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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]

Earth's core is likely the largest reservoir of carbon (C) in the planet, but its C abundance has been poorly constrained because measurements of carbon's preference for core versus mantle materials at the pressures and temperatures of core formation are lacking. Using metal-silicate partitioning experiments in a laser-heated diamond anvil cell, we show that carbon becomes significantly less siderophile as pressures and temperatures increase to those expected in a deep magma ocean during formation of Earth's core. Based on a multistage model of core formation, the core likely contains a maximum of 0.09(4) to 0.20(10) wt% C, making carbon a negligible contributor to the core's composition and density. However, this accounts for ~80 to 90% of Earth's overall carbon inventory, which totals 370(150) to 740(370) ppm. The bulk Earth's carbon/sulfur ratio is best explained by the delivery of most of Earth's volatiles from carbonaceous chondrite-like precursors. [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 distributions of major and minor elements in Earth’s core and mantle were primarily established by high pressure, high temperature metal–silicate partitioning during core segregation. The partitioning behaviors of moderately siderophile elements can be used to constrain the pressure–temperature conditions of core formation and the core’s composition. We performed experiments to study the partitioning of Ni, Co, V, Cr, Si, and O between silicate melt and Fe-rich metallic melt in a multianvil press and diamond anvil cell, up to 100 GPa and 5700 K. Combining our new results with data from 18 previous studies, we parameterized the effects of pressure, temperature, and metallic melt composition on partitioning. Ni and Co partitioning are insensitive to composition. At low pressures, these elements become less siderophile with increasing temperature, with this trend reversing above ∼45 GPa. V and Cr partitioning are much more sensitive to metallic melt composition and less sensitive to pressure. Partitioning of Si and O are insensitive to pressure, but with strong and moderate temperature dependences, respectively. Our new parameterizations of Ni and Co partitioning suggest that the Earth’s distributions of these elements can be matched by single-stage core–mantle equilibration at 54 ± 5 GPa and 3300–3400 K. These conditions would result in 8.5 ± 1.4 wt% Si and 1.6 ± 0.3 wt% O in the core, compatible with the core’s measured density. However, this single-stage model matches the Earth’s V and Cr distributions less well. We also incorporated our parameterizations into models of multi-stage core formation over evolving pressure–temperature-oxygen fugacity conditions, reproducing the Earth’s Ni and Co distributions while simultaneously producing a core whose light element composition is consistent with its density. [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]

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]