Identification of complex anions in La1 − x A x MnO3 manganites (A = Ca, Sr) from neutron diffraction data and refinement of their structures on the basis of raman spectroscopy data

Abstract

The consideration of the Mn(-O…Mn)6 structure unit of the LaMnO3 orthorhombic phase constructed on the basis of neutron diffraction data revealed the presence of atomic groups in the form of (MnO2)− complex anions and separate O2− ions. The Raman spectra have demonstrated that complex anions have a nearly linear O=Mn-O− structure. Comparison of the Raman spectra of the rhombohedral CaMnO3 and BaTiO3 phases and pyramidal molecules of the ZXY2 type has shown that the CaMnO3 phase is composed of pyramidal MnO32− molecules with one Mn=O double bond and two Mn-O− ordinary bonds. The complex anions are linked by O=Mn-O− intermolecular bonds to form planar zigzag Mn=O…Mn chains similar to C=C-C planar chains in some organic compounds exhibiting high electrical conductivity. With decreasing temperature, the Mn…O intermolecular distances decrease, and the Mn=O…Mn chains become even more similar to the C=C-C chains and exhibit high electrical conductivity.