Crystal structures of cation non-stoichiometric RMn3O6 (R = Gd, Er, and Tm) manganites belonging to A-site columnar-ordered quadruple perovskite family

Abstract

High-pressure RMn3O6 manganites (belonging to a family of A-site columnar-ordered quadruple perovskites A2A′A″B4O12, where A′ is a site with a square-planar coordination and A″ is a site with a tetrahedral coordination) are prone to cation non-stoichiometry: they are rare-earth-deficient, R1−xMn3+xO6, for R = Ho, Y, Er, and Tm and manganese-deficient, R1+xMn3−xO6, for R = Gd. Here we investigated their crystal structures using single-crystal X-ray diffraction and synchrotron X-ray powder diffraction at room temperature. We found that the rare-earth deficiency is formed by doping of Mn2+ into the A site: [Er3+1.78Mn2+0.22]A[Mn3+]A′[Mn2+]A′′[Mn3+2]B1[Mn3.61+2]B2O12. The manganese deficiency is realized through doping of a small amount of Gd3+ into the A″ site: [Gd3+2]A[Mn3+]A′[Mn2+0.87Gd3+0.13]A′′[Mn3+2]B1[Mn3.44+2]B2O12. They all crystallize in space group Pmmn with layered charge ordering at the B sites. They are structurally related to CaFeTi2O6. Cation non-stoichiometry is supported by variations of the oxidation states of manganese at one of the B sites.