Temperature- and pressure-dependent Raman scattering experiments have been performed to study the phonon behavior in $\beta$-LiI${\mathrm{O}}_3$. The temperature dependence of Raman-active phonons does not show anomalous frequency behavior in the temperature range 10-650 K; however, at least two $B_g$ phonons, with frequencies 344 and 460 ${\mathrm{cm}}^{-1\mathrm{}}$, and one $A_g$ phonon, with frequency 249 ${\mathrm{cm}}^{-1\mathrm{}}$, show anomalous increases in linewidth with temperature. The high-pressure Raman scattering experiments of $\beta$-LiI${\mathrm{O}}_3$ were performed at room temperature with a sapphire-anvil cell for hydrostatic pressures up to 95 kbar. The spectra show a discontinuous reversible change for all orientations at about 50 kbar. This change is interpreted as a phase transition in LiI${\mathrm{O}}_3$ at 50 kbar. The Raman spectra of this new $\delta$ phase are quite different from those of the three other known phases of LiI${\mathrm{O}}_3$ and reveal a lowering of symmetry upon passing from the $\beta$ to the $\delta$ phase. High-pressure x-ray measurements were performed to characterize the structural phase transition in $\beta$-LiI${\mathrm{O}}_3$, which transforms from tetragonal symmetry $\frac{P4\mathrm{}}{n} \left(C_{4h}^3\right)$ to monoclinic symmetry $\frac{P2\mathrm{}}{n} \left(C_{2h}^4\right)$ above 50 kbar. The complex behavior at high pressure appears to be analogous to that of some perovskite compounds, in which there exists an antipathetic lattice coupling between $\mathit{a}$ and $\mathit{c}$ of a tetragonal phase. The pressure and temperature dependence of Raman spectra of $\beta$-LiI${\mathrm{O}}_3$ were combined to analyze the explicit and implicit contributions to the isobaric temperature variation of phonons. The results show that at least two modes, the $B_g$ phonon at 460 ${\mathrm{cm}}^{-1\mathrm{}}$ and the $A_g$ phonon at 249 ${\mathrm{cm}}^{-1\mathrm{}}$, are highly anharmonic with increasing temperature. It is concluded from pressure Raman work that there exists strong mixing of internal and external modes in $\beta$-LiI${\mathrm{O}}_3$. Such a mixing of internal- and external-mode characters is also apparent from the study of fractional implicit and explicit contributions to the frequency variations of phonons with temperature.

• Received 8 June 1984

DOI:https://doi.org/10.1103/PhysRevB.30.7212