Structural,elastic and electronic properties of XNCa3 (X = Ge,Sn and Pb) compounds

Using ab initio calculations, we have studied the structural, elastic and electronic properties of XNCa₃, with X=Ge, Sn and Pb. Geometrical optimization of the unit cell are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site and momentum projected densities, charge transfer and total valence charge density shows that the chemical bonding in XNCa₃ compounds is of covalent–ionic nature with the presence of metallic character. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal polycrystalline XNCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that XNCa₃ compounds are brittle in nature. We estimated the Debye temperature of XNCa₃ from the average sound velocity.     

First principles study of the structural,elastic, electronic and optical properties of CaSrTt (Tt=Si,Ge, Sn and Pb)

We present an ab initio study of the structural, elastic, electronic and optical properties of CaSrTt (Tt=Si, Ge, Sn and Pb) compounds. To more-accurately describe the properties of these materials, the calculations were based on the DFT theory with the generalized gradient approximation (GGA). In particular, the calculated lattice constants are in good agreement with the experimental results, with a deviation less than 0.67%, 2.74% and 1.7% for a, b and c, respectively. For the equilibrium volume, the deviation does not exceed 4.7%. Single-crystal elastic stiffness (C_ij) values were calculated and the polycrystalline elastic moduli (B and G) were estimated utilizing Voigt, Reuss and Hill’s approximations. The electronic band-structure calculations indicate that these compounds are semiconductors, in agreement with the literature data on their Ae₂Tt analogues. The dielectric function, refractive index, extinction coefficient, reflectivity spectrum and electron energy loss are calculated over a spectral range from 0 to 45 eV.Unfortunately, there is no available previous study for comparison.     

Structural and dielectric properties of Pb5(Ge,Si)O11

The ferroelectric lead germanate (Pb₅Ge₃O₁₁) and its isomorphous compounds are important because of their uses as pyroelectric and electro-optic devices. Comparison of inter-planar d-spacings of Pb₅Ge_3−x Si _x O₁₁ (x=0, 0.3, 0.7 and 1.00) suggests that there is no change in basic structure of Pb₅Ge_3−x Si _x O₁₁ when Si is substituted for Ge in small quantity (x<1). The dielectric properties of the Si-substituted compounds have been studied as a function of temperature (30 to 200°C). The ferroelectric-paraelectric phase transition has been observed at 185°C. The Si doping causes (a) Curie point to shift towards low temperature, (b) peak value of the dielectric constant to decrease and (c) phase transition diffuse. The fast increase in dielectric constant of pure Pb₅Ge₃O₁₁ with temperature (beyond transition temperature) may be attributed to the development of space charge polarization in the system.     

Structural and electronic properties of neutral clusters Ga12X (X=C, Si, Ge, Sn, and Pb) and their anions from first principles

The structural and electronic properties of neutral and negatively charged Ga₁₂X (X=C, Si, Ge, Sn, and Pb) clusters are calculated by the first-principles method. The results show that the most stable symmetry depends on the doped atom rather than the geometry structure. However, the geometry symmetry plays an important role in calculating the energy gap. In addition, in the anionic clusters, the added electron would reduce the energy gap by about 0.4 eV. As for the density of states (DOS), clusters with the same symmetry show a similar trend of DOS. The major impact on DOS by adding an electron is the occurrence of relative energy shift.     

Structural,electronic, optical,elastic and thermal properties of ZnXAs<Subscript>2</Subscript> (X = Si and Ge) chalcopyrite semiconductors

We report first principles calculations of solid state properties of ZnSiAs₂ and ZnGeAs₂ chalcopyrite semiconductors. The structural properties are calculated using a Full Potential Linearized Augmented Plane Wave method (FP-LAPW) of the Density Functional Theory (DFT). A Generalized Gradient Approximation (GGA) scheme proposed by Wu and Cohen (WC) has been chosen to calculate electronic and optical properties. Optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients and optical conductivities were calculated for photon energies up to 30 eV. The elastic constants at equilibrium in tetragonal structure are also determined. Temperature effect on the volume, thermal expansion, heat capacity, Debye temperature, entropy, Grüneisen parameter and bulk modulus were calculated employing the quasi-harmonic Debye model at different temperatures and pressures and the silent results were interpreted. Finally using semi-empirical relation, we determined the hardness of the materials which attributed to different covalent bonding strengths.     

