DYNAMIC STUDY OF THE STRUCTURAL AND MAGNETIC PHASES IN PR2NIO4 SINGLE-CRYSTALS BY RAMAN-SPECTROSCOPY

Polarized Raman spectra of stoichiometric Pr2NiO4 single crystals have been obtained for temperatures between 74 and 300 K in a spectral range of 6000 cm-1. The lattice dynamics of the orthorhombic-to-tetragonal low-temperature phase transition has been characterized. The intensity associated with the in-plane Ni-O(1) modes is much lower than the corresponding modes related to motions along the c axis. Five A(g) modes have been identified with the corresponding atomic vibrations. The frequencies and widths of some particular modes show a discontinuous behavior at the transition temperature (T(c) congruent-to 117 K). Below this temperature, additional modes appear with very low intensities. Polarized two-magnon scattering is observed as wide bands whose intensity increases with decreasing temperature as does the magnetization. The characteristics of the observed two-magnon spectra do not correspond to classical two-spin scattering; their selection rules indicate that the Ni interlayer coupling plays an important role in the process.