Lattice parameters and orthorhombic distortion of CaMnO3

CaMnO3 is a parent compound for numerous multicomponent manganese perovskite oxides. Its crystallographic data are of primary importance in the science and technology of functional CaMnO3-based materials. In the present study, data were collected for a CaMnO3 sample at 302 K. The crystal structure refinement yields accurate absolute values of lattice parameters, a =5.281 59(4) angstrom, b=7.457 30(4) angstrom, and c=5.267 48(4) angstrom, leading to orthorhombic distortion of (c/a, root 2c/b) = (0.997 33 0.998 95). The orthorhombic distortion of the CaMnO3 structure is discussed on the basis of comparison Of Our unit-cell size with data already published. At a graphical representation of the distortion, it is observed that there is a considerable scatter of the distortion values among the literature data but, interestingly, a considerable fraction of experimental results (including the present one) for stoichiometric samples are grouped around the distortion (c/a, root 2c/b)=(0.9973,0.9990), which lies close to a maximum in the extent of orthorhombicity. The influence of off-stoichiometry on the orthorhombic distortion is discussed on the basis of available experimental data. Simulations, employing a mean-field approach for low temperatures, predict an increase in cell volume and structural distortions with the concentration of oxygen vacancies when the additional electrons are localized on the manganese. A simple model of delocalization produced the opposite effect, which is expected to combine with lattice vibrations to recover the cubic phase at high temperatures. (C) 2010 International Centre for Diffraction Data. [DOI: 10.1154/1.3314256]