The high-pressure behavior of optical phonons in wurtzite zinc oxide $(w-\mathrm{ZnO})$ is studied using room-temperature Raman spectroscopy and ab initio calculations based on a plane-wave pseudopotential method within the density-functional theory. The pressure dependence of the zone-center phonons ${(E}_2,A_1,$ and $E_1)$ was measured for the wurtzite structure up to the $\mathrm{hexagonal}\to \mathrm{cubic}$ transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the $E_2^{\mathrm{low}}$ mode. $E_1\left(\mathrm{LO}\right)$ and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born’s transverse dynamic charge $e_T^{*}$ is experimentally found to increase under compression like $e_T^{*}\mathrm{}\left(P\right)=2.02+6.4\mathrm{}\times {10}^{-3}P,$ whereas calculations give $e_T^{*}\mathrm{}\left(P\right)=2.09\mathrm{}–2.5\times {10}^{-3}P$ (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of an anomaly in the $E_2$ and $A_1$ mode behaviors before the phase transition.

Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on ${\mathrm{CaWO}}_4$ and ${\mathrm{SrWO}}_4$ up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of ${\mathrm{CaWO}}_4$, under hydrostatic conditions ${\mathrm{SrWO}}_4$ undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the $A{\mathrm{O}}_8$ polyhedra of $AB{\mathrm{O}}_4$ scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_4$ at pressures of up to $56\mathrm{GPa}$ and $24\mathrm{GPa}$, respectively. $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4$ is found to undergo a pressure-driven phase transition at $7.1\mathrm{GPa}$ from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_4$ at $9\mathrm{GPa}$. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4\text{−}\mathrm{II}$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_4\text{−}\mathrm{III}$ (space group $P{2}_1∕n$). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is, however, metastable and can only occur if the transition to the $P{2}_1∕n$ phases were kinetically inhibited. Our experiments in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4$ indicate that it becomes amorphous beyond $47\mathrm{GPa}$.

Room-temperature Raman scattering has been measured in barium tungstate $\left(\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4\right)$ up to $16\mathrm{GPa}$. We report the pressure dependence of all the Raman active first-order phonons of the tetragonal scheelite phase ($\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4\text{−}\mathrm{I}$, space group $I{4}_1∕a$), which is stable at normal conditions. As pressure increases the Raman spectrum undergoes significant changes around $6.9\mathrm{GPa}$ due to the onset of the structural phase transition to the monoclinic $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4\text{−}\mathrm{II}$ phase (space group $P{2}_1∕n$). This transition is only completed above $9.5\mathrm{GPa}$. A further change in the spectrum is observed at $7.5\mathrm{GPa}$ related to a scheelite-to-fergusonite transition. The scheelite, $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_4\text{−}\mathrm{II}$, and fergusonite phases coexist up to $9.0\mathrm{GPa}$ due to the sluggishness of the $\mathrm{I}\to \mathrm{II}$ phase transition. Further to the experimental study, we have performed ab initio lattice dynamics calculations that have greatly helped us in assigning and discussing the pressure behavior of the observed Raman modes of the three phases.

We have studied the vibrational properties of $\mathrm{Cu}\mathrm{Al}{\mathrm{O}}_2$ by means of Raman scattering in ambient conditions, at low temperature, and also at high pressure. Results are discussed in the framework of an ab initio calculation. Raman active modes have wave numbers ${\sigma }_{E_g}=418.1\pm 0.2{\mathrm{cm}}^{-1}$ and ${\sigma }_{A_{1g}}=767.2\pm 0.3{\mathrm{cm}}^{-1}$. Polarized measurements with single crystals have confirmed their symmetry. We present and discuss the phonon-dispersion curves. Below $200\mathrm{K}$, the temperature dependence of the Raman active modes is almost linear, with coefficients $\partial {\sigma }_{E_g}∕\partial T=(-4\pm 1)\times {10}^{-3}{\mathrm{cm}}^{-1}∕\mathrm{K}$ and $\partial {\sigma }_{A_{1g}}∕\partial T=(-1.0\pm 0.2)\times {10}^{-2}{\mathrm{cm}}^{-1}∕\mathrm{K}$. Most of the temperature shift is associated with thermal expansion. The pressure dependence is given by $\partial {\sigma }_{E_g}∕\partial P=2.72\pm 0.07{\mathrm{cm}}^{-1}∕\mathrm{GPa}$ and $\partial {\sigma }_{A_{1g}}∕\partial P=4.96\pm 0.12{\mathrm{cm}}^{-1}∕\mathrm{GPa}$. We observe a reversible phase transition at $34\pm 2\mathrm{GPa}$, which, as already has been shown to happen in $\mathrm{Cu}\mathrm{Ga}{\mathrm{O}}_2$, we relate to the existence of a dynamical instability.

In this letter, we discuss the electronic structure of copper aluminate (${\mathrm{CuAlO}}_2$ ) on the basis of absorption measurements at low temperature and under high pressure in single crystals and thin films, combined with ab initio electronic structure calculations. The indirect character of the fundamental transition could be clearly established through the photon energy dependence of the absorption edge as measured in single crystals, yielding a band gap (plus a phonon) of $2.99\pm 0.01\mathrm{eV}$ at room temperature. An indirect excitonic structure is also observed at low temperature. The lowest energy direct allowed transition occurs near $3.53\pm 0.01\mathrm{eV}$ and is identified through its low-energy tail in single crystals and its excitonic peak in thin films. Comparing our results with ab initio calculations available in the literature, we conclude that ${\mathrm{CuGaO}}_2$ and ${\mathrm{CuInO}}_2$ can hardly be considered as wide gap semiconductors, even if they are transparent in thin films.