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.

The electronic structure of four ternary-metal oxides containing isolated vanadate ions is studied. Zircon-type YVO4, YbVO4, LuVO4, and NdVO4 are investigated by high-pressure optical-absorption measurements up to 20 GPa. First-principles calculations based on density-functional theory were also performed to analyze the electronic band structure as a function of pressure. The electronic structure near the Fermi level originates largely from molecular orbitals of the vanadate ion, but cation substitution influences these electronic states. The studied orthovanadates, with the exception of NdVO4, undergo a zircon-scheelite structural phase transition that causes a collapse of the band-gap energy. The pressure coefficient dEg/dP shows positive values for the zircon phase and negative values for the scheelite phase. NdVO4 undergoes a zircon-monazite-scheelite structural sequence with two associated band-gap collapses.

The structural and vibrational properties of bismuth selenide (Bi2Se3) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30~GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral (R-3m) phase ($\alpha${}-Bi2Se3) with sixfold coordination for Bi to a monoclinic C2/m structure ($\beta${}-Bi2Se3) with sevenfold coordination for Bi above 10~GPa. The equation of state and the pressure dependence of the lattice parameters and volume of $\alpha${} and $\beta${} phases of Bi2Se3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in $\alpha${}-Bi2Se3 is discussed. Raman measurements evidence that Bi2Se3 undergoes two additional phase transitions around 20 and 28~GPa, likely toward a monoclinic C2/c and a disordered body-centered cubic structure with 8-fold and 9- or 10-fold coordination, respectively. These two high-pressure structures are the same as those recently found at high pressures in Bi2Te3 and Sb2Te3. On pressure release, Bi2Se3 reverts to the original rhombohedral phase after considerable hysteresis. Symmetries, frequencies, and pressure coefficients of the Raman and infrared modes in the different phases are reported and discussed.

The present work employs the total-energy pseudopotential technique within the local density approximation to determine the electronic structure and the full set of elastic constants of cubic BN which have not been directly established experimentally. We obtain ${\mathit{C}}_{11}$=8.44, ${\mathit{C}}_{12}$=1.9, and ${\mathit{C}}_{44}$=4.83 Mbar and we find that a cutoff of 160 Ry is needed to have converged results. We also present a study of the electronic band structure of BP and c-BN.

The electronic band-structure and band-gap dependence on the d character of A2+ cation in AWO4 wolframite-type oxides is investigated for different compounds (A = Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled d levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled d electron shells, give rise to direct and wide band gaps, Mn, with a half-filled d shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO4, ZnWO4, and CdWO4 blue-shift linearly with pressure, with a pressure coefficient of approximately 13~eV/GPa. However, the band gap of multiferroic MnWO4 red-shifts at $-${}22~meV/GPa. Finally, in MnWO4, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO6 complexes and how it affects their band-gap closure.

We use an ab initio total energy pseudopotential technique within the local-density aproximation to determine the full set of first-order elastic constants of BeS, BeSe, and BeTe, which have not been established experimentally. For BeS we obtain ${\mathrm{C}}_{11}$=1.84, ${\mathrm{C}}_{12}$=0.75, and ${\mathrm{C}}_{44}$=0.99 Mbar. For BeSe and BeTe we obtain ${\mathrm{C}}_{11}$=1.49, 1.11; ${\mathrm{C}}_{12}$=0.59, 0.43; and ${\mathrm{C}}_{44}$=0.81, 0.60 Mbar, respectively. We also calculate the bulk modulus, the optical phonon frequency at Γ, and we present a study of the electronic band structure of those compounds. The calculated bulk moduli agree well with previous experimental and theoretical values.