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Monoclinic α-MoP2, with the OsGe2-type structure (space group C2/m, Z = 4) and lattice parameters a = 8.7248(11) Å, b = 3.2322(4) Å, c = 7.4724(9)Å, and β = 119.263°, was synthesized under a pressure of 4~GPa at a temperature between 1100 °C and 1200 °C. The structure of α-MoP2 and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic β-MoP2 (space group Cmc21) is denser in structure than α-MoP2. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from β-MoP2 to α-MoP2, suggesting that α-MoP2 is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.

Orthorhombic (space group: Pnma) Nb2P5 is a high-pressure phase that is quenchable to ambient pressure, which could viewed as the zigzag infinite P chain-inserted NbP2. We report herein the high-pressure crystal growth of Nb2P5 and the discovery of its superconducting transition at Tc ~ 2.6 K. The electrical resistivity, magnetization, and specific heat capacity measurements on the high-quality crystal unveiled a conventional type-II weakly coupled s-wave nature of the superconductivity, with the upper critical field Hc2(0) ~ 0.5 T, the electron-phonon coupling strength {\lambda}ep ~ 0.5 - 0.8, and the Ginzburg-Landau parameter \k{appa} ~ 100. The ab initio calculations on the electronic band structure unveiled nodal-line structures protected by different symmetries. The one caused by band inversion, for example, on the {\Gamma}-X and U-R paths of the Brillouin zone, likely could bring nontrivial topology and hence possible nontrivial surface state on the surface. The surface states on the (100), (010) and (110) surfaces were also calculated and discussed. The discovery of the phosphorus-rich Nb2P5 superconductor would be instructive for the design of more metal phosphides superconductors which might host unconventional superconductivity or potential technical applications., Comment: 8 pages,8 figures in the main text + 4 pages, 3 figures, 1 table in the supplementary information

We observed an isostructural phase transition in the solid nitrogen λ-N2 at approximately 50 GPa accompanied by anomalies in lattice parameters, atomic volume and Raman vibron modes. The anomalies are ascribed to a slight reorientation of the nitrogen molecules, which does not seem to affect the monoclinic symmetry (space group P21/c). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the λ-N2 phase. In addition, a new high-pressure amorphous phase of η′-N2 was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of λ-N2. Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals (H2, O2, N2, etc).

The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report results of synchrotron X-ray diffraction (SXRD) and electrical transport measurements on TaAs2 under pressure up to ~ 37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could be related to the nearly stable electronic structure unveiled by the ab initio calculations. The discovery would expand the potential use of XMR even under high pressure., Comment: 15 pages, 7 figures including those in supporting information

We present a systematic experimental study on the phase transition, lattice microstrain, and order–disorder of cations for garnets in the majorite–pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson–Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg2+ + Si4+ = 2Al3+) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain.

Pressure effect on the electrical conductivity of San Carlos olivine was investigated by the newly installed electrical conductivity measurement system at China University of Geosciences. Electrical conductivity of San Carlos olivine aggregates was measured up to 12 GPa and 1475 K using the Walker-type multi-anvil apparatus equipped with eight WC cubes as the second-stage anvils. The pressure generation against applied load for the experimental assemblage was examined by phase transition of Bi, quartz, forsterite under different P-T conditions. To check the data validity of this new system, electrical conductivities of the serpentinites and talc samples were measured. The results are consistent with the published data of the same samples. Electrical conductivity (σ) of the San Carlos olivine aggregates and temperature (T) satisfy the Arrhenian formula: σ=σ 0exp[−(ΔE+PΔV)/kT]. The pre-exponential factor (σ 0), activation energy (ΔE) and activation volume (ΔV) yield value of 7.74 S/m, 0.85 eV and 0.94 cm3/mol, respectively. Electrical conductivities of the San Carlos olivine aggregates decline with increasing pressure at same temperatures. The negative pressure effect can be interpreted by strain energy model of defect energy together with the lattice deformation. In addition, the electrical conductivity-depth 1-D profile of the upper mantle was constructed based on our results and some assumptions. The calculated profile is concordant with the geophysical observation at the depth of 180–350 km beneath Europe, which indicates that the upper mantle beneath Europe might be dry.

Elastic wave velocities of synthetic Fe 3 Al 2 Si 3 O 12 almandine have been determined at simultaneous high pressure and temperature up to 19 GPa and 1700 K by the ultrasonic technique in conjunction with in situ synchrotron X-ray diffraction in a multi-anvil apparatus. Velocities of almandine are found substantially lower than those of other major end-member garnets such as pyrope, grossular, and MgSiO 3 majorite, while their pressure and temperature derivatives are comparable to those of the latter garnets. The observed density, and compressional ( V P ) and shear ( V S ) velocities were combined and fitted to functions of the Eulerian strain EoS, yielding a adiabatic bulk modulus K S 0 = 174.2 (12) GPa and a shear modulus G 0 = 94.9 (7) GPa, and their pressure and temperature derivatives ∂K S /∂P = 4.61 (14), ∂G / ∂P = 1.06 (6), ∂K S /∂T = −2.67 (7) × 10 −2 GPa K −1 , and ∂G/∂T = −1.31 (8) × 10 −2 GPa K −1 . The pressure derivative of the bulk modulus of almandine is similar to those of other garnet end-members, which is in contrast to the substantially higher value ( ∂K S / ∂P = 6.2 (5)) reported for pure almandine in an earlier study based on experiments up to 3 GPa. The present new results combined with those of pyrope, grossular, and MgSiO 3 majorite are successfully used to reproduce the sound velocities of majoritic garnet in the pyrolite composition.

