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${\mathrm{WN}}_{6}$ phase discovered at 126--165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group $R\overline{3}m$ which is consistent with the ${\mathrm{WN}}_{6}$ structure with armchairlike hexazine (${\mathrm{N}}_{6}$) rings, while strong ${A}_{1g}$ Raman mode confirms its $\mathrm{N}─\mathrm{N}$ single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent ${\mathrm{N}}_{6}$ rings, and repulsions between ${\mathrm{N}}_{6}$ rings that make ${\mathrm{WN}}_{6}$ ultrastiff and tough. The ${\mathrm{WN}}_{6}$ phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

Elastic moduli (${C}_{ij}$) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The ${C}_{12}$ of stishovite converges with the ${C}_{11}$ at $\ensuremath{\sim}55\text{ }\text{ }\mathrm{GPa}$, where the transverse wave ${V}_{S1}$ propagating along [110] also vanishes. Landau modeling of the ${C}_{ij}$, ${B}_{1g}$ optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in ${V}_{S}$ and Poisson's ratio in silica-bearing subducting slabs in the mid-lower mantle.

Because of their weak interlayer bonding, van der Waals (vdW) solids are very sensitive to external stimuli such as strain. Experimental studies of strain tuning of thermal properties in vdW solids have not yet been reported. Under $\ensuremath{\sim}9%$ cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed an increase of cross-plane thermal conductivity in bulk ${\mathrm{MoS}}_{2}$ from 3.5 to about $25\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{m}}^{\ensuremath{-}1}\text{ }{\mathrm{K}}^{\ensuremath{-}1}$, measured with a picosecond transient thermoreflectance technique. First-principles calculations and coherent phonon spectroscopy experiments reveal that this drastic change arises from the strain-enhanced interlayer interaction, heavily modified phonon dispersions, and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The contribution from the change of electronic thermal conductivity is negligible. Our results suggest possible parallel tuning of structural, thermal, and electrical properties of vdW solids with strain in multiphysics devices.

Brillouin light scattering and impulsive stimulated light scattering have been used to determine the full elastic constants of magnesiosiderite [(Mg_{0.35}Fe_{0.65})CO_{3}] up to 70 GPa at room temperature in a diamond-anvil cell. Drastic softening in C_{11}, C_{33}, C_{12}, and C_{13} elastic moduli associated with the compressive stress component and stiffening in C_{44} and C_{14} moduli associated with the shear stress component are observed to occur within the spin transition between ∼42.4 and ∼46.5 GPa. Negative values of C_{12} and C_{13} are also observed within the spin transition region. The Born criteria constants for the crystal remain positive within the spin transition, indicating that the mixed-spin state remains mechanically stable. Significant auxeticity can be related to the electronic spin transition-induced elastic anomalies based on the analysis of Poisson's ratio. These elastic anomalies are explained using a thermoelastic model for the rhombohedral system. Finally, we conclude that mixed-spin state ferromagnesite, which is potentially a major deep-carbon carrier, is expected to exhibit abnormal elasticity, including a negative Poisson's ratio of -0.6 and drastically reduced V_{P} by 10%, in Earth's midlower mantle.

Pressure dependence of the Ce valence in ${\mathrm{CeCu}}_{2}{\mathrm{Ge}}_{2}$ has been measured up to 24 GPa at 300 K and to 17 GPa at 18--20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in ${\mathrm{Ce}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Si}}_{2}$ at 20 K. A very weak, monotonic increase of the valence with $x$ was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ and ${\mathrm{Yb}}_{0.88}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$, using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the $x=0.88$ sample below 20 K. In contrast, the nearly stoichiometric ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe $3d$ electrons and valence instability of the Yb $4f$ electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}$ and 17 GPa for ${\mathrm{Yb}}_{0.88}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$.

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe(4)Sb(12) and Yb(0.88)Fe(4)Sb(12), using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x=0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe(4)Sb(12) is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe(4)Sb(12) and 17 GPa for Yb(0.88)YbFe(4)Sb(12).