First principles study of electronic structure and optical properties of double perovskite Ba2(InM)O6 [M=Nb,Ta]

The structural, electronic and optical properties of double perovskite oxides Ba₂(InNb)O₆ and Ba₂(InTa)O₆ are investigated by full-potential linearized augmented plane wave method based on density functional theory under generalized gradient approximation. The real and imaginary parts of the dielectric function and the optical constant (refractive index) are calculated. The calculated spectra are compared with the experimental results and are found to be in reasonable agreement. The interband contributions to the optical properties of these perovskite oxides have been analyzed.     

First-principles calculations of electronic,optical and elastic properties of Ba2MgWO6 double perovskite

The structural, electronic, optical and elastic properties of the cubic double perovskite Ba₂MgWO₆ were calculated using the ab initio plane wave method and compared with the available experimental data. The pressure effects were modeled by optimizing the crystal lattice structure and calculating the band gap at elevated hydrostatic pressures. The calculated values of the relative change of a unit cell volume with pressure are in excellent agreement with the recent experimental measurements [S. Meenakshi et al, J. Phys. Chem. Solids 72 (2011) 609]. The pressure coefficients of the lattice constant and the WO, MgO, BaO bonds variations were all evaluated. Elastic anisotropic properties were analyzed by calculating all independent components of the elastic constants tensor; the greatest and the smallest values of the Young's moduli were determined.     

Relaxor-like dielectric behavior in La2NiMnO6 double perovskite ceramics

The dielectric and conductive characteristics of La₂NiMnO₆ double perovskite ceramics were investigated together with the crystal structure. La₂NiMnO₆ ceramics crystallized in the monoclinic P2₁/n structure in which the Ni²⁺ and Mn⁴⁺ ions ordered periodically. Relaxor-like dielectric behavior combined with a giant dielectric constant step was observed in the present ceramics, and these unique dielectric characteristics should be attributed to the charge ordering of Ni²⁺ and Mn⁴⁺. The dielectric relaxation was well fitted by the modified Debye equation and Arrhenius law with the activation energy of 0.17 eV. The dc conductivity of La₂NiMnO₆ could be well fitted using a variable-range hopping mechanism instead of a band conduction mechanism.     

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite

The origin of dielectric anomalies and magnetodielectric response of La(2)MnCoO(6) has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn(4+)/Co(2+) valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency-temperature region where extrinsic effects dominate the dielectric response.     

Impedance spectroscopy of dielectric properties of perovskite ceramics Bi(Mg1/2Ti1/2)O3

Physics of the Solid State - The dielectric properties of metastable high-pressure perovskite ceramics Bi(Mg1/2Ti1/2)O3 were studied by impedance spectroscopy at frequencies of 25 Hz-1 MHz at...     

First principles calculations ofthe structural and electronic properties of(CdSe)n clusters

The structural and electronic properties of (CdSe)n(1≤n≤5) clusters are calculated using density functional theory within the pseudopotential and generalized gradient approximations. The calculated binding energies and highest occupied molecular orbital lowest unoccupied molecular orbital gaps are compared with those obtained within local density approximation.     

A possible coupling mechanism between magnetism and dielectric properties in EuTiO3

The dielectric constant of an incipient ferroelectric EuTiO₃ exhibits a sharp decrease at about 5.5K, at which the antiferromagnetic ordering of the Eu spins simultaneously appears. This fact indicates the existence of a coupling between the magnetism and dielectric properties of EuTiO₃. We propose a possible coupling mechanism between the magnetic and electrical subsystems as -gdsum_l dsum_〈i,j〉q²_lvec S_i·vec S_j. In the framework of soft-mode theory, we have obtained analytically a dielectric constant expression related to the spin correlation of nearest neighbours of Eu ions.     

Magnetic properties of the double perovskite compound Sr_2YRuO_6

We study the magnetic properties of the double perovskite ruthenate compound Sr_2YRuO_6 using Monte Carlo simulations(MCS).We elaborate the ground state phase diagrams for all possible and stable configurations.The magnetizations and the susceptibilities as a function of temperature for the studied system are also reported.The effects of the exchange coupling interactions and the crystal field are examined and discussed.On the other hand, since the compound Sr_2YRuO_6 exhibits an antiferromagnetic behavior, we find its Néel temperature, T_N≈ 31 K, which is in good agreement with the experimental results in the literature.To complete this study, the hysteresis loops and the coercive field as a function of the external magnetic field are also obtained for fixed values of the physical parameters.     

