PbMnO 2.75—a high-pressure phase having a new type of crystallographic shear structure derived from perovskite

Abstract

PbMnO 2.75, prepared at 7.8 GPa and 880°C, has monoclinic symmetry, A2/ m, with cell parameters a=32.232(5)) Å, b=3.831(1)) Å, c=35.671(5) Å, β=130.00(1)°. Electron diffraction patterns of the phase show features characteristic of ReO 3-based crystallographic shear (CS) structures, and a model for the structure was developed on this basis which gave a good fit to powder X-ray diffraction and neutron diffraction patterns. The structure can be formally derived from the perovskite structure by periodic shear on (704) p planes, giving blocks of perovskite that are displaced relative to one another by the CS vector of 1 2 [110] p . Ordered oxygen vacancies occur at the CS planes which convert the MnO 6 octahedra to MnO 5 square pyramids, resulting in the formation of pseudo-hexagonal channels along [010]. Pb atoms occupy these channels and form pyramidal coordination to oxygen. The PbO 4 pyramids edge-share with the MnO 5 square pyramids to form elements with the topology of tetragonal PbO. Pb atoms also occupy the A-cation sites in the perovskite blocks. This is the first example of a structure based on CS of the ReO 3 structure where the A-cation sites are occupied, thus giving a perovskite CS phase.