Identification of Complex Anions in La_(1-x)A_xMnO_3 Manganites (A = Ca, Sr) from Neutron Diffraction Data and Refinement of Their Structures on the Basis of Raman Spectroscopy Data

摘要

The consideration of the Mn(—O...Mn)_6 structure unit of the LaMnO_3 orthorhombic phase constructed on the basis of neutron diffraction data revealed the presence of atomic groups in the form of (MnO_2)_ complex anions and separate O~(2-) ions. The Raman spectra have demonstrated that complex anions have a nearly linear 0=Mn—O~- structure. Comparison of the Raman spectra of the rhombohedral CaMnO_3 and BaTiO_3 phases and pyramidal molecules of the ZXY_2 type has shown that the CaMnO_3 phase is composed of pyramidal MnO_3~(2-) 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.