IR vibrational modes and spin-phonon interplay in magnetoelectric LiNiPO4.

LiNiPO₄, multifunctional material with potential ionic current and magnetoelectric applications, was studied by IR optical spectroscopy and lattice dynamic shell-model calculations. We present IR phonon parameters of lithium-nickel-phosphate, both experimental (at T = 30 K) and calculated ones. Multiferroic properties of lithium-nickel-phosphate were confirmed by temperature evolution of phonon band parameters due to spin-phonon interaction revealed for a number of phonon modes. The most pronounced effect was registered for B_2u IR-vibrational mode near 202 cm⁻¹, which demonstrates anomalous temperature behavior in the vicinity of magnetic ordering (T_N = 21.8 K) indicating the coupling to the magnetic subsystem. This mode is assigned to the complex normal vibration including the librations of the PO₄ tetrahedra, out-of-plane vibrations of the Li atoms, and the in-plane vibrations of the magnetic Ni atoms along the b axis. The detected phonon shift at T_N can be explained by the striction phenomena. [ABSTRACT FROM AUTHOR]