Crystal structures and phase transitions of Sr2CrSbO6

Sr2CrSbO6 was synthesized by the conventional solid-state reaction process. X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) has been used to reinvestigate the structure at room temperature and to study the phase transitions at high- and low-temperature. Rietveld analysis revealed that Sr2CrSbO6 crystallizes at room temperature in a monoclinic system having a space group I2/m, with a=5.5574(1) Å; b=5.5782(1) Å; c=7.8506(2) Å and β=90.06(2), no P21/n space group as was previously reported. The high-temperature study (300–870 K) has shown that the compound presents the following temperature induced phase-transition sequence: I2/m–I4/m–Fm-3m. The low-temperature study (100–300 K) demonstrated that the room-temperature I2/m monoclinic symmetry seems to be stable down to 100 K.

The components of the GM3+ mode, responsible for the breaking of the symmetry to the I4/mmm tetragonal space group from the Fm-3m, observed experimentally at high. This mode involves the movements of all the oxygen atoms in the octahedra: those located in the (00z) positions move to the center (or out of) of the octahedra, as shown in (a); and the oxygen atoms located in the xy plane move outwards (inwards) along the diagonals of the basal plane of the octahedra (b). As the octahedra are corner sharing if one octahedra stretches the other one expands. (c) On the other hand, the mode GM4+ is responsible for the breaking of the symmetry down to the I4/m space group, and involves movements only of the oxygen atoms located in the xy plane (c): those displacements can be viewed as rotations (tilts) of the octahedra around the tetragonal axis.Figure optionsDownload as PowerPoint slide