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Nitrogen, the most abundant element in Earth's atmosphere, is also a primary component of solid nitride minerals found in meteorites and on Earth's surface. If they remain stable to high pressures and temperatures, these nitrides may also be important reservoirs of nitrogen in planetary interiors. We used synchrotron X‐ray diffraction to measure the thermal equation of state and phase stability of titanium nitride (TiN) in a laser‐heated diamond anvil cell at pressures up to ∼70 GPa and temperatures up to ∼2,500 K. TiN maintains the cubic B1 (NaCl‐type) crystal structure over the entire pressure and temperature range explored. It has K0 = 274 (4) GPa, K0′ = 3.9 (2), and γ0 = 1.39 (4) for a fixed V0 = 76.516 (30) Å3 (based on experimental measurements), q = 1, and θ0 = 579 K. Additionally, we collected Raman spectra of TiN up to ∼60 GPa, where we find that the transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical phonon modes exhibit mode Grüneisen parameters of 1.66(17), 0.54(15), and 0.93 (4), respectively. Based on our equation of state, TiN has a density of ∼5.6–6.4 g/cm3 at Earth's lower mantle conditions, significantly more dense than both the mantle of the Earth and the estimated densities of the mantles of other terrestrial planets, but less dense than planetary cores. We find that TiN remains stable against physical decomposition at the pressures and temperatures found within Earth's mantle, making it a plausible reservoir for deep planetary nitrogen if chemical conditions allow its formation. Plain Language Summary: Understanding the incorporation of nitrogen into Earth's interior could account for discrepancies in Earth's nitrogen budget. Solid nitride materials have been suggested to store nitrogen in the mantle and/or core. We performed high pressure, high temperature experiments on the refractory nitride mineral osbornite titanium nitride (TiN) to explore its stability at Earth's mantle conditions. We measured its density and crystal structure at high pressures and temperatures, finding that TiN does not change structure or melt at deep Earth conditions. Additionally, we performed Raman spectroscopy measurements to further explore the properties of TiN at high pressures. We find that TiN is a plausible mineral for nitrogen storage within the deep Earth, provided that it is chemically stable within the mantle. Key Points: We measured the thermal equation of state and vibrational properties of (titanium nitride) TiNTiN remains in the cubic rock salt structure with no melting or decomposition up to at least ∼70 GPa and ∼2,500 KFormation of TiN in the mantle may sequester N within Earth's interior, potentially solving the "missing nitrogen" problem [ABSTRACT FROM AUTHOR]

Synchrotron X‑ray diffraction data were obtained to simultaneously measure unit-cell volumes of W and WO2 at pressures and temperatures up to 70 GPa and 2300 K. Both W and WO2 unit-cell volume data were fit to Mie-Grüneisen equations of state; parameters for W are KT = 307 (±0.4) GPa, K′T = 4.05 (±0.04), g0 = 1.61 (±0.03), and q = 1.54 (±0.13). Three phases were observed in WO2 with structures in the P21/c, Pnma, and C2/c space groups. The transition pressures are 4 and 32 GPa for the P21/c-Pnma and Pnma-C2/c phase changes, respectively. The P21/c and Pnma phases have previously been described, whereas the C2/c phase is newly described here. Equations of state were fitted for these phases over their respective pressure ranges yielding the parameters KT = 238 (±7), 230 (±5), 304 (±3) GPa, K′T = 4 (fixed), 4 (fixed), 4 (fixed) GPa, g0 = 1.45 (±0.18), 1.22 (±0.07), 1.21 (±0.12), and q = 1 (fixed), 2.90 (±1.5), 1 (fixed) for the P21/c, Pnma, and C2/c phases, respectively. The W-WO2 buffer (WWO) was extended to high pressure using these W and WO2 equations of state. The T-f,O2 slope of the WWO buffer along isobars is positive from 1000 to 2500 K with increasing pressure up to at least 60 GPa. The WWO buffer is at a higher fO2 than the iron-wüstite (IW) buffer at pressures lower than 40 GPa, and the magnitude of this difference decreases at higher pressures. This implies an increasingly lithophile character for W at higher pressures. The WWO buffer was quantitatively applied to W metal-silicate partitioning by using the WWO-IW buffer difference in combination with literature data on W metal-silicate partitioning to model the exchange coefficient (KD) for the Fe-W exchange reaction. This approach captures the non-linear pressure dependence of W metal-silicate partitioning using the WWO-IW buffer difference. Calculation of KD along a peridotite liquidus predicts a decrease in W siderophility at higher pressures that supports the qualitative behavior predicted by the WWO-IW buffer difference, and agrees with findings of others. Comparing the competing effects of temperature and pressure the results here indicate that pressure exerts a greater effect on W metal-silicate partitioning. [ABSTRACT FROM AUTHOR]

