Pressure Dependence Of Crystal Structure Research Articles

Superconductivity in topological insulator β-PdBi2 under pressure

The topological insulator PdBi2 exhibits two different crystal phases at ambient pressure, i.e., ‘α-PdBi2’ and ‘ -PdBi2’. The pressure dependence of crystal structure and superconductivity of α-PdBi2 has been fully elucidated thus far. However, the physical properties of β-PdBi2 crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of β-PdBi2 under pressure based on synchrotron x-ray diffraction (XRD) patterns. The temperature dependence of β-PdBi2 indicates its superconductivity with a superconducting transition temperature (T c) as high as 4.10 K, and its crystal structure is tetragonal [space group of I4/mmm (no. 139)]. The XRD patterns at 0–22.0 GPa indicate no structural phase transitions, and the unit cell volume shrinks monotonically with pressure, unlike the behavior of α-PdBi2. Furthermore, α-PdBi2 transformed to β-PdBi2 under pressure. This suggests that β-PdBi2 is stable under pressure. The superconductivity is clearly observed at 0–11.8 GPa, and the value of T c is almost constant at ∼4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicates that the superconductivity is not attributed to a simple s-wave dirty limit but an s-wave clean or p-wave polar model. This is the first systematic study of superconductivity of topological insulator β-PdBi2 under pressure.

Superconducting behavior of a new metal iridate compound, SrIr2, under pressure

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, Tc, as high as 6.6 K at ambient pressure, in order to complete the Tc—pressure (p ) phase diagram. Very recently, this sample’s superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample’s superconductivity in a wide pressure range. The Tc value decreased with an increase in pressure, but the onset superconducting transition temperature, , increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R − T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

Pressure dependence of crystal structure of Cu2O by TOF powder neutron diffraction

Abstract Neutron diffraction measurements were performed on powder Cu 2 O with different samples that are pressure released after a pressing force of 295 K with iMATERIA (IBARAKI Materials Designed Diffractometer) installed at MLF in J-PARC. Peak widths of Bragg lines increased depending on the pressing force. To clarify the relation between the crystallite size and the microstrain effect, Rietveld refinement analysis on a TOF neutron diffraction pattern of Cu 2 O was performed by assuming a cubic cuprite type structure (space group Pn-3m ). Strain and crystallite size effects were estimated from Lorentzian terms in the Pseudo-Voigt profile function with a pair of Back-to-Back exponential types based on the Stephenes model. It is considered that the increasing peak width according to the dependence of pressing force is mainly derived from strain effects. And this result is consistent with the previous studies of Ag 2 O where the Williamson–Hall plot method by X-ray diffraction was applied.

Pressure dependence of crystal structure and molecular packing in anthracene

Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated values.

Theoretical elastic stiffness, structure stability and thermal conductivity of La 2Zr 2O 7 pyrochlore

Elastic stiffness and electronic structure of La 2Zr 2O 7 were calculated by means of the first-principles pseudopotential total energy method. The equation of state (EOS), elastic parameters (including the full set of second-order elastic coefficients, bulk modulus and Young’s modulus) and elastic anisotropy were reported. Furthermore, pressure dependence of crystal structure, electronic structure, and bond strengths were investigated. It is found that, although the La 2Zr 2O 7 lattice is stable at high pressures, its electronic structure and atomic bonding are definitely disturbed by the applied pressure. The crystal structure of La 2Zr 2O 7 approaches that of the fluorite-type lattice at high pressures. The strengths of different interatomic bonds in La 2Zr 2O 7 are examined by considering bond-length contractions at various pressures. In addition, the results based on quantum-mechanical-scale calculation clarify the nature of low thermal conductivity of La 2Zr 2O 7 at elevated temperatures.

Pressure study of structural properties and phonons in YBa{sub 2}Cu{sub 3}O{sub 7}

The pressure dependence of crystal structure and vibrational properties of YBa{sub 2}Cu{sub 3}O{sub 7} is investigated by first-principles calculations based on density functional theory. They have determined the optimized geometry for different unit cell volumes by relaxing the atoms to their force-free positions. The tendency of altered bond strength with pressure is consistent with experimental investigations and previous c-axis strain calculations. The pressure effect on the three A{sub 1g} phonon frequencies dominated by oxygen vibrations is in very good agreement with experimental investigations showing constant linear pressure coefficients d{omega}/dp over a wide range of pressure values.

Pressure-induced phase transition and pressure dependence of crystal structure in low (α) and Ca/Al-doped cristobalite

The phase stability and atomic-level compression mechanisms for both SiO2 cristobalite, and for cristobalite partially stabilized by Ca/Al doping (Cax/2 Si2−xAlxO4), have been investigated. A phase transition to a lower symmetry phase, observed with in situ high-pressure energy-dispersive x-ray diffraction, occurs at about 1.2 GPa. Structure models of the low-pressure phase were obtained by Rietveld analysis of neutron powder-diffraction data from powdered samples contained in a gas pressure apparatus. These data were collected at pressures up to 0.6 GPa and at 298 and 60 K. The results suggest collapse of the corner-connected framework from rotations of the rigid SiO4 tetrahedra at high pressures and low temperatures as the dominant mechanism for the densification of both materials. Compared to pure SiO2 cristobalite at the same pressure and temperature, the Ca/Al-doped material has a larger unit-cell volume. It also has a larger Si-O-Si bending angle and a more expanded framework as evidenced by the smaller rotations of the rigid SiO4 tetrahedra. The rate of change of these parameters as a function of pressure and temperature is the same for both pure and Ca/Al-doped cristobalite. These observations are consistent with Ca occupying positions within the cavities formed by the (Si, Al)-O framework and bracing it against collapse.

Pressure dependence of crystal structures in the system MgOAl 2O 3SiO 2H 2O at pressures up to 30 kilobars

The stability relations, as determined by experiment, of the following quaternary and anhydrous ternary phases of the system MgOAl 2O 3SiO 2H 2O are reviewed in the light of their crystal structures: Cordierite, gedrite, aluminous enstatite, boron-free kornerupine, sapphirine, yoderite, Mg-staurolite, pyrope, aluminous talc, and chlorite. With the exception of the last two phases, there is a consistent relationship between density, oxygen packing and coordination of Al on one side, and stability with respect to pressure on the other side: Densely packed phases with only octahedral Al like pyrope and Mg-staurolite are only stable at high pressures, whereas loosely packed cordierite with only tetrahedral Al is only stable at low pressures. The phases gedrite, aluminous enstatite, boron-free kornerupine, sapphirine, and yoderite exhibiting intermediate degrees of packing and (except yoderite) both tetrahedral and octahedral Al are stable at intermediate pressures. The phyllosilicates talc and chlorite are pressure-indifferent within the range considered, although they have low densities and contain Al both in tetrahedral and octahedral coordination.