Powder Diffraction

Technical Articles

Lattice parameters and orthorhombic distortion of CaMnO3

Wojciech Paszkowicza1 c1, Jarosław Piętosaa1, Scott M. Woodleya2, Piotr A. Dłużewskia3, Mirosław Kozłowskia3 and Christine Martina4

a1 Institute of Physics, Polish Academy of Sciences, Lotnikow 32/46, 02-668 Warsaw, Poland

a2 Department of Chemistry, University College London, 3rd Floor, Kathleen Lonsdale Building, Gower Street, London WC1E 6BT, United Kingdom

a3 Institute of Physics, Polish Academy of Sciences, Lotnikow 32/46, 02-668 Warsaw, Poland

a4 Laboratoire CRISMAT-ENSICAEN, UMR CNRS 6805, CNRS, 6, Bld. Maréchal Juin, 14050 Caen Cedex 04, France

Abstract

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) Å, b=7.457 30(4) Å, and c=5.267 48(4) Å, leading to orthorhombic distortion of (c/a, √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, √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.

(Received December 07 2009)

(Accepted January 19 2010)

Key Words:

  • calcium manganese oxide;
  • perovskite;
  • orthorhombic distortion;
  • oxygen deficiency

Correspondence:

c1 Author to whom correspondence should be addressed. Electronic mail: paszk@ifpan.edu.pl