Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Structural instability of PbSe/SnSe superlattices under pressure

Abstract

PbSe/SnSe superlattice, phase transition, high pressure, SR x-ray diffraction)

Synchrotron x-ray diffraction experiments have revealed successive phase transitions in epitaxially-grown PbSe/SnSe superlattices. The transition pressures from the low-pressre cubic B1- to the high-pressure orthorhombic B16-type structures are observed to vary systematically depending upon thickness of the PbSe layer. For example, a [PbSe(36A)/SnSe(12A)]₁₉, with the B1 structure in both layers stabilized in its asgrown state, undergoes the [B1/B1]-to-[B1/B16] and [B1/B16]-to-[B16/B16] structural transitions at 1.9 and 3.8GPa, respectively. This result is in contrast to their bulk data that the B1-to-B16 transition takes place at 5.3GPa in PbSe while the B16 phase is stable in SnSe at atmospheric pressure.     

Anisotropy of Linear Thermal Expansion and Compressibility of Y 2 Fe 17 Under Pressure and its Correlation to Magnetic Structure

A complex temperature dependence of a.c. susceptibility of Y 2 Fe 17 under high pressures together with recent neutron diffraction studies under pressure proved the instability of the collinear ferromagnetic structure and the development of a non-collinear magnetic arrangement of Fe moments in Y 2 Fe 17 . To study the correlation between magnetic structure and volume in more detail we performed compressibility and linear thermal expansion studies under high pressures up to 100 kbar and 10 kbar, respectively. The compressibility in the paramagnetic state s P (above 10 kbar at room temperature) was determined from the Murnaghan equation of state using the X-ray data, \kappa_{P}=0.80\ {\rm Mbar}^{-1} . The linear thermal expansion and compressibility in the ferromagnetic state at low temperatures are highly anisotropic. As a consequence, the c/a ratio decreases with increasing pressure. The magnetic phase diagram of Y 2 Fe 17 compound was compiled up to 20 kbar.     

Theoretical investigation on structures,stability and properties of [P,X, Y] (X=C,Si; Y = O,S) isomers

The structures, energetics and stability of the [P, X, Y] (X?=?C, Si; Y?=?O, S) radicals are explored by means of the density functional theory and ab initio levels. Seventeen [P, X, Y] isomers and 14 interconversion transition states are obtained at the B3LYP/6-311G(d) level. At the CCSD(T)/6-311?+?G(2df)//QCISD/6-311G(d)?+?ZPVE level, the lowest-lying isomers are the linear PCO 1a (0.0?kcal/mol), PCS 1b (0.0) and the three-membered ring cPSiO 1c (0.0), cPSiS 1d (0.0) on their respective potential energy surfaces. These four isomers exhibit considerably not only thermodynamic but also kinetic stabilities. Additionally, the cyclic cPCS 2b (32.8) and linear PSiS 2d (18.6) possess also high kinetic stability. All of six isomers 1a, 1b, 2b, 1c, 1d and 2d are considerably stabilized by a barrier of at least 20?kcal/mol, and may be detected in the laboratory or interstellar space. Their valence bond structures and possible formation strategies in the laboratory and space are discussed in detail. Finally, the similarities and discrepancies on structures and stabilities between [P, X, Y] (X?=?C, Si; Y?=?O, S) isomers are compared. These predicted results are highly expected to be informative for the future identification of [P, X, Y] (X?=?C, Si; Y?=?O, S) in the laboratory and space.     

Volume dependence of the raman frequencies in Y Ba2Cu3Ox single crystals with different oxygen content

Abstract

The Raman spectra of Y B _a2C_u3O_x, single crystals (x=6.25; 6.75; 7.0) were measured at pressures up to 22 GPa (35 GPa for x=6.25) at room temperature in nearly hydrostatic conditions. The frequency-volume curves for most of the Raman-active fundamental vibrations were derived from the present data making use of a previous high-pressure study of the equations of state of Y Ba₂ Cu ₃ O _x compounds.     

