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Sulfur and lanthanum hydrides under compression display superconducting states with high observed critical temperatures. It has been recently demonstrated that carbonaceous sulfur hydride displays room temperature superconductivity. However, this phenomenon has been observed only at very high pressure. Here, we theoretically search for superconductors with very high critical temperatures, but at much lower pressures. We describe two of such sodalite-type clathrate hydrides, YbH6 and LuH6. These hydrides are metastable and are predicted to superconduct with T c ∼ 145 K at 70 GPa and T c ∼ 273 K at 100 GPa, respectively. This striking result is a consequence of the strong interrelationship between the f states present at the Fermi level, structural stability, and the final T c value. For example, TmH6, with unfilled 4f orbitals, is stable at 50 GPa, but has a relatively low value of T c of 25 K. The YbH6 and LuH6 compounds, with their filled f-shells, exhibit prominent phonon “softening”, which leads to a strong electron-phonon coupling, and as a result, an increase in T c.

Few-layered gallium selenide (GaSe) is obtained by using the mechanical exfoliation method, and its properties are characterized by photoluminescence and Raman spectroscopy. The pressure-dependent phonon scatterings of bulk, few-layered, oxidized few-layered GaSe are characterized up to 30 GPa by using a diamond anvil cell with inert argon used as the pressure transmission medium. All the GaSe samples processed a phase transition around 28 GPa. A new vibration mode at 250 cm−1 is found in oxidized few-layered GaSe by Raman spectra, which is indexed as the Raman vibration mode of α-Se.

Pressure generation to a higher pressure range in a large-volume press (LVP) denotes our ability to explore more functional materials and deeper Earth’s interior. Pressure generated by normal tungsten carbide (WC) anvils in a commercial way is mostly limited to 25GPa in LVPs due to the limitation of their hardness and design of cell assemblies. We adopt three newly developed WC anvils for ultrahigh pressure generation in a Walker-type LVP with a maximum press load of 1000 ton. The hardest ZK01F WC anvils exhibit the highest efficiency of pressure generation than ZK10F and ZK20F WC anvils, which is related to their performances of plastic deformations. Pressure up to 35GPa at room temperature is achieved at a relatively low press load of 4.5MN by adopting the hardest ZK01F WC anvils with three tapering surfaces in conjunction with an optimized cell assembly, while pressure above 35GPa at 1700K is achieved at a higher press load of 7.5MN. Temperature above 2000K can be generated by our cell assemblies at pressure below 30GPa. We adopt such high-pressure and high-temperature techniques to fabricate several high-quality and well-sintered polycrystalline minerals for practical use. The present development of high-pressure techniques expands the pressure and temperature ranges in Walker-type LVPs and has wide applications in physics, materials, chemistry, and Earth science.

Silane (SiH4) is a promising hydrogen-rich compound for pursing high temperature superconducting. Previous high pressure measurements of Raman, x-ray diffraction and theoretical studies on SiH4 mainly focused on its polymorphic structures above 50 GPa, while the structure and the stability under lower pressure range are still unclear. Here we report an investigation of condensed SiH4 by Brillouin scattering measurements at high temperature up to 407 K and high pressure up to 18 GPa. Brillouin scattering frequencies of fluid SiH4 under compression are obtained under isothermal conditions of 300 K, 359 K and 407 K. The SiH4 becomes unstable with increasing temperature and subsequently decomposes into silicon and H2. We find that compression at room temperature induces two velocity anomalies corresponding to a fluid-solid state transition and a phase IV to phase V transition, respectively. Brillouin scattering spectra has been a powerful tool to investigate the fruitful phases and structure transitions in the hydrogen-rich compound under extreme conditions.

The incomplete decomposition product of metastable hydrazine (N2H4) instead of the energetically favorable ammonia (NH3) upon decompression is one drawback in applications of energetic material oligomeric hydronitrogens. We explore the stability of hydrazine molecules in hydrazine hydrate (N2H4·H2O) under pressure in diamond anvil cells (DACs) combined with in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD) measurements. The results show that one NH2 branch forms NH3 group by hydrogen bonds between hydrazine and water molecules after the sample crystallizes at 3.2 GPa. The strengthening hydrogen bonds cause the torsion of hydrazine molecules and further dominate a phase transition at 7.2 GPa. Surprisingly, the NN single bonds are strengthened with increasing pressure, which keeps the hydrazine molecules stable up to the ultimate pressure of 36 GPa. Furthermore, the main diffraction patterns show continuous shift to higher degrees in the whole pressure range while some weak lines disappear above 8.2 GPa. The present peak-indexing results of the diffraction patterns with Materials Studio show that the phase transition occurs in the same monoclinic crystal system. Upon decompression, all of the hydrazine molecules extract from hydrazine hydrate crystal at 2.3 GPa, which may provide a new way to purify hydrazine from hydrate.

Based on density functional theory calculations, it is found that for substitutional N in diamond the C 3v symmetry structure is more stable, while C 3v and D 2d symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant (the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.

We choose nano-Pt in hydrogen environment to explore the size effect on the formation of metal hydrides. At 30 GPa, a phase transition in the metal lattice from the cubic to hexagonal phase is observed characterized by a drastically increased volume per metal atom, indicating the formation of PtH-P63 /mmc. We find that nano-Pt could form PtH at a lower pressure than the bulk Pt due to its high specific surface and structure defects. The present work provides the possible route to new metal hydrides under mild conditions.

Simple molecular solids have been an important subject in condensed matter physics, particularly for research of pressure-induced molecular dissociation. We re-explore the structural changes of element bromine through pressure-induced decomposition of solid HBr. The phase changes in HBr are investigated by Raman spectroscopy and synchrotron x-ray diffraction up to 125 GPa at room temperature. By applying pressure, HBr decomposes into solid bromine in the pressure range of 18.7–38 GPa. The solid bromine changes from molecular phase to incommensurate phase at 81 GPa, and finally to monatomic phase at 91 GPa. During the process of pressure-induced molecular dissociation, the intermediate incommensurate phase of element bromine is confirmed for the first time from the x-ray diffraction studies. The decomposition of HBr is irreversible since HBr cannot form again upon pressure decompression.

Raman spectra of layered ferroelectrics Ba2+xBi4-xTi5-xNbxO18 (0 ≤ x ≤ 2) at room temperature in the range 20-1000 cm-1 were investigated. Vibration modes were assigned with the use of internal mode approach. Two doping-induced structure phase transitions were observed at about x = 0.50 and x = 1.25, respectively. Raman spectra variations were analyzed according to the selection rules of BO6 octahedra in various symmetry. Polycrystalline samples were synthesized by solid state reaction method.

Within a movable and orientable rigid rotor model, four interaction potentials of solid deuterium (hydrogen) system from combinations of two isotropic and two anistropic parts are tested by the path integral Monte Carlo method in a constant-pressure ensemble. With the best one of the potentials, the equation of state of deuterium is quantitatively in agreement with the experimental data. For understanding the quantum nature of the solid, the energies of the system at different temperatures are also calculated, the results show strong zero-point motions exist in the solid. As would be expected, the quantum corrections to both the rotational and the translational kinetic energies decrease with increasing temperature, and increase with pressure.

Using density-functional-theory calculations, a monoclinic metallic post-ζ phase (space group C2/c) is predicted at 215 GPa. The calculated phonon dispersion curves suggest that this structure is stable at least up to 310 GPa. Oxygen remains a molecular crystal and there is no dissociation in the related pressure range. Moreover, it is found that the phase transition from ζ to post-ζ phase is attributed to phonon softening. The significant change in the optical properties can be used to identify the phase transition.