Half-metallic ferromagnetic nature of the double perovskite Pb2FeMoO6 from first-principle calculations

The structural, electronic and magnetic properties of the double perovskite Pb₂FeMoO₆ have been studied by using the first-principle projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). Similar to Sr₂FeMoO₆ and Pb₂FeReO₆, the optimized crystal structure of Pb₂FeMoO₆ is a body-centered tetragonal (BCT) with the lattice constants a=b=5.60 Å and c=7.94 Å. The two axial TM?O distances are slightly larger than the four equatorial TM?O distances, which shows that the Jahn–Teller structural distortion exists in FeO₆ and MoO₆ octahedra. The half-metallic ferromagnetic nature implies a potential application of this new compound in spintronics devices. The Fe³⁺ and Mo⁵⁺ ions are in the states (3d⁵, S=5/2) and (4d¹, S=1/2) with magnetic moments 3.87 and ?0.38μ_B respectively and thus there exists an antiferromagnetic coupling via oxygen between them.     

Hole doping effects in (Sr2-xNax)FeMoO6 double perovskite

Hole-doped double perovskite compounds (Sr,Na)₂FeMoO₆ with the Na content of x=0,0.02,0.03,0.07 and 0.17 have been synthesized by sol–gel method. Effects of hole doping on the crystal structure and magnetic properties of Sr₂FeMoO₆ have been investigated by means of X-ray powder diffraction (XRD) and magnetic measurements. The XRD pattern indicates that all the samples are of single phase and belong to the space group I4/m. Due to the smaller ionic radius of Na⁺ than that of Sr²⁺ ions, the lattice constants and unit cell volume of the compound decrease slightly with x. The degree of cation ordering in the Na-doped Sr₂FeMoO₆ compounds shows a non-monotonic variation with the doping level, increasing from x=0 to x=0.03 and decreasing slightly with further increase of the doping. In contrast to the composition dependence of the degree of ordering, the Curie temperature of the compound decreases at low doping level and increases at high doping level. The saturation magnetization of the compound increases with x for x<0.17. Similar to the electron-doped Sr₂FeMoO₆, provided that the doped hole enters selectively the spin-down band, the variation of the saturation magnetization can be explained in light of the ferrimagnetic model (FIM). PACS 61.72.Ww; 75.50.-y; 75.30.Cr; 75.50.Gg     

First principles study on lattice vibration and electrical properties of layered perovskite Sr_2M_2O_7(M=Nb,Ta)

In this paper, we performed calculations to investigate the dielectric, piezoelectric properties, Born effective charge(BEC), and spontaneous polarization of Sr_2M_2O_7, the method used in our study was a well-known density functional theory based on first-principles. The optimized results were in good agreement with previous experiments and calculations,which indicates that our calculated method is reasonable. The research we have done suggested that greater piezoelectric components of Sr_2Nb_2O_7 were e_(31) and e_(33), and the contributions were derived from the A_1. By studying the Born effective charge, it could be seen that the valence of ions changed, and the O of Sr_2Nb_2O_7 were most obviously that caused by the covalent character of ions and the hybridization of O-2 p and Nb-4 d. The spontaneous polarization of Sr_2Nb_2O_7 in the [001]direction is 25 μC/cm~2, while for Sr_2Ta_2O_7, there was no spontaneous polarization in the paraelectric state. Finally, the effect of pressure on the piezoelectric properties were also investigated, the polarization of Sr_2Nb_2O_7 decreased linearly with the increase after pressure. All our preliminary results throw light on the nature of dielectric, piezoelectric properties,Born effective charge, and spontaneous polarization of Sr_2M_2O_7, it was helpful for experimental research, the development of new materials, and future applications.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

First principles study of the vibrational,dielectric and thermal properties of SrClF

SrClF is an important optical crystal and has many technological applications. In this work, vibrational, dielectric and thermal properties of SrClF were investigated by density functional perturbation calculation. The calculated Born effective charges are close to their nominal ionic charges, revealing the ionic characteristic of SrClF. Group theory analysis indicates that there are two E _u and A _2u infrared modes at the Brillouin zone center of SrClF. The LO-TO splitting of these infrared modes were calculated and discussed and their vibrational modes were sketched. Static dielectric constants were studied, which show that SrClF has a larger ionic dielectric contribution than its electrons. Its birefringence was calculated and the infrared reflectance spectra were simulated, which can be used to explain the experimental findings. Based on the computed phonon dispersion curves, the lattice heat capacity, the Grüneisen parameter and the thermal expansion coefficient as functions of temperature were predicted.     

Structural and dielectric properties of (La, Bi) modified PZT ceramics

Polycrystalline samples of Pb_0.9(La_1−zBi_z)_0.1(Zr_0.55Ti_0.45)_0.975O₃ [referred as PLBZT] (z=0.0, 0.3, 0.5 and 0.7) have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis suggests the formation of single-phase compounds with monoclinic structure. The dielectric studies of the compounds as a function of temperature (room temperature, RT to 350 °C) at frequency (1, 10 and 100 kHz) show that the compounds undergo a phase transition of diffuse type. Diffusivity (γ) study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ) as a function of temperature (RT to 350 °C) at a constant biasing field suggests the compounds have negative temperature coefficient of resistance.     

Preparation, structural analysis and dielectric properties of Bi x La1−x FeO3 perovskite

Rare earth element substituted bismuth ferrites (BiFeO₃) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi _x La_1−x FeO₃ (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.     

(Nb,N)共掺杂锐钛矿电子结构和光学性质的第一性原理研究

二氧化钛（TiO₂）作为一种性能优良的光催化剂已经受到越来越多的关注.本研究采用密度泛函理论的第一性原理和广义梯度近似+U方法,对锐钛矿结构TiO₂晶体三种可能的（Nb,N）共掺杂TiO₂的几何结构、形成能、能带结构、电子密度和光吸收系数进行了研究,并与单掺杂（Nb/N）体系进行了对比.对掺杂后体系的几何结构进行的计算表明杂质原子掺入后晶格发生了不同程度的畸变.此外,（Nb,N）共掺杂体系与纯TiO₂相比,其禁带宽度和吸收边较小.同时,与N掺杂TiO₂相比,N的2p态在共掺杂情形下变为完全占据,从而减少了电子空穴对的复合.而且共掺杂体系的形成能比N单掺杂体系低,因而更加稳定.因此,（Nb,N）共掺杂可以很好地提升锐钛矿型TiO₂在可见光波段的光催化性能.     

First principles calculations on the effect of pressure on SiH4(H2)2

The effect of pressure on the strength of H₂ covalent bond in the molecular solid SiH₄(H₂)₂ has been investigated using quantum molecular dynamics simulations and charge density analysis. Our calculations show, in agreement with the implications of the experimental results, that substantial elongation of H₂ bond can be achieved at low pressures, with the onset of rapid changes close to 40 GPa. Model calculations show redistribution of charge from bonding to antibonding states to be responsible for the behavior. Our computed Raman spectra confirm the dynamic exchange of hydrogen atoms speculated to be operative in SiH₄–D₂ mixture by experiments. This exchange is shown to be a three step process driven by thermal fluctuations.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Synthesis, crystal structure, dielectric and optical properties of a new rare earth double perovskite: Ca2CeNbO6

In order to develope and understand the phenomena involved in producing advanced materials, a rare earth double perovskite oxide calcium cerium niobate, Ca₂CeNbO₆ (CCN) is synthesized for the first time. The x-ray diffraction pattern of CCN at room temperature (300K) shows orthorhombic perovskite structure, with the lattice parameters, a=9.36Å, b=6.61Å and c=5.88Å and α=β=γ= 90°. A scanning electron micrograph shows the formation of grains with average size ∼2μm. Impedance spectroscopy and Fourier transform infrared spectroscopy are applied to investigate the dielectric and optical properties of CCN. The frequency-dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. The experimental data of real part of dielectric permittivity (ε′) and imaginary part of electric modulus (M″) are fitted with Davidson-Cole equation to explore the idea of dielectric relaxation (conduction) mechanism in CCN. The frequency-dependent conductivity spectra follow a power law. The scaling behaviour of imaginary electric modulus (M″) suggests that the relaxation describes the same mechanism at various temperatures.     

Magnetic ordering in double perovskites R2CoMnO6 (R = Y, Tb) investigated by high resolution neutron spectroscopy

We have investigated low energy nuclear spin excitations in double perovskite compounds R(2)CoMnO(6) (R=Y, Tb) by inelastic neutron scattering with a high resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 μeV for Y(2)CoMnO(6) and also for Tb(2)CoMnO(6) at T = 2 K in both energy-loss and energy-gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of the (59)Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature T(C). The energy of the low energy excitations decreases continuously and becomes zero at T(C) ≈ 75 K for Y(2)CoMnO(6) and T(C) ≈ 100 K for Tb(2)CoMnO(6). For Tb(2)CoMnO(6), which contains magnetic rare earth ions, additional quasielastic scattering due presumably to the fluctuations of large Tb magnetic moments was observed. The present study reveals the magnetic ordering of the Co sublattice. The results of this investigation along with that obtained by us for other compounds indicate the presence of unquenched orbital moments in some of the Co compounds.