PbMnO2.75 -: a high-pressure phase having a new type of crystallographic shear structure derived from perovskite

PbMnO2.75, prepared at 7.8 GPa and 880degreesC, has monoclinic symmetry, A2/m, with cell parameters a = 32.232(5)), Angstrom, b = 3.831 (1)) Angstrom, c = 35.671(5) Angstrom, beta = 130.00(l)degrees. Electron diffraction patterns of the phase show features characteristic of ReO3- 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 MnO6 octahedra to MnO5 square pyramids, resulting in the formation of pseudo-hexagonal channels along [0 10]. Pb atoms occupy these channels and form pyramidal coordination to oxygen. The PbO4 pyramids edge-share with the MnO5 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 ReO3 structure where the A-cation sites are occupied, thus giving a perovskite CS phase. (C) 2002 Elsevier Science (USA). All rights reserved.