Compounds with the formula ${\mathit{R}}_2$BaM${\mathrm{O}}_5$ where R=rare earth and M=Co, Ni, or Cu, have been studied with Raman spectroscopy. The lattice dynamics of the three different crystallographic phases adopted by these systems (space groups Immm, Pnma, and P4/mbm) is discussed in relation to the transition-metal–oxygen coordination, and to the binding M-O and R-O distances. An almost complete identification of the normal modes has been performed for the compounds with the P4/mbm structure and an estimation of the parameters of the M-O and R-O stretching force constants has been obtained for the Immm and P4/mbm structures.

• Received 25 January 1993

DOI:https://doi.org/10.1103/PhysRevB.47.14898