Thermodynamic properties and enhancement of diamagnetism in nitrogen doped lutetium hydride synthesized at high pressure.

Nitrogen doped lutetium hydride has drawn global attention in the pursuit of room-temperature superconductivity near ambient pressure and temperature. However, variable synthesis techniques and uncertainty surrounding nitrogen concentration have contributed to extensive debate within the scientific community about this material and its properties. We used a solid-state approach to synthesize nitrogen doped lutetium hydride at high pressure and temperature (HPT) and analyzed the residual starting materials to determine its nitrogen content. High temperature oxide melt solution calorimetry determined the formation enthalpy of LuH1.96N0.02 (LHN) from LuH2 and LuN to be -28.4 ± 11.4 kJ/mol. Magnetic measurements indicated diamagnetism which increased with nitrogen content. Ambient pressure conductivity measurements observed metallic behavior from 5 to 350 K, and the constant and parabolic magnetoresistance changed with increasing temperature. High pressure conductivity measurements revealed that LHN does not exhibit superconductivity up to 26.6 GPa. We compressed LHN in a diamond anvil cell to 13.7 GPa and measured the Raman signal at each step, with no evidence of any phase transition. Despite the absence of superconductivity, a color change from blue to purple to red was observed with increasing pressure. Thus, our findings confirm the thermodynamic stability of LHN, do not support superconductivity, and provide insights into the origins of its diamagnetism

In situ electrical resistivity and viscosity measurements of iron alloys under pressure using synchrotron X-ray radiography

This project performed at the APS synchrotron aimed at developing a new experimental setup that allowed us to perform simultaneous electrical conductivity measurements, falling sphere experiments (to determine viscosity) and XRD to see in-situ changes in the material. We carried out these experiments on Fe-Si-Ni-O powders

Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe-S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field

Transformation kinetics for olivine with ~75 ppm H_2O into ringwoodite

The existence of metastable olivine (MO) has been debated as a possible trigger of deep focus earthquakes, but of equal importance its existence may constrain the amount of hydrogen being brought to Earth’s mantle transition zone (MTZ) via subduction. The rate which olivine (ol) transforms into wadsleyite and ringwoodite (rw) is dependent on hydrogen content, and determines the likelihood that metastable olivine could persist into the MTZ. Previous results indicate that 300 ppm of H_2O is too much to allow for MO [1,2]. How much is too much? We present rw rim growth rates for olivine containing as little as 75 ppm of H_2O

Ringwoodite growth rates from olivine with ~ 75 ppmw H_2O: Metastable olivine must be nearly anhydrous to exist in the mantle transition zone

It has been previously demonstrated that as little as 300 ppmw H_2O increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis. To further test this hypothesis, we present new ringwoodite growth rate measurements from olivine with ∼75 ppmw H_2O at 18 GPa and 700, 900, and 1100 °C. These growth rates are nearly identical to those from olivine with ∼300 ppmw H_2O, and significantly higher than those from nominally anhydrous olivine. We infer that transformation of olivine with 75–300 ppmw H_2O is primarily enhanced by hydrolytic weakening of reaction rims, which reduces the elastic strain-energy barrier to growth. We present a new method for fitting non-linear nominally anhydrous data, to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine. Based on previous thermokinetic modeling, these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone. Metastable persistence of olivine into the mantle transition-zone would therefore require <75 ppmw H_2O

Crystal structure of CaSiO3 perovskite at 28-62 GPa and 300 K under quasi-hydrostatic stress conditions

To find the thermodynamically stable crystal structure of CaSiO3 perovskite (CaPv) at high pressure and 300 K, we have conducted synchrotron X-ray diffraction (XRD) on thermally stress-annealed samples in a Ne pressure medium in the diamond-anvil cell at 28-62 GPa. Rietveld refinements of the diffraction patterns are significantly improved in fitting the positions and intensities of the split lines of CaPv if the starting model is a tetragonal perovskite-type structure with the SiO6 octahedral rotation around the tetragonal c-axis. The result is in contrast with other previous experiments, but is consistent with first-principles calculations, reconciling the discrepancy between computations and experiments on the crystal structure of CaPv. We attribute the observed difference to the formation of the thermodynamically more stable phase under improved stress conditions in our experiments. Our fitting shows that the bulk modulus of CaPv is 223 +/- 6 GPa when its pressure derivative fixed to 4, which is also consistent with first-principles calculations. The previous observations of the diffraction patterns of CaPv inconsistent with the first-principles studies could be due to the formation of a metastable crystal structure of CaPv under elevated deviatoric stresses

Large volume multianvil cell assembly for hydrothermal synthesis and conversions up to 6.5 GPa and 400°C

A multianvil cell assembly with octahedral edge length 25 mm has been adapted for high pressure investigations involving water-rich environments up to 6.5 GPa and 400°C. Water-rich samples are confined in Teflon containers with a volume up to 300 mm3. Applicability tests were performed between 250 and 400°C by investigating the transformation of amorphous titania particles close to the rutile–TiO2-II (∼5 GPa) phase boundary, and the transformation of amorphous silica particles close to the quartz–coesite (∼2.5 GPa) and coesite–stishovite (∼7 GPa) phase boundaries. The performed experiments employed 25.4 mm tungsten carbide anvils with a truncation edge length of 15 mm. The sample pressure at loads approaching 820 t was estimated to be around 6.5 GPa. The large volume multianvil cell is expected to have broad and varied application areas, ranging from the simulation of geofluids to hydrothermal synthesis and conversion/crystal growth in aqueous environments at gigapascal pressures

The Bridgmanite–Akimotoite–Majorite Triple Point Determined in Large Volume Press and Laser-Heated Diamond Anvil Cell

The bridgmanite&ndash;akimotoite&ndash;majorite (Bm&ndash;Ak&ndash;Mj or BAM) triple point in MgSiO3 has been measured in large-volume press (LVP; COMPRES 8/3 assembly) and laser-heated diamond anvil cell (LHDAC). For the LVP data, we calculated pressures from the calibration provided for the assembly. For the LHDAC data, we conducted in situ determination of pressure at high temperature using the Pt scale at synchrotron. The measured temperatures of the triple point are in good agreement between LVP and LHDAC at 1990&ndash;2000 K. However, the pressure for the triple point determined from the LVP is 3.9 &plusmn; 0.6 GPa lower than that from the LHDAC dataset. The BAM triple point determined through these experiments will provide an important reference point in the pressure&ndash;temperature space for future high-pressure experiments and will allow mineral physicists to compare the pressure&ndash;temperature conditions measured in these two different experimental methods