The presence of magnesium silicate perovskite coexisting with ringwoodite and a very small amount of calcium silicate perovskite in a harzburgite composition was observed at pressures of 22.5–23.5 GPa and a fixed temperature of 1600 °C. The silicate perovskites are products of pressure-induced phase transformation in non-olivine component in the harzburgite. The postspinel transition occurs at 24 GPa in the olivine component and the harzburgite consists of the silicate perovskites and magnesiowustite above 24 GPa. The possible presence of magnesium silicate perovskite in harzburgite at lowermost mantle transition zone may contribute to high velocity anomalies in stagnant slabs.

Elastic wave velocities of MgSiO3 akimotoite polycrystalline samples have been measured at pressures up to 25.7 GPa and temperatures to 1500 K by a combination of in situ X-ray diffraction and ultrasonic interferometry techniques in a large volume Kawai-type multianvil apparatus (KMA). The elastic moduli of akimotoite and their pressure and temperature dependences are determined by a 2D linear fitting analysis of the present data, yielding: KS = 219.4(7) GPa, ∂KS/∂P = 4.62(3), ∂KS/∂T = −0.0228(4) GPa/K, G0 = 132.1(7) GPa, ∂G/∂P = 1.63(4), ∂G/∂T = −0.0225(4) GPa/K. The bulk and shear moduli at ambient conditions are generally consistent with the result of a previous Brillouin study. However, significant nonlinear behaviors of the elastic moduli were observed at higher temperatures, indicating that the velocities derived from the linear fitting analysis are overestimated for the actual mantle conditions. Using the present new experimental data, we compared the elastic velocities of various high-pressure forms of MgSiO3 under the mantle conditions. The results demonstrate a large velocity difference between akimotoite and perovskite, which may be relevant to the complex seismic structures near the bottom of the mantle transition zone.

High-pressure polymorphs of olivine (wadsleyite and ringwoodite) are major minerals in the mantle transition zone (MTZ). Phase transformations in olivine are important for a series of geodynamic problems such as the mineralogical and evolutionary history of the mantle, mantle convection patterns, and deep focus earthquakes in subduction zones. In this study, we examine phase transformations in olivine with two compositions, namely Mg2SiO4 (Fo100) and (Mg0.9Fe0.1)2SiO4 (Fo90), at pressures between 14.1 and 20 GPa and a constant temperature of 1400°C, using the newly installed multi-anvil system at the Laboratory for Studies of the Earth’s Deep Interior (SEDI), China University of Geosciences (Wuhan). At 14.1 GPa, Fo90 transformed completely into the wadsleyite structure (β), while Fo100 remained as olivine (α). Between 14.8 and 15.6 GPa, both Fo100 and Fo90 transformed into the wadsleyite structure. Wadsleyite crystals were identified by two characteristic Raman peaks between 722 and 723 and 917 and 919 cm−1. They exhibit a bimodal grain size distribution: large-crystals with average grain sizes greater than 100 μm and microcrystals less than 10 μm. The population of microcrystals increased with pressure, apparently due to the increase in over-pressure (the difference between the experimental pressure condition and the equilibrium transformation pressure at 1400°C), which promotes nucleation and retards grain growth. All run charges contained large numbers of wadsleyite microcrystals, because of the low activation energy of the nucleation process. The experimentally observed microstructure may shed light on the morphology of wadsleyite observed in shocked meteorites. At 19.5 GPa, wadsleyite coexisted with ringwoodite (γ) in Fo100, but was absent in Fo90. At 20 GPa, both samples transformed completely into ringwoodite, which was characterized by the 798 and 840 cm−1 Raman lines. Ringwoodite crystals are euhedral grains (average grain size 10–20 μm), with well-developed triple junctions. The complex upper mantle structure in eastern China determined from seismological studies cannot be explained by the simple transformation sequence of the olivine system alone. Phase transformations in other pyroxene-normative components (including pyroxenes and garnets) and the interaction of these components with olivine may be responsible for the complex structure. High-pressure and high-temperature experimental studies on complex systems (e.g. olivine-pyroxene), combined with data from geophysical exploration, may help in establishing a more realistic geological-petrological model for eastern China and further our understanding of the possible physical mechanisms that are responsible for the complex structure. Such studies will have profound implications for understanding the dynamic processes in the deep Earth interior.