Raman study of anharmonic effects inα-LiIO3

We present a detailed study of the Raman spectrum of $\alpha$-LiI${\mathrm{O}}_3$ as a function of temperature and pressure. Measurements under hydrostatic pressure were performed at 300 and 380 K. Comparison of these results among each other and with previous results from uniaxial stress measurements performed at 77 K reveal that mode Grüneisen parameters, ${\gamma }_j$, are essentially temperature independent. These parameters are strongly dependent on mode frequency, ${\omega }_j$, obeying an approximate scaling law, ${\gamma }_j\propto {\omega }_j^{-2\mathrm{}}$—a behavior characteristic of that observed in molecular crystals. Data from the pressure and temperature dependence of ${\omega }_j$ are combined to separate the intrinsic (thermal expansion) from the extrinsic contributions to the latter. This procedure reveals the existence of anomalous anharmonic behavior affecting only the lattice mode which involves translations of ${\mathrm{Li}}^{+}$ ions along the $c$ axis. We suggest that this is indicative of the existence of a link between this mode and the anomalous increase in the ionic conductivity along the $c$ axis occurring in this material at high temperatures.