Pressure Dependence Of Thermodynamic Properties Research Articles

Investigation of electronic structure, magnetic stability, spin coupling, and thermodynamic properties of novel antiferromagnets XMn2Y2 (X = Ca, Sr; Y = P, As)

In the present work, the structural, electronic, magnetic, and thermodynamic properties of ternary compounds of composition XMn2Y2 (X = Ca, Sr; Y = P, As) are reported. The calculations are performed using the full potential augmented plane wave method (FP-APW) within the framework of density functional theory (DFT) as integrated with the WIEN2k code. The volume optimization is performed for different spin configurations to obtain the optimized unit cell structure, using the minimum energy considerations. It is found that the studied systems favor the antiferromagnetic (AFM) structure. Calculated structural parameters are in good agreement with the existing data. Based on their band structures and density of states (DOS), CaMn2P2, SrMn2P2, and SrMn2As2 have a metallic character, while CaMn2As2 is a semiconductor with a narrow band gap of 0.1161 eV on using GGA and as metallic when GGA+U is employed. The density of states diagrams show that the Mn-3d state contributes majorly to the valence band and the lower part of the conduction band. From stable geometries of XMn2Y2 compounds, it is evident that the Mn atoms are coupled antiferromagnetically in all the compounds. The large value of TC for CaMn2As2 shows strong correspondence among the magnetic atoms. Owing to their suitable magnetic characteristics, these compounds can be used in applications such as Colossal magnetoresistance (CMR) or Giant magnetoresistance (GMR), single-molecule magnets, read heads, spin filters, spin valves, magnetic sensors, and in spintronics applications. Furthermore, temperature and pressure dependence of thermodynamic properties of these materials have been examined in the ranges (0–800 K) and (0–18 GPa), respectively.

Compressible models of equilibrium polymerization

Flory-Huggins-type models of equilibrium polymerization are extended to describe compressible systems and, hence, the pressure dependence of thermodynamic properties. The theory is developed for three different mechanisms of equilibrium polymerization (the free association, monomer-activated polymerization, and chemically initiated polymerization models). In contrast to previous approaches for describing the pressure dependence, the theory delineates the thermodynamic consequences of the size disparities between solvent molecules, unpolymerized monomers, and the monomers within polymers. Basic thermodynamic properties (the extent of polymerization, density, heat capacities C(P) and C(V), etc.) are calculated analytically as functions of pressure, temperature, and composition of the associating species. Illustrative calculations refer to systems that polymerize upon cooling and demonstrate general agreement with numerous experimental trends. Comparisons with results from other theories are also discussed.

Molecular Dynamics Simulation of Solid C60 under Progressively Raised Pressure

Recent molecular dynamics investigations have shown that C60 fullerites can be investigated by means of simple potentials and few calculations have focused on the pressure dependence of thermodynamic properties and the induced order–disorder phase transition. The aim of this work is to perform molecular dynamics simulation of the C60 crystal lattice in the face centred cubic structure under progressively raised pressure, to illustrate the limits of the crude pseudo atom approach and the validity of the potential in the rigid molecule, when seeking for possible phase transitions. For that purpose, several thermodynamic and structural properties are calculated with different potential models in the pseudo atom approach and as a rigid freely rotating C60 molecule. Calculated properties are compared to experimental data. It will be demonstrated that though the pseudo atom approach is reasonable in the low-pressure region (>4 kbar), the rigid molecule is in better agreement with experiments depending on the choice of the model potential and the pressure applied. Orientational transition pressures are observed at 5 and 260 K, the distorted lattice and the cell contraction are measured and the density of phonon states is reported at different pressures.

Molecular Dynamics Simulation of Solid C60 under Progressively Raised Pressure

Recent molecular dynamics investigations have shown that C60 fullerites can be investigated by means of simple potentials and few calculations have focused on the pressure dependence of thermodynamic properties and the induced order–disorder phase transition. The aim of this work is to perform molecular dynamics simulation of the C60 crystal lattice in the face centred cubic structure under progressively raised pressure, to illustrate the limits of the crude pseudo atom approach and the validity of the potential in the rigid molecule, when seeking for possible phase transitions. For that purpose, several thermodynamic and structural properties are calculated with different potential models in the pseudo atom approach and as a rigid freely rotating C60 molecule. Calculated properties are compared to experimental data. It will be demonstrated that though the pseudo atom approach is reasonable in the low-pressure region (>4 kbar), the rigid molecule is in better agreement with experiments depending on the choice of the model potential and the pressure applied. Orientational transition pressures are observed at 5 and 260 K, the distorted lattice and the cell contraction are measured and the density of phonon states is reported at different pressures.

Temperature and pressure dependence of thermodynamic properties and stability of the C 70 fullerite

Following the correlative method of unsymmetrized self-consistent field for strongly anharmonic crystals, we investigate thermodynamic properties of the high-temperature modifications of fullerites taking into account the intramolecular degrees of freedom. Here we calculate them for C 70 fullerite as functions of the temperature and pressure up to 20 kbar. We use the potentials of Verheijen et al. and of Girifalco together with the available experimental information about the vibrational spectrum of this molecule. At any constant pressure its equation of state has two roots a 1( T)< a 2( T), which coalesce at some (maximal) pressure that depends on the temperature. At the lower branch, B T ( T, a 1)>0, while at the upper one, B T ( T, a 2)<0; the latter represents the absolute instability of the states a 2( T). Along the lower branches of the isobars we calculate a complete set of equilibrium properties,including the components of the elastic tensors. We also study the thermodynamic stability of this fullerite and the mechanism of loss of stability depending on the pressure.

Pressure dependence of thermodynamic properties of NaBr, NaI and AgCl

Various state-equations, derived by expanding energy as a function of volume in Taylor series and using different order Pade’s approximants, have been combined with quasi-harmonic approximation for free energy to reproduce the pressure dependence of thermodynamic properties of NaBr, NaI and AgCl crystals. We have used these state-equations to compute the reduced volume, the isothermal bulk modulus and the pressure derivative of isothermal bulk modulus for the three crystals at various pressures (up to 80 kbar) and at room temperature (T=298 K). The results obtained are reasonably good lending support to the state-equations and the technique used to extend their applicability. The significant results obtained in the present study include the unified reduced equation of state for the three crystals which generates almost a single curve for theP – V behaviour at room temperature.

Gutzwiller-Hubbard lattice-gas model with variable density: Application to normal liquid 3He.

The results of the Gutzwiller approach to a Hubbard lattice-gas model with a variable density of particles is used to describe the pressure dependence of thermodynamic properties of the ground state of normal liquid $^{3}\mathrm{He}$. The molar volume of the liquid is given by that of the underlying lattice and the filling factor n=1-\ensuremath{\delta} of the band, where \ensuremath{\delta} describes the deviation from half-filling. If the lattice is taken as incompressible, one finds that there exists a critical pressure at which a transition to a localized state occurs. The transition is accompanied by a disappearance of \ensuremath{\delta}, i.e., the transition only takes place at exactly half-filling. As the transition is approached, \ensuremath{\delta} and the density of doubly occupied sites are found to scale. The pressure dependence of the effective mass, the spin susceptibility and the compressibility is calculated. In a second model, the lattice is assumed to be compressible, shifting the critical pressure to much higher values. The on-site repulsion U is related to the microscopic soft-core potential ${f}_{0}^{s}$(r), which allows one to calculate the pressure dependence of the effective mass and the spin susceptibility. The absence of a localization transition for pressures of the order of the melting pressure of $^{3}\mathrm{He}$ leads to a smooth pressure dependence of the calculated quantities which are qualitatively borne out by experiment.