Intrinsic structural distortion in orthorhombic perovskite oxides

Octahedra in the orthorhombic perovskite structure are not rigid. The bond-length splitting at an octahedral site can be described by two degenerate vibration modes, the orthorhombic Q(2) and the tetragonal Q(3). A polar plot of r=rho=(Q(2)(2)+Q(2)(2))(1/2) and phi=tan(-1)(Q(3)/Q(2)) has been made to map out the site distortion as a function of the rare-earth ionic size for three families, RFeO(3), RVO(3), and RMnO(3). The octahedral-site distortion deduced from RFeO(3) is found to be intrinsic to orthorhombic perovskites and to strongly bias orbital ordering and spin ordering in compounds that have the same structure.