Although it has previously been considered to be essentially anhydrous, Al‐free stishovite can contain up to ∼1.3 wt % of H₂O, perhaps through the direct substitution ( ), according to recent studies. Yet the stability of such substitution and its impact on the properties of silica and rutile‐structured hydrous phases (such as δ‐AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al‐free stishovite samples at 723 K and 9 GPa, and 1473 K and 10 GPa, respectively. Synchrotron X‐ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1 bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt % H₂O). At 300 K, we found a lower and broader transition pressure from rutile type to CaCl₂ type (28–42 GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica‐rich heterogeneities, such as subducted oceanic crust.

We determined the single‐crystal elasticity of Fe³⁺‐bearing grossular using Brillouin scattering up to 22 GPa and 1,000 K. Using the obtained results, we modeled mantle velocity profiles to investigate the origin of low‐velocity zones (LVZs) in the bottom 100 km of the mantle transition zone. Our modeling revealed that although a water‐saturated transition zone could be seismically detected as the LVZs, it would also cause a negative velocity jump across the 520‐km discontinuity, which has not been seismically detected. However, mid‐ocean ridge basalts (MORB) with an FeO content of 10–17 wt.% in garnet will have V_S 2.0(5)%–3.0(5)% lower than that in the pyrolitic mantle. Oxidizing (FeO = 17 wt.%, Fe³⁺/(Fe²⁺ + Fe³⁺) = 0.3–0.8) or lowering the Ca content of garnet to 1.4 wt.% can further decrease the V_S of MORB. Such Fe‐enriched, oxidized MORB or Ca‐depleted, oxidized MORB, with a 3.5(5)%–7.6(5)% lower V_S and distinct dln ρ/dln V_S ratio, have a seismic signature consistent with the observed LVZs in the region.

Melamine (C₃H₆N₆; 1,3,5-triazine-2,4,6-triamine) is an aromatic substituted s-triazine, with carbon and nitrogen atoms forming the ring body, and amino groups bonded to each carbon. Melamine is widely used to produce laminate products, adhesives, and flame retardants, but is also similar chemically and structurally to many energetic materials, including TATB (2,4,6-triamino-1,3,5- trinitrobenzene) and RDX (1,3,5-trinitroperhydro-1,3,5-triazine). Additionally, melamine may be a precursor in the synthesis of superhard carbon-nitrides, such as β-C₃N₄. In the crystalline state melamine forms corrugated sheets of individual molecules, which are stacked on top of one another, and linked by intra- and inter-plane N-H hydrogen bonds. Several previous high-pressure X-ray diffraction and Raman spectroscopy studies have claimed that melamine undergoes two or more phase transformations below 25 GPa. Our results show no indication of previously reported low pressure polymorphism up to approximately 30 GPa. High-pressure crystal structure refinements demonstrate that the individual molecular units of melamine are remarkably rigid, and their geometry changes very little despite volume decrease by almost a factor of two at 30 GPa and major re-arrangements of the intermolecular interactions, as seen through the Hirshfeld surface analysis. A symmetry change from monoclinic to triclinic, indicated by both dramatic changes in diffraction pattern, as well as discontinuities in the vibration mode behavior, was observed above approximately 36 GPa in helium and 30 GPa in neon pressure media. Examination of the hydrogen bonding behavior in melamine’s structure will allow its improved utilization as a chemical feedstock and analog for related energetic compounds.

Water is an essential chemical compound for living organisms, and twenty of its different crystal solid forms (ices) are known. Still, there are many fundamental problems with these structures such as establishing the correct positions and thermal motions of hydrogen atoms. The list of ice structures is not yet complete as DFT calculations have suggested the existence of additional and – to date – unknown phases. In many ice structures, neither neutron diffraction nor DFT calculations nor X-ray diffraction methods can easily solve the problem of hydrogen atom disorder or accurately determine their anisotropic displacement parameters (ADPs). Here, accurate crystal structures of H₂O, D₂O and mixed (50%H₂O/50%D₂O) ice VI obtained by Hirshfeld atom refinement (HAR) of high-pressure single-crystal synchrotron and laboratory X-ray diffraction data are presented. It was possible to obtain O—H/D bond lengths and ADPs for disordered hydrogen atoms which are in good agreement with the corresponding single-crystal neutron diffraction data. These results show that HAR combined with X-ray diffraction can compete with neutron diffraction in detailed studies of polymorphic forms of ice and crystals of other hydrogen-rich compounds. As neutron diffraction is relatively expensive, requires larger crystals which can be difficult to obtain and access to neutron facilities is restricted, cheaper and more accessible X-ray measurements combined with HAR can facilitate the verification of the existing ice polymorphs and the quest for new ones.

The phase transitions of natural San Carlos Fe‐, Al‐ and Ca‐bearing orthopyroxene and two synthetic orthopyroxenes containing Al (up to 12 wt. % Al₂O₃) and Fe (up to 15 wt. % FeO) were investigated at high pressures and temperatures up to ∼30 GPa and 700 K using synchrotron‐based single‐crystal X‐ray diffraction. These orthopyroxenes undergo α‐opx → β‐opx → γ‐opx phase transitions with increasing pressure and temperature. We quantitatively described how the composition influences the pressures of the phase transitions. The results indicated that the variations in Mg, Fe, and Al contents do not significantly influence the pressure of the α‐opx → β‐opx transition (P_α‐β). For the β‐opx → γ‐opx phase transition, increasing Fe content decreases the P_β‐γ while increasing Al content increases the P_β‐γ. The incorporation of Ca increases the P_α‐β and decreases the P_β‐γ. The thermal equations of states of α‐opx and β‐opx for these three orthopyroxenes were determined using the unit‐cell volume collected at high pressures and temperatures, which have been used to establish a density model to interpret the cold stagnant slabs. Our experimental results have indicated that β‐opx and γ‐opx are possible metastable phases of natural orthopyroxenes within cold stagnant slabs in the mantle transition zone.