Raman and infrared-active phonons in hexagonal HoMnO3 single crystals:: magnetic ordering effects

Polarized first- and second-order Raman scattering and infrared reflection spectra of hexagonal HoMnO3 single crystals in the temperature range 10-300 K are reported. Based on the symmetry analysis and comparison with the results of lattice dynamics calculations the observed lines are assigned to the lattice eigenmodes. The magnetic ordering of Mn ions, which occurs below T-N = 76 K, is shown to affect Raman- and infrared-active phonons, which modulate Mn-O-Mn bonds and, consequently, the Mn-Mn exchange interaction.