RAMAN-ACTIVE PHONONS OF THE SPIN-1 CHAIN ANTIFERROMAGNETS R2BANIO5 (R=GD AND Y)

We report Raman spectra of single-crystalline Gd2BaNiO5 and Y2BaNiO5, concentrating on identification and assignment of the phonons at room temperature. All nine of the expected Ramanactive phonons (3 A(g) + 1B 1g + 2B2g + 3B3g) are observed. The three Gd(Y) related modes of B2g, A(g), and B3, symmetry are observed at 123(160), 175(233), and 204(269 ) cm-1, respectively. The six O(1) vibrational modes of the Gd(Y) comp ound occur at 183(184), 312(328), 335(340), 422(440), 518(540), and 56 2(584) cm-1, having B3g, B2g, B1g, B3g, A(g), and A(g) symmetries, res pectively. A strong mode polarized along the Ni-O(2) chain axis at app roximately 726(755) cm-1 in the Gd(Y) compound may arise from magnetic excitations along the spin chains or from a Raman-forbidden, infrared -allowed vibrational mode along the Ni-O(2) chains.