Computational and spectroscopic comparative analysis of Raman phonon spectra of LiNiPO4

We have performed Raman studies of a LiNiPO4 single crystal at temperatures below and above the phase transition to a magnetically ordered state at 5 and 25 K, respectively. A shift of some phonon lines during the transition to a magnetically ordered state was observed that is explained by a significant spin-phonon interaction. We identified the anomalous Raman lines using the shell-model calculation. It was found that with an increase in temperature, some external vibrational modes shift to the low-frequency region, and the internal vibrational modes of the (PO4)3– tetrahedra shift to the high-frequency region. This anomalous behavior of the vibrational modes is explained by the presence of magnetostriction that can selectively act on the bonds between ions in the crystal upon the transition to a magnetically ordered state.