Raman and x-ray studies of a high-pressure phase transition inβ-LiIO3and the study of anharmonic effects

Abstract

Temperatureand pressure-dependent Raman scattering experiments have been performed to study the phonon behavior in P-Lil03. The temperature dependence of Raman-active phonons does not show anomalous frequency behavior in the temperature range 10-650 K; however, at least two Bg phonons, with frequencies 344 and 460 cm ', and one Ag phonon, with frequency 249 cm show anomalous increases in linewidth with temperature. The high-pressure Raman scattering experiments of P-Lil03 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 LiIO3 at 50 kbar. The Raman spectra of this new 6 phase are quite different from those of the three other known phases of LiIO3 and reveal a lowering of symmetry upon passing from the P to the 5 phase. High-pressure x-ray measurements were performed to characterize the structural phase transition in P-LiIO3, which transforms from tetragonal symmetry P4/n (C4~) to monoclinic symmetry PZ/n (Czq) 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 a and e of a tetragonal phase. The pressure and temperature dependence of Raman spectra of P-Lil03 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 Bg phonon at 460 cm ' and the Ag phonon at 249 cm are highly anharmonic with increasing temperature. It is concluded from pressure Raman work that there exists strong mixing of internal and external modes in P-Lil03. Such a mixing of internaland external-mode characters is also apparent from the study of fractional implicit and explicit contributions to the frequency variations of phonons with temperature.

High-Pressure Spectroscopy Study of Zn(IO3)2 Using Far-Infrared Synchrotron Radiation

“…Table 2 shows clearly the change in the pressure dependences at 3.6 GPa and 8.8 GPa as well as the changes in frequencies and pressure dependencies at 13 GPa, supporting the existence of the proposed transitions. The pressure coefficients in the low-pressure phase are comparable to pressure coefficients in Fe(IO 3 ) 3 [13], LiIO 3 [28], and KIO 3 [29] in the same frequency region, indicating that Zn(IO 3 ) 2 is extremely compressible, similar to these other iodates. Additionally, the pressure coefficients for the different HP phases are of the same order of magnitude as in the low-pressure phase, suggesting that the changes in the coordination polyhedra are not drastic and that the transition is probably related to gradual changes in cation coordination number [30].…”