Pressure induced phase transformations in alkali metal hydrides

Abstract

A three body potential approach(Singh and Singh, Phys. Rev. B39,761(1989)) has been described and applied to investigate the pressure induced phase transformation from B1 (NaCl-Structure) to B2 (CsCl-Structure) phases for the partially ionic alkali-metal hydrides (LiH, NaH and KH). Our calculated values of the phase transition pressures and the associated volume collapses for NaH and KH are in reasonably good agreement with their experimental data. The variations of the relative volumes (V(P)/V(O)) with pressure for both B1 and B2 phases, have shown no phase transition upto 140 GPa in LiH. This feature is in agreement with the experimental observations.     

First-Principles Calculations of Elastic Properties of Cubic Ni2MnGa

Dependence of bulk modulus on both pressure and temperature, the elastic constants Cij and the pressure and temperature dependence of normalized volume V/Vo of cubic Ni2MnGa alloy are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 800 K and obtain the relationships between bulk modulus B and pressures at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the temperature dependences of the Debye temperature are also analysed. The calculated results are in agreement with the available experimental data and the previous theoreticM results.     

Structural phase transition (zincblende-rocksalt) and elastic properties in AlY (Y=N, P and As) compounds: Pressure-induced effects

The present paper addresses the pressure-induced structural aspects of ZnS-type (B3) to NaCl-type (B1) structure in AlY (Y=N, P, As). An effective-interionic interaction potential (EIoIP) with long-range Coulomb and three-body interactions and the Hafemeister-and-Flygare-type short-range overlap repulsion extended up to the second-neighbour ions and the van der Waals (vdW) interaction is developed. Emphasis has been given on evaluating the vdW coefficients by the Slater-Kirkwood variational method, as both the ions are polarizable. The lattice model calculations have revealed reasonably good agreement with the available experimental data on the phase-transition pressures (P_t=16, 14, 7.5 GPa) and the elastic properties of AlY (Y=N, P, As). The equation of state curves (plotted between V(P)/V(0) and pressure) for both the B3 and B1 structures obtained are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also close to their observed data. Further, the variations of the second-order elastic constants with pressure follow a systematic trend that is almost identical to that exhibited by the observed data measured for other semiconducting compounds with B3→B1 structural phase transitions.     

Ultrahigh pressure equation of state of CaS to 1.5 Mbar

Abstract

Energy Dispersive X-ray Diffraction (EDXD) was performed at room temperature to gather structural data on CaS between approximately 1.7 GPa to nearly 150GPa. In these experiments, CaS retained the B1 structure up to approximately 40 GPa above which it began to transform to the B2 structure. The B2 structure remained stable to the highest pressure reached, 149 GPa, where the relative volume V/V₀ was 0.490. Previous studies on CaS extended only up to 52 GPa, which is barely 10 GPa after the B1 phase changes to the B2 structure. Thus it was not possible to accurately extrapolate the equation of state (EOS) for the B2 phase region to significantly higher pressures. In the present study EOS data for CaS was collected to 150 GPa and no other structural change was observed. EOS parameters for the B1 and B2 phase regions agree well with values reported in the literature.     

Equation of State and Crystal Structure of NaAlSiO 4 with Calcium-Ferrite Type Structure in the Conditions of the Lower Mantle

The full profile refinement of the structure of the calcium-ferrite type NaAlSiO 4 was carried out at pressures up to 40 GPa. This high-pressure modification of NaAlSiO 4 is known to be stable at least to 75 GPa and 2450 K that corresponds to a middle part of the Earth's lower mantle. A zero-pressure unit cell volume V 0 =36.58(2) cm 3 /mol was obtained from a sample synthesized from natural albite. In situ pressure-volume data (with found V 0 ) was fitted to the Birch-Murnaghan equation of state, and it yielded the bulk modulus equal to 220(1) GPa and its pressure derivative equal to 4.1(1).     

Response to ‘Comment on a recent conjectured solution of the three-dimensional Ising model’

This is a Response to a recent Comment [F.Y. Wu, B.M. McCoy, M.E. Fisher et al., Phil. Mag. 88 (2008)] on the conjectured solution of the three-dimensional (3D) Ising model [Z.D. Zhang, Phil. Mag. 87 5309 (2007)]. Several points are made: (1) Conjecture 1, regarding the additional rotation, is understood as performing a transformation for smoothing all the crossings of the knots. (2) The weight factors in Conjecture 2 are interpreted as a novel topologic phase. (3) The conjectured solution and its low- and high-temperature expansions are supported by the mathematical theorems for the analytical behavior of the Ising model. The physics behind the extra dimension is also discussed briefly.     

