Frequency dependence of the quasi-soft Raman-active modes in rotationally distorted R(3+)B(3+)O3 perovskites (R(3+)-rare earth, B(3+)=Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga)

Abstract

The structural data and Raman spectra of distorted R(3+)B(3+)O(3) perovskites (R(3+)-rare earth, B(3+)=Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga) with BO(6) octahedral tilts were critically reviewed in order to obtain the frequency (ω) versus BO(6) tilt angle (α) dependence of the quasi-soft vibrational modes. It was found that the relation ω = ϰα is satisfied in a good approximation for each RBO(3) family with fixed B atom, despite the different spatial tilt orientations for compounds of the same series with either orthorhombic Pnma or rhombohedral R ̅3c structure. The dependence of the proportionality coefficient ϰ on the averaged <B-O> octahedral distance can be described using different functions (power, linear and exponential ones). The established dependences can be used for analysis of Raman data and heuristic prediction of structural second-order transitions.