A new high-pressure phase of ZnSe with B9-type structure

High-pressure and high-temperature behavior of ZnSe was investigated by energy dispersive X-ray diffraction method up to 14 GPa and 800°C. A new high-pressure phase with B9 (HgS)-type structure is found near the B3-B1 phase boundary at room temperature, as predicted by an ab-initio calculation. The property and observed pressure region of the B9-type phase are in good agreement with the ab-initio calculation. At high-temperature condition above 300°C, only the direct transitions are observed between the B3 and B1 phases. The B3-B1 phase boundary is also determined to be P (GPa)=12.21−0.0039T (°C) for the temperature range between 300 and 800°C.     

Crystal structure of the high-pressure modification of LaNbO4

Abstract

The crystal structure of the metastable LaNbO₄ high-pressure modification (HPM) has been studied by X-ray powder diffraction. It was found that the crystal structure of HPM LaNbO₄ belongs to- the room temperature BaMCF₄ -type structure (space group Cmc2₁, a = 3,941(1)Å, b= 14,460(4) Å, c = 5,681(2) Å, Z = 4).     

Crystal structure prediction of ReN at high pressure: a new incompressible phase

Based on the swarm-intelligence-based CALYPSO method the NbO, R3m and NiAs phases for ReN are predicted. The R3m phase of ReN at high pressure is firstly found. The structural, mechanical and electronic properties of ReN with the three phases are studied systematically. Moreover, it is also firstly found that pressure stimulated ReN to undergo twice phase transitions, from NbO to R3m phase at 43.3?GPa and from R3m to NiAs phase at 53.6?GPa. The three phases of ReN are verified to be mechanically stable and a promising low-compressible material at ambient conditions. According to the electron density of states and electron localization functions we have found that their structural stability and high hardness is on account of the strong covalent bonding of Re-N and N-N.     

Apparently complex high-pressure phase of gallium as a simple modulated structure

The phase of gallium GaII, with symmetry C222(1) and 104 atoms per unit cell, has been recently reported as an example of structural complexity under high pressure. It is shown here that this phase is a simple modulated distortion of an average structure of Fddd symmetry with all atoms structurally equivalent. The modulation can be described with only 4 parameters and satisfies symmetry properties described by a centrosymmetric superspace group. The structural distortion is dominated by a frozen transversal mode associated with a single irreducible representation of Fddd, with a wave vector on the line Q, at an edge of the Brillouin zone. The average structure can be related with an hcp configuration through simple sliding of hcp layers, reminiscent of the hcp-bcc Bürgers mechanism.     

Crystal structure prediction of uranium hydrides at high pressure: A new hydrogen-rich phase

Uranium hydride is a novel hydrogen-rich system which contains 5f electrons. Uranium hydride can not only be used in the nuclear fuel industry, but also be a candidate of high superconducting-temperature materials. In this paper, we have searched the stable uranium hydride structures by using particle swarm optimization method and first-principles calculations. Besides UH₈ and UH₉, we find that UH₁₇, which contains larger hydrogen content than most hydride materials reported before, is also stable at high pressure. The atomic structures, electronic structures and phase diagram of uranium hydrides are provided, and we find that all of the discovered uranium hydrides are metals with negligible magnetic moments.     

Domain structure in a superconducting ferromagnet

The domain structure is inherent to all ferromagnets and the recent discovery of the superconducting ferromagnets raises the question of the modification of this domain structure by superconductivity. In the framework of the general London theory, applicable to both singlet and triplet superconductors, we demonstrate that superconductivity leads to a dramatic shrinkage of the domain width. The presence of this dense domain structure has to be taken into account for all magnetic measurements on superconducting ferromagnets, and the study of the domain structure evolution could provide important information on the mechanisms of superconductivity and magnetism interplay.     

On the high-pressure phase transition in

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO₄ at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a =7.306?, b =5.887? and c =5.124?. Received: 7 October 1997 / Received in final form: 17 November 1997 / Accepted: 18 November 1997     

A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti₂O₃ on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti₂O₃ was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti₂O₃ can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å³ at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å³ at ambient conditions. The Birch–Murnaghan equation of state yields K ₀=206 (3) GPa and K′₀=4 (fixed) for corundum phase, and K ₀=296 (4) GPa and K′₀=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti₂O₃ can be identified as Th₂S₃-type rather than U₂S₃-type structure. The transition from corundum-type to Th₂S₃-type structure accompanies the drastic change of the form of polyhedron: from TiO₆ octahedron in the corundum-type to TiO₇ polyhedron in the Th₂S₃-type structures.     

Calculation of the electronic band structure for the hole-metal state of iodine in high-pressure phase

It is shown by calculation of an electronic energy band structure that iodine in high-pressure phase (above 210 kbar) is a “hole-metal”, in which the conduction is due to holes. Properties of such states are discussed on the basis of the calculated band and analysis of measurements of a reflectance and a thermoelectric power recently reported.     

Correlations between structure and the superconducting transition temperature in Chevrel phase molybdenum chalcogenides

The superconducting transition temperatures for Chevrel phase materials, M_xMo₆X₈(M=Pb, Sn, rare eart …, X = S, Se, Te) show wide variability even for a single phase, e.g. PbMo₆S₈ with reported T_c's of 9.8–14.7 K. The T_c is shown to be a linear function of the hexagonal crystallographic $\text{c}\text{a}$ ratio when materials are grouped according to the formal charge on the ternary element M. This correlation is discussed in terms of the electron-phonon coupling and electronic density of states.     

Crystal structure of a new high pressure polymorph of GaS

Single crystal structure determinations on GaS by X-ray diffraction show a new high pressure phase above 3.0 GPa. The space group of this new phase is P6₃/mmc with the Ga atoms at the site 4(e) and the S atoms at the site 4(f).     

Nucleation and collapse of the superconducting phase in type-I superconducting films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis.     

Calculation of the vortex structure in a superconducting alloy

We calculate properties of an isolated vortex in a dirty BCS superconductor at zero temperature by numerical integration of a diffusion equation. This is supplemented by a self-consistency equation for the order parameter, and we show that a simple trial form gives quite a good solution of it. The electronic density of states and low temperature specific heat are also calculated for extreme type II material. The difference between the density of states at frequencies ω and 0 is proportional to ω² for small ω, and the specific heat is similar to that predicted by previous approximate theory.     

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi2B2C

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.     

Crystal field effects in superconducting tunneling

A comparison of the tunnel curves of superconducting $\text{La}$Pr, $\text{La}$Y and $\text{La}$Gd reveals that the character of the tunneling density of states of $\text{La}$Pr is between that of the BCS- and that of the Abrikosov-Gorkov theory. This is expected on theoretical grounds for pair-breaking caused by ions in a singlet ground state. The predicted crystal field induced structures in the tunnel curves could not be detected.