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 Raman-active phonons (3Ag+1B1g+2B2g+3B3g) are observed. The three Gd(Y) related modes of B2g, Ag, and B3g symmetry are observed at 123(160), 175(233), and 204(269) cm-1, respectively. The six O(1) vibrational modes of the Gd(Y) compound occur at 183(184), 312(328), 335(340), 422(440), 518(540), and 562(584) cm-1, having B3g, B2g, B1g, B3g, Ag, and Ag symmetries, respectively. A strong mode polarized along the Ni-O(2) chain axis at ∼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.