High-pressure structures of disilane (Si 2 H 6 ) are investigated extensively by means of first-principles density functional theory and a random structure-searching method. Three metallic structures with P-1, Pm-3m, and C2∕c symmetries are found, which are more stable than those of XY 3 -type candidates under high pressure. Enthalpy calculations suggest a remarkably wide decomposition (Si and H 2 ) pressure range below 135 GPa, above which three metallic structures are stable. Perturbative linear-response calculations for Pm-3m disilane at 275 GPa show a large electron-phonon coupling parameter λ of 1.397 and the resulting superconducting critical temperature beyond the order of 10 2 K. metallization | new phase | solid disilane I t is known that the highest superconductive critical temperature (T c ) found for a conventional superconductor is 39 K for MgB 2 (1) at ambient pressure. Cuprate superconductors have much higher critical temperatures. The cuprate superconductor discovered has a critical temperature of 93 K (2), and mercury-based cuprates have critical temperatures in excess of 130 K. Pressure causes extraordinary changes in materials and modifies their properties. This often provides a path for synthesis of novel materials. Applying BCS theory to hypothetic metallic hydrogen, Ashcroft realized that it is a conventional superconductor with a very high-T c (3). A T c of the order of 10 2 K was further proposed under very strong compression by quantitative calculations (4). This value compares favorably with those in cuprate superconductors. However, hydrogen remains insulating up to extremely high pressures, at least up to about 342 GPa (5).It was recently predicted that group IVa hydrides would also present a high superconducting critical temperature, while becoming metallic at lower pressures due to chemical precompression (6). Theoretical (7-12) and experimental (13-15) studies of silane, and theoretical studies of germane (16) and stannane (17, 18), have investigated possible metallization and superconducting phase transitions at high pressures. Indeed, the theoretical studies on germane and stannane have predicted very high T c of 64 K at 220 GPa (16) and 80 K at 120 GPa (17), respectively. These results sufficiently encouraged us to prompt studies on a wider range of hydrides to confirm the prediction (6). Disilane containing a large fraction (3∕4) of H atoms is also an important hydrogen-rich compound and leads to interesting properties under high pressure. Furthermore, it is more readily available for experimental studies because of the higher boiling and melting points than silane, germane, and stannane. However, studies on disilane are very scarce.Here, we have explored the crystal structures of disilane in a wide pressure range from 50 to 400 GPa, and three favored structures, i.e., P-1, Pm-3m, and C2∕c, are found above 135 GPa. Remarkably, the large T c of 80 K at 200 GPa for P-1 and 139 K at 275 GPa for Pm-3m are predicted by quantitative calculations. Up to now, the superconductive T c of...

With ever increasing interest in layered materials, molybdenum disulfide has been widely investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials such that high pressure studies can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E(1)2g and A1g modes. A further compression leads to a transition from the 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm(-1). This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms slided within a single layer like that of metastable 1T'-MoS2. However, unlike the non-photoluminescent 1T'-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure-induced blue shift up to 13.1 GPa. This pressure-dependent behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.

By adding small amounts of potassium to hydrogen, the energetically preferred compound KH 6 forms above 70 GPa and metalizes within 166 GPa. Two-layered C2/m and C2/c structures of KH 6 discovered here contain H 2 and H 3 units under high pressure. The metallic C2/c phase is found to be a one-dimensional conductor with a conducting hydrogen-bonded network and an insulating potassium chain. The layered hydrogen-dominant compound KH 6 is found to be a good example of pressure-induced metallization, which originates from a hydrogen sublattice at lower pressures than those in pure hydrogen system due to the "chemical precompression" from heavier element potassium.

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

The structural studies of lithium amide (LiNH2) have been performed by synchrotron X-ray diffraction measurements and ab initio density functional theoretical calculations up to 28.0 GPa. It is revealed that LiNH2 undergoes a reversible pressure-induced phase transitions from tetragonal phase (I-4) into the monoclinic phase (P21), which starts from about 10.3 GPa and completes at about 15.0 GPa. This transition is accompanied by about 11% large volume collapse, and this volume collapse is much larger than other complex ternary hydrides. The experimental pressure–volume data for the two phases of LiNH2 are fitted by third-order Birch–Murnaghan equation of state, yielding B 0 of 37.2 (1.7) GPa for the tetragonal phase and 7.6 (4.9) GPa for the monoclinic phase with the pressure derivatives at 3.5. We also have calculated the total and partial density of states of the two phases in order to explore the mechanism of the volume reduction.