The determination of the lifetime of hot electrons in metals by time-resolved two-photon photoemission: the role of transport effects, virtual states, and transient excitons

Experience has shown that theoretically determined lifetimes of bulk states of hot electrons in real metals agree quantitatively with the experimental ones, if theory fully takes into account the crystal structure and many-body effects of the investigated metal, i.e., if the Dyson equation is solved at the ab initio level and the effective electron–electron interaction is determined beyond the plasmon-pole approximation. Therefore the hitherto invoked transport effect [Knoesel et al.: Phys. Rev. B 57, 12812 (1998)] does not seem to exist. In this paper we show that likewise neither virtual states [Hertel: et al. Phys. Rev. Lett. 76, 535 (1996)] nor damped band-gap states [Ogawa: et al.: Phys. Rev. B 55, 10869 (1997)] exist, but that the hitherto unexplained d-band catastrophe in Cu [Cu(111), Cu(110)] can be naturally resolved by the concept of the transient exciton. This is a new quasiparticle in metals, which owes its existence to the dynamical character of dielectric screening at the microscopic level. This means that excitons, though they do not exist under stationary conditions, can be observed under ultrafast experimental conditions. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Monte Carlo simulations of primitive models for ionic systems using the Wolf method

Thermodynamic and structural properties of primitive models for electrolyte solutions and molten salts were studied using NVT and NPT Monte Carlo simulations. The Coulombic interactions were simulated using the Wolf method [D. Wolf, Phys. Rev. Lett. 68, 3315 (1992); D. Wolf, P. Keblinnski, S. R. Phillpot, and J. Eggebrecht, J. Chem. Phys. 110, 8254 (1999)]. Results for 1?:?1 and 2?:?1 charge ratio electroneutral systems are presented, using the restricted and non-restricted primitive models, as well as a soft PM pair potential for a monovalent salt [J.-P. Hansen and I. R. McDonald, Phys. Rev. A 11, 2111 (1975)] that has also been used to model 2?:?12 and 1?:?20 asymmetric colloidal systems, with size ratios 1?:?10 and 2?:?15, respectively [B. Hribar, Y. V. Kalyuzhnyi, and V. Vlachy, Molec. Phys. 87, 1317 (1996)]. We present the predictions obtained for these systems using the Wolf method. Our results are in very good agreement with simulation data obtained with the Ewald sum method as well as with integral-equation theories results. We discuss the relevance of the Wolf method in the context of variable-ranged potentials in molecular thermodynamic theories for complex fluids.     

Pressure Induced Disordering of Vacancies in RNi 2 Compounds

As known from earlier investigations, most of the RNi 2 compounds (R…rare earth element) do not crystallize in the cubic Laves phase structure (C15), but in a superstructure of C15 with ordered vacancies at the R sites. In the present work high pressure X-ray diffraction experiments on selected RNi 2 compounds (R=Tb, Sm, Gd and Y) are presented, showing that pressures of about 8-15 GPa lead to a disordering of the vacancies in all of them. Furthermore the studies show that the pressure-induced order-disorder transition is closely related to the earlier observed disordering of the vacancies at high temperatures between 430 K (TbNi 2 ) and 740 K (YNi 2 ), and that there is a pronounced variation of the transition pressures and temperatures among the investigated compounds.     

Electrical properties of modified TZP(+TiO2,FeOx)

1 to 24 mol% TiO₂ and FeO_X were added to 3.2 mol% Y₂O₃ doped ZrO₂ (Z3Y) to obtain tetragonal zirconia polycrystals (TZP) with modified electronic properties. The materials were prepared by coprecipitation which allows to obtain fine, homogeneous and sinteractive powders. The solubility of TiO₂ in Z3Y can reach 24 mol%, while the maximum solubility of FeO_x is about 8 mol%. The impedance results show a decrease of the bulk and total conductivity of titania doped TZP with increasing titania concentration, while those of FeO_x doped TZP show only minor changes. The Hebb-Wagner polarization method was applied to evaluate the partial hole and electron conductivities. Three models are presented to interpret the polarization curves. 1.6 mol% FeO_x doped TZP has higher hole conductivity in air, while titania doped TZP has higher electronic conductivity at low oxygen partial pressures as compared to pure Z3Y. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept.1994     

High pressure studies up to 50GPa of CuO

Abstract

Copper oxide has been studied at high pressure up to 50 GPa. A monoclinic structure was compatible with the measurements at all pressures, and no phase change was observed. A bulk modulus, B₀, = 98 GPa, and its pressure derivative B′₀ = 5.6 was obtained.     

