Ferroelectric phase transition in LiNaGe4O9 crystal studied by EPR of Mn4+ and optical absorption spectroscopy

Electron paramagnetic resonance (EPR) of Mn4+ ions and optical absorption spectra are studied in the lithium–sodium tetragermanate LiNaGe4O9 crystal in the range of a ferroelectric phase transition. Anisotropy of the EPR spectra evidences that the paramagnetic Mn4+ ions are substituted for Ge hosts within oxygen octahedra. Double splitting of EPR lines below the phase transition temperature (TC = 114 K) indicates lowering the symmetry (C2 → C1) of the Mn4+ sites. The components of the g factor and hyperfine interaction tensor A, axial D, and rhombic E crystal field parameters have been determined from the EPR spectra above and below TC. It is shown that at phase transition, the values of the spin-Hamiltonian parameters vary only slightly and the EPR spectra are changed mainly due to magnetic axes rotation. Based on this result, it is proposed that below TC, GeO6 octahedra rotate nearly around the crystal axis b. On cooling, the angle of octahedra rotation obeys the square root temperature dependence predicted by the mean field theory of phase transitions. The low-energy part of the optical absorption edge in the LiNaGe4O9 crystal has been described by Urbach's rule. Near TC, the parameters of Urbach's rule exhibit an anomalous deviation from the regular temperature behavior. The linear dependence of the forbidden band isoabsorption width Egα(T) changes the slope, whereas the parameter σ(T), determined by the strength of an electron–phonon interaction, demonstrates a sharp non-monotonic anomaly. The study of the EPR and optical absorption spectra evidences for second order of the ferroelectric phase transition in the LiNaGe4O9 crystal.