High‐quality single‐crystals of (Al,Fe)‐bearing bridgmanite, Mg_0.88 Fe³⁺_0.065Fe²⁺_0.035Al_0.14Si_0.90O₃, of hundreds of micrometer size were synthesized at 24 GPa and 1800 °C in a Kawai‐type apparatus from the starting hydrous melt containing ~6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion free in micrometer‐ to nanometer‐spatial resolutions. Nanosecondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show ~1,020(±70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier‐transform infrared spectroscopy (FTIR) analyses with two pronounced OH‐stretching bands at ~3,230 and ~3,460 cm⁻¹. Our results indicate that lower‐mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water content exceeding ~0.1 wt%.

${\mathrm{WN}}_6$ 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 $\sim 55\mathrm{GPa}$, 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.

Brillouin light scattering and impulsive stimulated light scattering have been used to determine the full elastic constants of magnesiosiderite [$({\mathrm{Mg}}_{0.35}{\mathrm{Fe}}_{0.65}){\mathrm{CO}}_3$] 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 $\sim 42.4$ and $\sim 46.5\mathrm{GPa}$. 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.