OPTICAL PHONONS IN R2BAMO5 OXIDES WITH M = CO, NI, CU, AND R = A RARE-EARTH

Compounds with the formula R2BaMO5 where R=rare earth and M =Co, Ni, o r Cu, have been studied with Raman spectroscopy. The lattice dynamics of the three different crystallographic phases adopted by these system s (space groups Immm, Pnma, and P4/mbm) is discussed in relation to th e 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 esti mation of the parameters of the M-O and R-O stretching force constants has been obtained for the Immm and P4/mbm structures.