The Earth's iron-rich inner core is seismically anisotropic, which may be due to the preferred orientation of Fe-rich hexagonal close packed (hcp) alloy crystals. Elastic anisotropy in a hexagonal crystal is related to its c/a axial ratio; therefore, it is important to know how this ratio depends on volume (or pressure), temperature, and composition. Experimental data on the axial ratio of iron and alloys in the Fe-Ni-Si system from 15 previous studies are combined here to parameterize the effects of these variables. The axial ratio increases with increasing volume, temperature, silicon content, and nickel content. When an hcp phase coexists with another structure, sample recovery and chemical analysis from each pressure-temperature point is one method for determining the phase's composition and thus the position of the phase boundary. An alternate method is demonstrated here, using this parameterization to calculate the composition of an hcp phase whose volume, temperature, and axial ratio are measured. The hcp to hcp+B2 phase boundary in the Fe-FeSi system is parameterized as a function of pressure, temperature, and composition, showing that a silicon-rich inner core may be an hcp+B2 mixture. These findings could help explain observations of a layered seismic anisotropy structure in the Earth's inner core. [ABSTRACT FROM AUTHOR]

The ability to produce high pressures and temperatures ( P – T ), and study the physical and chemical properties of solids and melts at these conditions, is possible through the use of the laser-heated diamond anvil cell (LH-DAC). High P – T experiments are commonly performed at synchrotron radiation facilities, where in situ X-ray diffraction (XRD) permits the detection of melting, crystallographic structural analysis, and density measurements at extreme conditions. Here we present an overview of recent experimental advances in the use of high pressure techniques in combination with laser heating and in situ X-ray diffraction. We summarize state-of-the-art capabilities, including recent advancements, in sample preparation, pressure and temperature measurements, and other technical aspects. Two main applications of the LH-DAC are discussed: the study of the melting curves of elements and compounds at high pressures, and materials synthesis at extreme conditions. The melting curves of Fe, Ta, and MgO are used as examples in a discussion of experimental techniques, technical developments, and sources of discrepancies in melting data. High P – T syntheses of light molecular systems, superhard materials, nitrides, oxides, carbon compounds, and geologically important materials are reviewed. [ABSTRACT FROM AUTHOR]

Abstract: The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200GPa and ~145GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42GPa. The minimum temperature of an Fe–Si outer core is 4380K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, only the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases. [Copyright &y& Elsevier]

Abstract: Wüstite, Fe1−xO, is an important component in the mineralogy of Earth''s lower mantle and may also be a component in the core. Therefore the high pressure, high temperature behavior of FeO, including its phase diagram and equation of state, is essential knowledge for understanding the properties and evolution of Earth''s deep interior. We performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures. Wüstite was mixed with iron metal, which served as our pressure standard, under the assumption that negligible oxygen dissolved into the iron. Our data show a positive slope for the subsolidus phase boundary between the B1 and B8 structures, indicating that the B1 phase is stable at the P–T conditions of the lower mantle and core. We have determined the thermal equation of state of B1 FeO to 156GPa and 3100K, finding an isothermal bulk modulus K0 =149.4±1.0GPa and its pressure derivative K0′=3.60±0.4. This implies that 7.7±1.1wt.% oxygen is required in the outer core to match the seismologically-determined density, under the simplifying assumption of a purely Fe–O outer core. [Copyright &y& Elsevier]

Iron oxide (FeO) is an important component in the mineralogy of Earth's lower mantle and possibly its core, so its phase diagram is essential to models of the planet's interior. The melting curve of wüstite, Fe0.94O, was determined up to 77 GPa and 3100 K in a laser-heated diamond anvil cell. Melting transition temperatures were identified from discontinuities in the emissivity vs. temperature relationship within the laser-heated spot. The melting curve exhibits no obvious kinks that could be related to a subsolidus transition in wüstite, but there is evidence for a two-phase loop at pressures below 30 GPa. Comparison of these results to previous studies on Fe, Fe-O, and Fe-S confirms that the melting point depression in the Fe-O system remains significantly less, by a factor of 2 or more, than that in the Fe-S system up to pressures exceeding 80 GPa. [ABSTRACT FROM AUTHOR]