Structural stability,mechanical, electronic and thermal behaviour of Ru2CrZ (Z=Sb,Si, Pb,Ge) Heusler alloys

In this paper, the structural, elastic, electronic properties of Ru2CrZ (Z=Si, Ge, Pb, Sn) are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory. With the help of the quasi-harmonic Debye model, we also investigate the variation of normalized volume V/V0, the heat capacities CV and CP, thermal expansivity, and Debye temperature of Ru₂CrZ (Z=Si, Ge, Pb, Sn). Results show that the Cu₂MnAl type structure is more stable then Hg₂CuTi type structure. The four compounds in the ground state are predicted to be nearly half-metal behavior with total magnetic moment near to the integer value. To provide a comparative and complementary study to future researches, we investigated the elastic and thermodynamic properties.     

Magnetic properties and hyperfine fields of some Eu-compounds with Si,Ge and Sn

Mössbauer effect and magnetization measurements are reported for the intermetallic compounds Eu-(Si, Ge, Sn) and Eu-(Si₂, Ge₂). The equiatomic compounds show a complex magnetic behaviour while EuSi₂ and EuGe₂ are antiferromagnetic. The hyperfine fields at the Eu and Sn sites in EuSn have the same temperature dependence following aS=7/2 Brillouin function.     

Interlattice displacements and elastic constants of the A-15 compounds V3Si,V3Ge and Nb3Sn

General expressions for the interlattice displacements of the A-15 structure compounds are obtained in terms of the strain components making use of the deformation theory. The nature of the interlattice displacements of all the 8 atoms in the unit cell is discussed. It is found that the interlattice displacements occur in such a way that the pair of atoms along any linear chain move in opposite directions with equal magnitudes. Expression for the strain energy of these compounds is developed using deformation theory and this is compared with the strain energy expression from continuum theory to obtain the elastic constants. The theoretical values of the elastic constants fairly agree with the experimental values for V₃ Si, V₃Ge and Nb₃ Sn.     

Mössbauer spectroscopic studies of MIIMIVS3 (MII=Sn,Pb; MIV=Sn,Ge, Hf) and PbSnS2Se

Temperature-dependent Mössbauer experiments have been carried out to examine the lattice dynamical properties of tin sites in Sn₂S₃ and its related compounds. The differences in the lattice parameters, estimated from the temperature dependence of the area intensities for the compounds, are attributed to the characteristics of their crystal structures.     

Structural,elastic, electronic and optical properties of the cubic perovskites CaXO3 (X=Hf and Sn)

The structural, elastic, electronic and optical properties of CaXO₃ compounds with the cubic perovskites structure have been investigated, by employing a first principles method, using the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA). The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal monocrystalline and for polycrystalline CaXO₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are indirect energy band gap (R-G) semiconductors; the analysis of the site and momentum projected densities, valence charge density bond length, bond population and Milliken charges, shows that bonding is of covalent–ionic nature. We have found that the elastic constants C₁₁, C₁₂, C₄₄ are in good correlation with the bonding properties. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV.     

6Li elastic scattering ON 12C, 16O, 40Ca, 58Ni, 74Ge, 124Sn, 166Er and 208Pb at E(6Li) = 50.6 MeV

The elastic scattering of ⁶Li ions from a variety of targets, A = 12 to 208, has been measured at a bombarding energy of 50.6 MeV. The angular distributions are characteristic of strongly absorbed particles, such as ³He and heavy ions, and less diffractive than for ⁴He. A simple optical model with Woods-Saxon real and imaginary volume potentials is adequate to fit the data. Spin-orbit effects are not apparent in the data.     