The bandstructure of Pd(110) above the fermi level

The band-structure of Pd(110) was studied by k-resolved inverse photoemission (IPE). The data are interpreted on the basis of a relativistic band-structure calculation. Apart from direct bulk band transitions, weak surface umklapp and surface emission features are observed. Four different surface state bands could be detected in the gaps at X and Y. An anomalous temperature behaviour is found for a transition into the d-bands close to the Fermi level. Finally, structures resulting from LEED reflections are identified in the IPE spectra and their characteristics are discussed.     

Structure-toxicity relationships of thiono and seleno phosphoramidate compounds: new type of acetylcholinesterase inhibitors

Novel thiono and seleno phosphoramidate compounds with the general formula (X)(Y)P(C₆H₅)₂; (X = NMe₂ & Y = S, 1a; X = NEt₂ & Y = S, 2a; X = NMe(CH₂Ph) & Y = S, 3a; X = NH(CH₂Ph) & Y = S, 4a; X = NEt(CH₂Ph) & Y = S, 5a; X = N(C(Me)₃) (CH₂Ph) & Y = S, 6a; X = N(CH₂Ph)₂ & Y = S, 7a; X = NMe₂ & Y = Se, 1b; X = NEt₂ & Y = Se, 2b; X = NMe(CH₂Ph) & Y = Se, 3b; X = NH (CH₂Ph) & Y = Se, 4b; X = NEt(CH₂Ph) & Y = Se, 5b; X = N(C(Me)₃)(CH₂Ph) & Y = Se, 6b and X = N(CH₂Ph)₂ & Y = Se, 7b) were prepared and characterized by ¹H, ³¹P and ¹³C NMR and IR spectroscopy and elemental analysis. ³¹P chemical shift of thiono and seleno derivatives didn’t show significant different because of their little difference in electronegativity sulfur and selenium. Hydrophobic parameter of compounds was determined by measurement of octanol-water partition coefficient by shake-flask technique. Determination of human erythrocyte acetylcholinesterase (hAChE) activity was carried out according to the Ellman’s modified kinetic method. IC₅₀ values of the selected thiono and seleno compounds varied from 3.4 to 0.11 and 9.9 to 5.1 mM, respectively. The seleno compounds show lower affinities for hAChE relative to the thino compounds. These results demonstrate that hydrophobic and electronic factors of the organophosphorus compounds play a key role on the inhibitory potency.     

Determination of the Mode Grüneisen Parameter of AlN using different Fits on Experimental High Pressure Data

To investigate the pressure dependence of the AlN phonon frequencies Raman spectra of single-crystalline bulk AlN under hydrostatic pressure up to 10 GPa were recorded. The Raman peak positions of the A 1 (TO), E 1 (TO), E 2 (high), A 1 (LO) and quasi-longitudinal optical (QLO) phonons were plotted as a function of pressure. The experimental data was fitted using the traditional parabolic fit (M. Kuball et al . (2001) Appl. Phys. Lett., 78, 724 [1]) and fits to physical models, density, volume, etc. The mode Grüneisen parameters of the different phonons were determined for each fit and significant differences are found between the various fits. Results are compared with recent theoretical calculations (J.-M. Wagner et al . (2000) Phys. Rev. B, 62, 4526 [2]).     

Cotunnite-Structured Titanium Dioxide and the Hardest known Oxide

Combined theoretical and experimental investigations led to the discovery of a new polymorph of titanium dioxide with titanium nine-coordinated to oxygen in the cotunnite (PbCl 2 ) structure. Hardness measurements on the cotunnite-structured TiO 2 synthesized at pressures above 60 GPa and temperatures above 1000 K reveal that this material is the hardest oxide yet discovered. Furthermore, it is one of the least compressible (with a measured bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa) polycrystalline materials studied thus far.