In this article, we review the advances that have been made on the understanding of the high-pressure (HP) structural, vibrational, and electronic properties of wolframite-type oxides since the first works in the early 1990s. Mainly tungstates, which are the best known wolframites, but also tantalates and niobates, with an isomorphic ambient-pressure wolframite structure, have been included in this review. Apart from estimating the bulk moduli of all known wolframites, the cation–oxygen bond distances and their change with pressure have been correlated with their compressibility. The composition variations of all wolframites have been employed to understand their different structural phase transitions to post-wolframite structures as a response to high pressure. The number of Raman modes and the changes in the band-gap energy have also been analyzed in the basis of these compositional differences. The reviewed results are relevant for both fundamental science and for the development of wolframites as scintillating detectors. The possible next research avenues of wolframites under compression have also been evaluated.

We have investigated the origin of the bandgap of BaMoO₄, PbMoO₄, and CdMoO₄ crystals on the basis of optical absorption spectroscopy experiments and ab initio electronic band structure, density of states, and electronic localization function calculations under high pressure. Our study provides an accurate determination of the bandgaps E_g and their pressure derivatives $d{E}_g/\mathit{dP}$ for BaMoO₄ (4.43 eV, −4.4 meV/GPa), PbMoO₄ (3.45 eV, −53.8 meV/GPa), and CdMoO₄ (3.71 eV, −3.3 meV/GPa). The absorption edges were fitted with the Urbach exponential model which we demonstrate to be the most appropriate for thick crystals with direct bandgaps. So far, the narrowing of the bandgap of distinct PbMoO₄ had been qualitatively explained considering only the presence of the Pb 6s levels at the top of its valence band. Its fast pressure dependent redshift and the occurrence of its direct bandgap away from Γ in contrast to the other scheelites had remained unsolved. Here, we show that in contrast to what had been proposed and different from the other scheelites, in PbMoO₄, the bandgap takes place between the Pb 6s levels at the top of the valence band and the antibonding O 2p levels at the bottom of the conduction band. For this reason, the direct bandgap is pushed away from the zone center in order to allow s–p mixing. Its pressure dependence is one order of magnitude faster than that in the other scheelites due to two effects: its delocalized character and the higher compressibility of dodecahedral units, PbO₈, compared to tetrahedral units, MoO₄.

International audienceNew data on the high-pressure melting curve of Ta up to 48GPa are reported. Evidence of melting from changes in sample texture was found in five different experiments using scanning electron microscopy. The obtained melting temperatures are in excellent agreement with earlier measurements using x-ray diffraction or the laser-speckled method but are in contrast with several theoretical calculations. The results are also compared with shock-wave data. These findings are of geophysical relevance because they confirm the validity of earlier experimental techniques that resulted in low melting slopes of the transition metals measured in the diamond-anvil cell, including iron

Scheme of the different phase transitions of (( R )-(−)-3-hydroxyquinuclidium)[FeCl ₄ ].

The P-T phase diagram of the hydrated magnesium carbonate nesquehonite (MgCO₃·3H₂O) has not been reported in the literature. In this paper, we present a joint experimental and computational study of the phase stability and structural behavior of this cementitious material at high-pressure and high-temperature conditions using in situ single-crystal and synchrotron powder X-ray diffraction measurements in resistive-heated diamond anvil cells plus density functional theory calculations. Our results show that nesquehonite undergoes two pressure-induced phase transitions at 2.4 (HP1) and 4.0 GPa (HP2) at ambient temperature. We have found negative axial compressibility and thermal expansivity values, likely related to the directionality of the hydrogen bonds. The equations of state of the different phases have been determined. All the room-temperature compression effects were reversible. Heating experiments at 0.7 GPa show a first temperature-induced decomposition at 115 °C, probably into magnesite and a MgCO₃·4H₂O phase.