Structural and thermodynamic properties of MgB2 from first-principles calculations

A first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of local density approximation is performed to calculate the lattice parameters and the equation of states (EOS) of superconducting MgB₂. Our calculations show that the ratio c/a of about 1.134 is the most stable structure for MgB₂, as is consistent with experiment and other theoretical results. Also, the isothermal and isobaric properties are discussed from energy-volume curves using a quasi-harmonic Debey model.     

Isothermal equation of state for the skutterudites CoSb3 and LaFe3CoSb12

The thermoelectric materials CoSb₃ and LaFe₃CoSb₁₂ with skutterudite structure were subjected to high pressures using a diamond anvil high-pressure cell up to 20 GPa. Energy-dispersive X-ray diffraction was used to determine the dependence of the lattice parameter on pressure. No major change in the X-ray diffraction spectra was observed for both compounds, constituting evidence that both compounds are stable within this pressure range, despite their relatively open structures. Three distinct isothermal equations of state for solids under high pressure were fitted to the experimental data to determine the bulk modulus for both compounds. The filled skutterudite showed a greater compressibility than the unfilled one, this difference can be understood in terms of the larger lattice parameter of the former.     

First-principles study of the electronic and optical properties of rutile TiO2

The electronic, structural properties and optical properties of the rutile TiO₂ have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our results including lattice parameter, bulk modulus, density of states, the reflectivity spectra, the refractive index and band gap are compared with the experimental data. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0).     

P-V relation for cuprous halides: Pseudopotential approach

Cuprous halides are found to show significantly different behaviour from that of alkali halides. This difference is mainly due to the presence of outermost d-electrons present in Cu⁺ ion. In the present paper, the ab initio pseudopotential formalism incorporating the effect of d-electrons is used to generate the equation of state (EOS) for group I-VII binary compounds from first-principles calculations. The expression for the EOS is derived for the first time within the pseudopotential framework. The computed isothermal compression curves for cuprous halides are compared with compression curves from other EOSs belonging to different classes and categories and have also been tested for the prediction of end point.     

Structural and elastic properties under pressure effect of the cubic antiperovskite compounds ANCa3 (A = P, As, Sb, and Bi)

Using first-principles density functional calculations, the effect of high pressures, up to 40 GPa, on the structural and elastic properties of ANCa₃, with A = P, As, Sb, and Bi, were studied by means of the pseudo-potential plane-waves method. Calculations were performed within the local density approximation and the generalized gradient approximation for exchange-correlation effects. The lattice constants are in good agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus, Poisson's ratio and Lamé's constants for ideal polycrystalline ANCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that ANCa₃ compounds are brittle in nature. We estimated the Debye temperature of ANCa₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of PNCa₃, AsNCa₃, SbNCa₃, and BiNCa₃ compounds, and it still awaits experimental confirmation.     

High pressure electrical transport measurements of Cs2MoS4 and KTbP2Se6 and X-ray diffraction study of Cs2MoS4

We report the results of electrical resistance measurements at high pressures on Cs₂MoS₄ and KTbP₂Se₆. The results of high pressure X-ray diffraction study of Cs₂MoS₄ are also presented. Interestingly, in the case of Cs₂MoS₄ the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct-indirect band crossing. In the case of KTbP₂Se₆,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs₂MoS₄.     

Size effects on the structure and phase transition behavior of baddeleyite TiO2

High-pressure structural transitions in nanocrystalline systems are of significant interest as models of first-order phase transitions. We demonstrate size-induced lattice expansion and significant atomic rearrangements in the crystal structure of nanocrystalline high-pressure baddeleyite-TiO₂. The α-PbO₂ structured TiO₂ recovered after dozens of pressure cycles in the α-PbO₂-baddeleyite pressure field displayed elongate 25-35 nm crystallites, compared to starting 34-nm anatase crystallites, suggesting crystallite coherency across anatase, baddeleyite, and α-PbO₂ structures and ‘single structural domain’ behavior of the nanocrystalline system.     

Pressure-induced phase transition in tysonite LaF3

We have studied the high-pressure phase stability of LaF₃ using full-potential linear augmented plane wave method. We have shown that experimentally observed orthorhombic phase is less stable compared to the theoretically predicted tetragonal structure above 25 GPa pressure. The structural transition is mainly due to the steric repulsion of ions and electrons to higher pressures.     

Shock Induced Emission from Sapphire in High-Pressure Phase of Rh2O3 （Ⅱ） Structure

A distinct optical emission from the Rh203 （Ⅱ） structural sapphire is observed under shock compression of 125, 132, and 143 GPa. The emission intensity continuously increases with the thickness of shocked sapphire. The colour temperature is determined to be about 4000 K, which is obviously smaller than the reported value of the alpha phase alumina at the pressures below 80 GPa. The present results suggest that the structural transformation will cause an obvious change of optical property in sapphire.     

Equation of state of aluminum carbide Al4C3

Quasi-hydrostatic compression of aluminum carbide, Al₄C₃ has been studied to 6 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit of the experimental p-V data to the Birch equation of state yields the values of the bulk modulus, B₀, of 130(5) GPa and the first pressure derivative of the bulk modulus, B′₀, of 4.6(9). The compression is found to be anisotropic, with the a-axis being more compressible than the c-axis.     

