Raman scattering study of a two-dimensional S = 1 quantum spin system Ni₅(TeO₃)₄Cl₂

The transition metal tellurium oxychloride, Ni5(TeO3)4Cl2, has been investigated by the Raman scattering method at temperatures of 15-300 K. Sixty two phonon modes (of the total 69 Raman-active phonon modes allowed for the monoclinic C6 2h structure) are identified in the spectra, and their behavior is analyzed. Several distinct features are observed in Raman spectra at low temperatures: (i) three low-frequency bands (22.3, 29.4, and 49.0 cm-1 at T = 15 K) with a temperature behavior characteristic of the one-magnon scattering and (ii) a band at 56.3 cm-1 with a pronounced temperature dependence. The origin ofthe low-temperature bands is discussed.