Average Elastic Wave Velocities Research Articles

Mechanical, electronic and thermodynamic properties of C14-type AMg2 (A = Ca, Sr and Ba) compounds from first principles calculations

Using first principles total energy calculations within the generalized gradient approximation (GGA), we have investigated the mechanical and thermodynamics properties of C14-type Laves phases AMg2 (A=Ca, Sr and Ba). Our results of equilibrium lattice parameters and formation enthalpy agree closely with previous experimental results. In particular we have discussed the thermodynamic stabilities and mechanical anisotropies of these phase structures. The polycrystalline elastic moduli have been deduced by Voigt–Reuss–Hill arithmetic approximation. Subsequently, the ductility and brittleness are characterized with the estimation from Pugh’s rule (B/G) and Poisson’s ratio. Additionally, the Debye temperature is calculated from the average elastic wave velocity obtained from bulk and shear moduli. The calculations of density of states and charge density difference are performed to reveal the underlying mechanism of electronic structure. Finally, variations of thermodynamic properties with temperature are predicted by calculating phonon frequencies combined within the quasi-harmonic approximation.

Investigations on the structural, electronic, elastic and thermodynamic properties of niobium silicide under high temperature and pressure

A first principles investigation on the structural, electronic and elastic properties of NbSi2 has been calculated using the plane wave pseudo-potential density functional theory (DFT) method. The lattice constants and elastic constants are obtained. The characters of the band structure and the density of states of niobium silicide (NbSi2) are analyzed, which shows that NbSi2 is metallic. The bulk, shear and Young’s modulus for NbSi2 are also calculated. The Debye temperature is calculated from the average elastic wave velocity obtained from the shear and bulk modulus. The thermodynamic properties including the heat capacity CV and CP, thermal expansionα, entropy S and Grüneisen parameterγhave been calculated at temperatures from 0 to 2100K and pressures from 0 to 30GPa using the quasi-harmonic Debye model. We also found that the present work can give a reference to those not to be experimentally investigated.

First-principles calculations of structural, elastic and electronic properties of Li2B12H12

We investigate the structural, elastic and electronic properties of Li2B12H12 using the first-principles method. Our calculations show that the lowest energy structure of Li2B12H12 is monoclinic C2/m type. We take the monoclinic C2/m Li2B12H12 as a representative to carry out the corresponding theoretical studies. The independent elastic constants are successfully obtained from the strain energy–strain curve calculations. The Shear and Young‘s moduli, as well as Poisson‘s ratio for ideal polycrystalline Li2B12H12 are calculated. The shear anisotropic factors and elastic anisotropy of Li2B12H12 are analyzed. The Debye temperature and the average elastic wave velocity are derived from theoretical elastic constants. According to the obtained results, the monoclinic C2/m Li2B12H12 is found to be mechanically stable and brittle at zero temperature and zero pressure. Furthermore, the density of states and electron charge density distributions are studied. The insulator Li2B12H12 is a technologically interesting indirect hydrogen storage material for further studies.

Theoretical studies of elastic properties of orthorhombic LiBH4

We have presented a study of the elastic properties of orthorhombic Pnma structural LiBH4 by first-principles calculations using a projected augmented plane-wave method. The single crystal elastic constants are successfully obtained from the stress–strain relationship calculations and the strain energy–strain curves calculations, respectively. The Shear and Young‘s moduli as well as Poisson‘s ratio for ideal polycrystalline LiBH4 are calculated. The bulk moduli are in good agreement with recent experimental and theoretical results. The linear compressibility and the directional dependent Young‘s modulus are also calculated. The shear anisotropic factors and the elastic anisotropy of Pnma LiBH4 are analyzed. The Debye temperature and the average elastic wave velocity are derived from theoretical elastic constants. According to the obtained results, the Pnma LiBH4 is found to be mechanically stable and ductile at zero temperature and zero pressure.

Investigations on the structural, elastic and electronic properties of the orthorhombic Zirconium–Nickel alloy under different pressure

A theoretical study of structural, elastic, mechanical and electronic properties of the orthorhombic crystal ZrNi is presented for the first time by density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA). The lattice constant, elastic constants and their pressure dependence are calculated. The bulk modulus, shear modulus, and Poisson’s ratio for ideal polycrystalline ZrNi are also successfully achieved. The shear and bulk modulus anisotropic factors are obtained from the single crystal elastic constants. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. Band structure and density of states are also given in this work, which are in good agreement with available theoretical data.

Pressure effects on elastic and thermodynamic properties of ZrB2

A theoretical formalism to calculate the single crystal elastic constants for hexagonal crystals from first principle calculations is described. The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. The calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline ZrB2 are also calculated and compared with corresponding experimental values. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential. It is found that the elastic constants and the Debye temperature of ZrB2 increase monotonically and the anisotropies weaken with pressure. The thermal properties including the equation of state, linear compressibility, ductility, and the heat capacity at various pressures and temperatures are estimated.

Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2

A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the experimental results. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. From the site and angular momentum decomposed density of states combined with a charge density analysis and the elastic anisotropies, the chemical bonding nature between the constituents in TiSi2 is analyzed. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential.