Vibrational and thermodynamic properties of orthorhombic CaSnO3 from DFT and DFPT calculations

Density functional theory (DFT) and density functional perturbation theory (DFPT) calculations were used to investigate the vibrational and thermodynamic properties of orthorhombic stannate CaSnO₃ compound. Our approach was based on the generalized gradient approximation with dispersion correction (GGA+D), considering the norm-conserved pseudopotentials. The phonon dispersion relation as well as theoretical peaks of the infrared (IR) and Raman spectrum in the frequency range of 100–800 cm⁻¹ was analyzed and assigned. The thermodynamic potentials and the specific heat at constant volume of the CaSnO₃ compound are also calculated, whose dependence with the temperature are discussed.     

多体作用对氦原子团簇He9压缩特性的贡献

 采用Lowdin方法计算了处于压缩状态的氦原子团簇He₉体心立方向型的排斥势及其多体分量，并由此计算出团簇微观压强及其多体分压强。结果发现，多体作用对氦原子团簇He₉压缩特性贡献较大。在相同体积下，由两体近似方法计算的体系压强比多原子体系实际压强偏高10%~30%；考虑三体分压强修正后，近似理论的计算值又会偏低1.5%~10%；只有同时考虑三体、四体修正后，近似理论计算值与实际多原子体系的压强值基本一致。团簇He₉所表现出的压缩特性与固态氦的实验结果相符合。     

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.     

MgCNi3超导体在静水压下的弹性性质

 在0.1 MPa到0.5 GPa压力范围内，采用脉冲回波重合法（Pulse-Echo Overlap Method）测量了MgCNi₃超导体的横波及纵波声速，计算了体弹模量B、剪切模量G等弹性参数以及德拜温度、比热等热力学参数随压力的变化。常压下的θ_D与c_V的值分别为454.82 K和111.30 J/(mol·K)。对B-p曲线作线性拟合，得到体弹模量B₀在零压附近的导数B′₀，由此建立了MgCNi₃的状态方程。     

Ab initio thermodynamic properties for different isomers of the AlO molecule

We present ab initio thermodynamic properties for seven different geometric isomers of molecular Al₂O₃ over a wide temperature range. The rigid rotator-harmonic oscillator approximation is used to calculate the partition function as it is generally applied in thermodynamic studies of polyatomic molecules. The molecular data employed in setting up the partition functions are taken from the theoretical results of ab initio MP2/6-31G(d) calculations. An analysis of the effects of the various contributions is made. The resulting thermodynamic functions of Al₂O₃ are consistent with the JANAF thermochemical data compilation. Some thermochemical implications are discussed. Received: 9 September 1998 / Received in final form: 2 October 1998     

First-Principles Calculations of Elastic and Thermal Properties of Molybdenum Disilicide

The first-principles plane-wave pseudopotential method using the generalized gradient approximation within the framework of density functional theory is applied to anaylse the equilibrium lattice parameters, six independent elastic constants, bulk moduli, thermal expansions and heat capacities of MoSi2. The quasi-harmonic Debye model, using a set of total energy versus cell volume obtained with the plane-wave pseudopotential method, is applied to the study of the elastic properties, thermodynamic properties and vibrational effects. The calculated zero pressure elastic constants are in overall good agreement with the experimental data. The calculated heat capacities and the thermal expansions agree well with the observed values under ambient conditions and those calculated by others. The results show that the temperature has hardly any effect under high pressure.     

Lattice Dynamics Study of Magnesium Chalcogenides

First-principles calculations,which are based on the plane-wave pseudopotential approach to density functional perturbation theory within the local density approximation,have been performed to investigate the structural,lattice dynamical and thermodynamic properties of zinc blende(B3) structure magnesium chalcogenides:MgS,MgSe and MgTe.The results of ground state parameters and phonon dispersion are compared and agree well with the experimental data available and other calculations.We obtain the change of Born effective charge and LO-TO splitting under hydrostatic pressure.Finally,by the calculations of phonon frequencies,some thermodynamic properties such as the entropy,heat capacity,internal energy,and free energy are also successfully obtained.     

Bulk Modulus and Electronic Band Structure of ZnGa2X4(X=S,Se): a First-Principles Study

First-principles local density functional calculations are presented for the compounds ZnGa₂X₄ (X=S,Se). We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters are optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa₂X₄ by fitting the Birch-Murnaghan's equation of state. The extended Cohen's empirical formula agrees well with our ab initio results.     

