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 have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (1987–2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (30–32° N, Morocco) and the tropics (10–13° N, Bamako region) close to the Atlantic coast (at 5–8° W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve.

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 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 $47\mathrm{GPa}$.

We have studied by means of high-pressure x-ray diffraction the structural stability of Ni2Mo3N, Co3Mo3N, and Fe3Mo3N. We also report ab initio computing modeling of the high-pressure properties of these compounds, Pd2Mo3N and Pt2Mo3N. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides such as $\gamma${}-Si3N4.

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.