Optical Spectroscopy of Diatomic Species: Copper with Group 14 Elements (Si, Ge, Sn, Pb)

Electronic band systems of the gaseous diatomic compounds of copper and various X elements of the 14th column (Si, Ge, Sn, Pb) have been observed by thermal excitation in the red part of the visible spectrum. Vibrational analysis of the two subsystems observed for each of these molecules (except for CuPb with only one system) are reported and assigned as (2)Sigma(+)-(2)Pi(3/2) and (2)Sigma(+)-(2)Pi(1/2) transitions. The variation of the spin-orbit splitting of the (2)Pi lower states from CuSi to CuSn follows closely that of the np shell spin-orbit parameters in the group 14 atoms. This fully corroborates previous ab initio calculations that predict a (2)Pi(r) ground state with the ionic Cu(+) (3d(10))X(-)(pvarsigma(2)ppi) configuration for these molecules. Copyright 2000 Academic Press.     

Comparative first-principles calculations of electronic, optical and elastic anisotropy properties of CsXBr3 (X=Ca,Ge,Sn ) crystals

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of CsCaBr₃, CsGeBr₃ and CsSnBr₃ crystals are presented in this paper. Based on the obtained results, CsCaBr₃ is characterized as a dielectric with an indirect band gap, whereas CsGeBr₃ and CsSnBr₃ are semiconductors with very narrow direct band gaps. The first theoretical estimations of the refractive indexes for all compounds are reported. Variations of the electron density difference distribution induced by changes of the second cation were analyzed and related to the type of chemical bonding between atoms. In addition, the complete set of elastic parameters (which includes the elastic constants, elastic compliance constants, bulk and Young’s moduli, elastic anisotropy) was obtained. Directional anisotropy of elastic properties was visualized; the directions in the crystal lattices, along which the maximal and minimal values of the Young’s moduli are realized, were identified.     

Thermoelectric performance of XI2 (X = Ge,Sn, Pb) bilayers

We study the thermal and electronic transport properties as well as the thermoelectric (TE) performance of three two-dimensional (2D) XI₂ (X=Ge, Sn, Pb) bilayers using density functional theory and Boltzmann transport theory. We compared the lattice thermal conductivity, electrical conductivity, Seebeck coefficient, and dimensionless figure of merit (ZT) for the XI₂ monolayers and bilayers. Our results show that the lattice thermal conductivity at room temperature for the bilayers is as low as ~1.1 W·m^{-1·K-1-1.7 W}·m^{-1·K-1, which is about 1.6 times as large as the monolayers for all the three materials. Electronic structure calculations show that all the} XI₂ bilayers are indirect-gap semiconductors with the band gap values between 1.84 eV and 1.96 eV at PBE level, which is similar as the corresponding monolayers. The calculated results of ZT show that the bilayer structures display much less direction-dependent TE efficiency and have much larger n-type ZT values compared with the monolayers. The dramatic difference between the monolayer and bilayer indicates that the inter-layer interaction plays an important role in the TE performance of XI₂, which provides the tunability on their TE characteristics.     

Transport properties of Mg2 X 0.4Sn0.6 solid solutions (X = Si, Ge) with p-type conductivity

The transport properties of Mg₂ X _0.4Sn_0.6 (X = Si, Ge) solid solutions are investigated. It is shown that these materials can be rendered p-type with a hole concentration of up to 4 × 10¹⁹ cm^?3. The Hall coefficient, thermopower, and electrical conductivity are measured over a wide temperature range. The mobility of holes in these solid solutions is less than that of electrons by a factor of 2 for Mg₂Si_0.4Sn_0.6 and by a factor of 1.5 for Mg₂Ge_0.4Sn_0.6. Solid solutions in the Mg₂Ge-Mg₂Sn system appear more promising for thermoelectric applications.     