Ground state analysis of XCNi3 (X=Mg, Zn, and Ga) from first-principles

The ground state properties of recent antiperovskite MgCNi₃ and related systems ZnCNi₃ and GaCNi₃ are studied using full-potential augmented plane waves method. The obtained quantities agree well with the recent experiments and theoretical data. The band structure is dominated by the Ni d band as shown in the charge density and density of states. Besides, the effect of external pressure has been addressed.     

Electronic structure of the new MgCNi3 superconductor and related intermetallic compounds

The band structure of the new perovskite-like MgCNi₃ superconductor is studied by the self-consistent FP-LMTO method. The superconducting properties of MgCNi₃ are associated with the occurrence of an intense peak in the density of Ni3d states at the Fermi level. The absence of superconductivity for nonstoichiometric MgC_1?x Ni₃ compositions is caused by the transition of the system to a magnetic state. The possibilities of finding superconductivity for ScBNi₃, InBNi₃, MgCCo₃, and MgCCu₃ isostructural with MgCNi₃ are discussed.     

First-Principles Calculations of Elastic and Thermal Properties of Lanthanum Hexaboride

The plane-wave pseudopotentiai method using the generaiized gradient approximation within the framework of density functional theory is applied to anaylse the bulk modulus, thermal expansion coefficient and heat capacity of LaB6. The quasi-harmonic Debye model, using a set of total energy versus volume obtained with the plane-wave pseudopotential method, is applied to the study of the thermal properties and vibrationai effects. We analyse the bulk modulus of LaB6 up to 1500 K. The elastic properties calculations show that our system is mechanically stable. For the heat capacity and the thermal expansion, significant differences in properties are observed above 300K. The calculated zero pressure bulk modulus is in good agreement with the experimental data. Moreover, the Debye temperatures are determined from the non-equilibrium Gibbs functions and compared to available data.     

Working Pressure Dependence of Properties of Al2O3 Thin Films Prepared by Electron Beam Evaporation

The effects of working pressure on properties of Al2O3 thin films are investigated. Transmittance of the Al2O3 thin film is measured by a Lambda 900 spectrometer. Laser-induced damage threshold （LIDT） is measured by a Nd：YAG laser at 355 nm with a pulse width of 7ns. Microdefects were observed under a Nomarski microscope. The samples are characterized by optical properties and defect, as well as LIDT under the 355 nm Nd：YAG laser radiation. It is found that the working pressure has fundamental effect on the LIDT. It is the absorption rather than the microdefect that plays an important role on the LIDT of Al2O3 thin film.     

An All-Thin-Film Electrochromic Device Composed of MoO3--LiBSO--NiOx Multilayer Structure

An all-thin-tilm （ATF） electrochromic device for modulating the optical transmittance is manufactured using magnetron sputtering. The devices consists of MoO3 as the main electrochromic layer, LiBO2 ＋Li2SO4 （LiBSO） as the ion conductor layer, and NiOx as the complementary electrochromic layer. Glass covered with indium tin oxide （ITO） is used as the substrate and the ITO film is used as the bottom electrode. The ITO film deposited on the top of the devices is used as the other electrode. The structure and morphology of the films are characterized by x-ray diffraction （XRD） and scanning electron microscopy （SEM）. The devices exhibit good optical properties with low transmittance values in the coloured state, and the optical modulation is measured by spectrophotometer in the wavelength range from 400 to 800nm. The average visible light transmittance reaches 50.2% and 3.7% in bleached and coloured state, respectively. The results indicate that such a monolithic system has great potential to be applied in smart windows.     

Giant Static Dielectric Constant of Strained PbTiO3

First-principles density functional perturbation calculations are employed to study the dielectric and piezoelectric properties of strained tetragonal PbTiO3. Lattice distortion, static dielectric constant, Born effective charge, zone-centre phonons, and piezoelectric constant are obtained. For the strained tetragonal PbTiO3, we obtain a giant static dielectric constant （3600） under a strain 0. 77%. Moreover, the calculated piezoelectric constant e15 of strained PbTiO3 reaches about 203 C/m^2 which is about 20 times of that of unstrained system. The giant static dielectric constant is mainly due to the softening of the lowest-frequency phonon mode and the reduce of Ti-O bond length. This work demonstrates a route to a giant static dielectrics for electrically microwave and other devices.     

