VIBRATIONAL STUDY OF R2BANIO5(R = Y, HO, ER OR TM), NIO6 ONE-DIMENSIONAL CHAINS, AND TM2BANIO5, NIO5 PYRAMIDS

Raman scattering experiments have been performed in R2BaNiO5 (R = Y, Ho, Er, or Tm) polycrystalline samples. All the compounds crystallize in the Immm space group. The Tm system is dymorphic and has also been obtained in the Pnma phase. Out of the 9 Raman active modes expected in the Immm phase, two modes associated with R ion motions and the two oxygen O(1) stretching modes have been identified. Three other weaker modes are also assigned to O(1) vibrations while the highest energy peak (740 cm-1) is the odd stretching mode of the short Ni-O(2) bond only allowed in infrared spectroscopy. An evaluation of the principal stretching force constants has been done and the non diagonal terms of the F matrix are found to be quite important. The low symmetry phase of the Tm compound shows a high number of Raman modes, however smaller than the 52 expected. Some correspondance in the energy zones of the modes has been found by comparison between both structural phases of the Tm2BaNiO5 dymorphic compound.