) angstrom, alpha = 90.56(3)degrees, beta = 90.65(2)degrees, gamma = 89.46(2)degrees). A third displacive phase transition towards another crystal structure in space group P (1) over bar occurs between P = 13.6 and P = 15.3 GPa (phase IV at P = 20.5 GPa: a = 7.889(5) angstrom, b = 5.028(3) angstrom, c = 6.462(3) angstrom, alpha = 90.99(3)degrees, beta = 91.01(3)degrees, gamma = 89.89(4)degrees).). A non-linear compression behaviour over the entire pressure range is observed, which can be described by three Vinet relations in the ranges from P = 0.21 to 4.4 GPa, from P = 5.07 to 13.55 GPa and from P = 15.26 to 20.5 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to K(0) = 48(1) GPa for phase II, K(0) = 56(2) GPa for phase III, and K(0) = 51(13) GPa for phase IV. The crystal structures of all phases are refined against X-ray powder diffraction data measured at several pressures between 15 and 20.5 GPa. The structural phase transitions of SnSO(4) are mainly characterized by a reorientation of the SO(4) tetrahedra, in order to optimize crystal packing. With increasing pressure, the lone pair which is localized at Sn(2+) increasingly adopts pure s-character.