Unique Magnetic Moment and Electronic Properties for Fe（MgO）n（n=1-8） Clusters： First-Principles Calculations

The geometries and electronic properties of Fe（MgO）n are systematically investigated by the density functional theory. The results show that the doped Fe atom is prone to bond with the O atom, and Fe almost does not disturb the frame of （MgO）n. The second-order energy difference, the fragmentation energies and the electron amnities show that Fe（MgO）4 and Fe（MgO）6 possess relatively higher stabilities. The HOMO-LUMO gaps of Fe（MgO）n decrease obviously as compared with （MgO）n. Almost equal unpaired electrons of the 3d state of the Fe atom in Fe（MgO）n result in a nearly equal magnetic moment of Fe（MgO）n.     

Electronic structure and optical properties of rutile RuO2 from first principles

The systematic trends of electrionic structure and optical properties of rutile (P42 /mnm) RuO2 have been calculated by using the plane-wave norm-conserving pseudopotential density functional theory (DFT) method within the generalised gradient approximation (GGA) for the exchange-correlation potential.The obtained equilibrium structure parameters are in excellent agreement with the experimental data.The calculated bulk modulus and elastic constants are also in good agreement with the experimental data and available theoretical calculations.Analysis based on electronic structure and pseudogap reveals that the bonding nature in RuO2 is a combination of covalent,ionic and metallic bonds.Based on a Kramers-Kronig analysis of the reflectivity,we have obtained the spectral dependence of the real and imaginary parts of the complex dielectric constant (ε1 and ε2,respectively) and the refractive index (n);and comparisons have shown that the theoretical results agree well with the experimental data as well.Meanwhile,we have also calculated the absorption coefficient,reflectivity index,electron energy loss function of RuO2 for radiation up to 30 eV.As a result,the predicted reflectivity index is in good agreement with the experimental data at low energies.     

The effect of Fe and Ru substitution on the superconductivity in MgCNi3

We report the effects of partial substitutions of the VIIIa elements: Fe (3d) and Ru (4d) in the intermetallic perovskite superconductor MgCNi₃. In both the MgCNi_3−xRu_x and MgCNi_3−xFe_x systems, the superconducting critical temperature (T_C) decreases monotonically as x is increased. T_C decreases more slowly in the case of Ru, suggesting that the 3d electrons in Fe are magnetically pair breaking. This is supported by the normal state magnetic susceptibility measurements (χ), which shows that χ increases with Fe and decreases with Ru doping. No ferromagnetism is observed in either system.     

Effects of Li Substitution and Sintering Temperature on Properties of Bi0.5（Na,K）0.5TiO3 Lead-Free Piezoelectric Ceramics

Bi0.5 （Na0.72K0.28- x Lix ）0.5 TiO3 （BNKLT- 100x） lead-free piezoelectric ceramics are synthesized by conventional solid state sintering techniques. The dielectric and piezoelectric properties of the BNKLT-100x ceramics as a function of Li content are systematically investigated. It is found that not only Li content but also the sintering temperature has a strong effect on the piezoelectric properties of BNKLT. The piezoelectric constant d33 Of BNKLT varies from 120 to 252pC/N in the Li content range from 0.03 to 0.16. In the sintering temperature range from 1080 to 1130℃, the d33 value of BNKLT-6 changes from 200pC/N to 252pC/N. The BNKLT-6 sample sintered at 1100℃ has the highest piezoelectric constant d33 of 252pC/N, with the electromechanical coupling factors kp of 0.32 and kt of 0.44.     

Mechanical Properties and Electronic Structures of Cotunnite TiO2

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO2. The calculated results show that cotunnite TiO2 has the highest bulk modulus （348 GPa） and hardness （32 GPa） among the high-pressure phases of TiO2, but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO2 can be understood from both the dense crystal structure （high valence electron density and short bond lengths） and the unusual mixtures of covalent and ionic bonding of Ti-O.     

Two Possible Configurations for Silver-C60-Silver Molecular Devices and Their Conductance Characteristics

Coherent electronic transport properties of silver-C60-silver molecular junctions in different configurations are studied using hybrid density function theory. The experimentally measured current flows of （760 molecules adsorbed on the silver surface are well reproduced by theoretical calculations. It is found that the current-voltage characteristics of the molecular junctions depend strongly on the configurations of the junctions. Transmission spectra combined with density of states can help us to understand in depth the transport properties. Different kinds of electrode construction are also discussed. With the help of the calculation, two possible configurations of silver-C60-silver molecular junctions are suggested.