First-Principles Study of Orthorhombic Perovskites MgSiO3 up to 120 GPa and Its Geophysical Implications

High-pressure behaviour of orthorhombic MgSiO3 perovskite crystal is simulated by using the density functional theory and plane-wave pseudopotentials approach up to 120 GPa pressure at zero temperature. The lattice constants and mass density of the MgSiO3 crystal as functions of pressure are computed, and the corresponding bulk modulus and bulk velocity are evaluated. Our theoretical results agree well with the high-pressure experimental data. A thermodynamic method is introduced to correct the temperature effect on the O-K first-principles results of bulk wave velocity, bulk modulus and mass density in lower mantle PIT range. Taking into account the temperature corrections, the corrected mass density, bulk modulus and bulk wave velocity of MgSiO3-perovskite are estimated from the first-principles results to be 2%, 4%, and 1% lower than the preliminary reference Earth model （PREM） profile, respectively, supporting the possibility of a pure perovskite lower mantle model.     

Ab initio study of pressure-induced magnetic transition in manganese pnictides

We report a density functional calculation on the NiAs-type Mn-based pnictides. The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin–tin orbital method by performing spin and non-spin polarized calculation. From the present study, we predict a magnetic-phase transition from ferromagnetic (FM) to non-magnetic (NM) around 49 and 35.7 GPa for MnAs and MnSb, respectively. The pressure-induced transition is found to be a second-order transition. The band structure and density of states (DOS) are plotted for FM and NM states. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and are compared with the available results. Under large volume expansion these compounds exist in zinc-blende (ZB) structure, which shows half metallicity. The magnetic moment and equilibrium lattice constants for ZB structure are obtained as well as band structure and DOS are presented.     

Pressure-induced structural phase transition in rare-earth trihydrides. Part II. SmH3 and compressibility systematics

High-pressure studies of samarium trihydride have been performed in a diamond anvil cell at pressures up to 30 GPa at room temperature. As a result, a reversible structural phase transformation from the hexagonal to the cubic phase has been observed. The transition pressure and the lattice parameter follow the tendency of the other lanthanide trihydrides investigated earlier [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 477; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 481]. The compressibility systematics of lanthanide and yttrium trihydrides has been established.     

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure： an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.     

Magnetic collapse and electronic phase transitions at high pressure in transition metal oxides

A review of electronic and magnetic phase transition in metal oxides with strong electron correlations (SEC) is given. The bandwidth control of the insulator gap is expected in the Hubbard model when the decreasing of the interatomic distance results in the bandwidth W(P) increase and at some critical value P_c, W(P_c)∼U and the Mott–Hubbard gap disappears. The other situation takes place in transition metal boroxides FeBO₃ and GdFe₃(BO₃)₄, where the increase of crystal field parameter Δ(P) results in the high spin–low spin crossover.     

Reentrant high-conduction state in CuIr2S4 under pressure

The results of electrical resistance and angle dispersive X-ray diffraction measurements at high pressures and ambient temperature on the chalcogenide spinel, CuIr₂S₄ are reported. The resistance increases gradually and reaches around 12 GPa a value that is approximately forty times the initial value. Above 15 GPa, the resistance decreases up to 30 GPa and on further pressure increase tends to saturate at a value slightly above the ambient pressure value. Thus, the material exhibits a reentrant high conducting phase under pressure. The behaviour of the electrical resistance exhibits a close correlation with the structural evolution with pressure.     

Equation of state of silicon nitride carbodiimide Si2CN4

The lattice parameters of silicon nitride carbodiimide Si₂CN₄ have been measured up to 8 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit of the experimental p-V data to the Birch-Murnaghan equation of state yields the values of the bulk modulus of 8.8(2) GPa and its first pressure derivative of 3.4(1). The compression is found to be anisotropic, with the b-axis being significantly more compressible than the a-and c-axes.     

Electronic structure and magnetic properties of Laves phase LuFe2

The electronic structure and magnetic properties of the Laves phase of LuFe₂ with C14, C15, and C36 structures has been investigated using the full-potential linearized augmented plane wave method. In order to study the stability of magnetic phases, nonmagnetic and spin-polarized calculations for ferromagnetic ordering were performed. It is found that the ferromagnetic hexagonal C14 phase is the ground-state structure and the C15 phase is an intermediate state between the C14 and C36 structures. There is an increase in the average magnetic moment on the Fe sites in the order of C15 →C14 →C36 structures, whereas the Lu-moment is not significantly different. We also find that there exist both localized and itinerant d electrons, resulting in antiferromagnetic ordering in the three structures. Their density-of-states, equilibrium volumes, and elastic properties are discussed, which is important for the understanding of the physical properties of LuFe₂ and may inspire future experimental research.     

High pressure X-ray diffraction and electrical resistance study of the quasi-one-dimensional sulfide InV6S8

The ambient structural details and the results of room temperature high pressure angle dispersive X-ray diffraction and electrical resistance measurements on the quasi-one-dimensional sulfide, InV₆S₈, to a pressure of 25 GPa are reported. The material does not undergo a phase transition in this pressure range, though an anomaly in the c/a ratio has been observed around 10 Gpa. A fit of the Murnaghan equation of state to the V/V₀ versus pressure data, with the value of the derivative of B₀ with respect to pressure, B′₀, fixed at 4 has yielded a value of the bulk modulus, B₀, of 110 GPa. We also present data of the pressure dependence of the lattice constants, a and c, the ratio c/a, and the resistance at room temperature.     

First-principles calculations on the origins of the gap bowing in InAs1-xPx alloys

We perform self-consistent ab-initio calculations to study the structural, electronic and thermodynamic properties of InAs_1-xP_x alloy. The full potential-linearized augmented plane wave (FP-LAPW) method was employed within density functional theory (DFT). The ground-state properties are determined for the bulk materials (InAs and InP) as well as for the different concentration of their alloys. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The gap bowing for the alloy of interest was found to be mainly caused by the charge-exchange contributions. In addition, the thermodynamic stability of InAs_1-xP_x alloy was investigated by calculating the excess enthalpy of mixing ΔH_m and the calculated phase diagram showed a broad miscibility gap with a high critical temperature.