Theoretical investigations on the structural, lattice dynamical and thermodynamic properties of XC (X=Si, Ge, Sn)

First-principles calculations, which is based on the plane-wave pseudopotential approach to the density functional perturbation theory within the local density approximation, have been performed to investigate the structural, lattice dynamical, and thermodynamic properties of SiC, GeC, and SnC. The results of ground state parameters, phase transition pressure and phonon dispersion are compared and agree well with the experimental and theoretical data in the previous literature. The obtained phonon frequencies at the zone-center are analyzed. We also used the phonon density of states and quasiharmonic approximation to calculate and predict some thermodynamic properties such as entropy, heat capacity, internal energy, and phonon free energy of SiC, GeC, and SnC in B3 phase.     

Ab-initio study of structural,elastic, thermal,electronic and magnetic properties of quaternary Heusler alloys CoMnCrZ (Z = Al,As, Si,Ge)

First-principles approach is used to study the structural, electronic and magneticproperties of CoMnCrZ (Z = Al,Si, Ge and As) quaternary Heusler compounds, using full-potential linearized augmentedplane wave (FP-LAPW) scheme within the generalized gradient approximation (GGA). Thecomputed equilibrium lattice parameters agree well with the available theoretical data.The obtained negative formation energy shows that CoMnCrZ (Z = Al, Si, Ge, As) compounds have strongstructural stability. The elastic constants C _ij are calculatedusing the total energy variation with strain technique. The polycrystalline elastic moduli(namely: the shear modulus, Young’s modulus, Poisson’s ratio, sound velocities, Debyetemperature and melting temperature were derived from the obtained single-crystal elasticconstants. The ductility mechanism for the studied compounds is discussed via the elasticconstants C _ij . Our calculationswith the GGA approximation predict that CoMnCrGe, CoMnCrAl, CoMnCrSi and CoMnCrAs arehalf-metallic ferrimagnets (HMFs) with a half-metallic gap E _HM of 0.03 eV, 0.19 eV,0.34 eV and 0.50 eV for, respectively. We also find that the half-metallicity ismaintained on a wide range of lattice constants.     

The Si,Ge, Sn,Pb doped (6,3) Chiral single-walled carbon nanotubes: A computational study

Electronic structure properties including bond lengths, bond angles, dipole moments (μ), energies, band gaps, NMR parameters of the isotropic and anisotropic chemical shielding parameters for the sites of various atoms were calculated using the density functional theory for Si, Ge, Sn, Pb doped (6,3) Chiral single-walled carbon nanotubes (SWCNTs). The calculations indicated that average bond lengths were as: Pb3C>Sn3C>Ge3C>Si3C>C3C. The dipole moments for Si, Ge, Sn, Pb doped (6,3) Chiral single-walled carbon nanotubes structures show fairly large changes with respect to the pristine model.     

Full potential calculation of electronic properties of rutile RO2 (R=Si, Ge, Sn and Pb) compounds via modified Becke Johnson potential

The electronic properties of RO₂ (R=Si, Ge, Sn and Pb; a group IVA element) compounds in rutile structure have been calculated using WIEN2k implementation of full potential linearized augmented plane wave (FPLAPW) method. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke Johnson (MBJ) potential as coupled with Local Density Approximation (LDA) for all the compounds except for PbO₂ where only Generalized Gradient Approximation (GGA) is considered for the same. We predict a direct band gap in all these compounds with continuous decrease as the atomic size of IVA element increases such that there is an appearance of semimetallic band structure for the last compound, PbO₂. The largest band gap (7.66 eV) has been found for SiO₂, which governs its insulating nature. We observe that MBJLDA results for band gaps of these compounds are far better than those obtained using GGA and Engel-Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gaps with corresponding experimental values as compared to other calculations. The electronic band structures are also analyzed in terms of contributions from various electrons.     

Electronic, magnetic and transport properties of Co2TiZ (Z=Si, Ge and Sn): A first-principle study

We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co₂TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co₂TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μ_B, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co₂TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.