Dielectric response in ferroelectric BaTiO3

The far-infrared optical and dielectric properties of ferroelectric perovskite titanate powder BaTiO₃ are reported. The terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low frequency dielectric response of BaTiO₃ is closely related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near at 180 cm⁻¹, which is verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon mode for ferroelectric materials. Combining terahertz TDS with Raman spectra, the overall low frequency optical phonon response of BaTiO₃ is presented in an extended spectral range from 6.7 to 1200 cm⁻¹.     

Orthorhombic YAlO3 – a novel many‐phonon SRS‐active crystal

In single crystals of orthorhombic YAlO₃, widely known as a host‐matrix for Ln³⁺‐lasant ions, many‐phonon stimulated Raman scattering interactions as well as different manifestations of cascaded and cross‐cascaded nonlinear χ⁽³⁾↔χ⁽³⁾ processes are initiated by picosecond laser pulses. The scientific and applicative potential of YAlO₃ crystals is considerably expanded by the demonstration of its SRS properties. In particular, the studies revealed the manifestation of eight χ⁽³⁾‐active vibrational modes. The corresponding Stokes and anti‐Stokes lines have been assigned and the steady‐state Raman gain coefficients related to the strongest phonon mode have been estimated. In addition, a short review presents the stimulated emission channels of its Ln³⁺‐ions together with some χ⁽³⁾‐nonlinear laser properties of crystals belonging to the binary Y₂O₃‐Al₂O₃ system.     

Lattice dynamics properties of XAs (X=Al,Ga and In) with zinc-blende structure from first-principle calculations

Band structures, density of states, dielectric and vibrational properties of XAs (X=Al, Ga and In) alloys with zinc-blende structure have been studied using the density functional theory (DFT). The calculated lattice constants, band gap, static dielectric constants and phonon frequencies are all in good agreement with the available experimental data and other theoretical results. The calculated results show that Born effective charges Z_B increase with cation mass. A similar tendency has been observed for phonon frequencies ω_TO and ω_LO. Calculation results prove that static dielectric constants ε(0) increase with atomic weight, i.e. in the sequences AlAs–GaAs–InAs, and show an inverse sequence for band gap.     

First-principles calculations of the dielectric and vibrational properties of ferroelectric and paraelectric BaAl2O4

First-principles calculations have been conducted to study the structural, dielectric, and vibrational properties of ferroelectric and paraelectric BaAl₂O₄. High-frequency and static dielectric constants, and phonon frequencies at the Brillouin zone center are reported. Both BaAl₂O₄ polymorphs are promising infrared-transparent materials due to their low electronic dielectric constants. The ferroelectric and paraelectric BaAl₂O₄ have much smaller permittivity compared to the classical ferroelectric materials. From an atomic nanostructure standpoint, the abnormally low permittivity of BaAl₂O₄ polymorphs is mainly related to low coordination numbers of Ba (9) and Al (4).     

Pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide AlAs with zinc-blende (B3) structure and nickel arsenide (B8₁) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B8₁ is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies are varying in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of transversal acoustic mode at X point supports the phase transition in AlAs.     

First principles lattice dynamical study of the cubic antiperovskite compounds AsNBa3 and SbNBa3

An ab initio pseudopotential plane wave method using linear response approach has been employed to study the lattice dynamics of two cubic antiperovskites AsNBa₃ and SbNBa₃. The bulk properties, elastic constants, phonon dispersion curves, phonon density of states and temperature dependent thermodynamic quantities of both antiperovskites are obtained. The calculated lattice constants, elastic and bulk properties are compared with the available theoretical data. This is the first systematic and quantitative prediction of phonon and thermodynamical properties of these antiperovskite compounds.     

Vibrational properties of single-wall BC3 nanotubes

The phonon dispersions of single-wall BC₃ nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC₃ tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom-atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC₃ sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC₃ tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC₃ tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Phonon dispersion and dielectric behaviour of mixed alkali halide crystal NaCl0.5Br0.5

In the present paper we have calculated the phonon dispersion curves and dielectric constants, their volume derivatives and the Gruneisen parameters of the mixed alkali halide NaCl_0.5Br_0.5, using the extended three body force shell model. The results of dielectric constants, their volume derivatives and Gruneisen parameters are found to agree reasonably well with the experimental values, which establishes the validity of the three body charge transfer parameter. With this concept the phonon dispersion curves are obtained in all the three symmetry directions (q,o,o), (q,q,o) and (q,q,q), which will be of interest for experimental workers, since no experimental values have so far been obtained for phonon frequencies.     

Cascaded and cross-cascaded χ(3) nonlinear optical effects in a new SRS-active YAlO3 crystal

Stimulated Raman scattering (SRS) is discovered under picosecond laser excitation in a YAlO₃ orthorhombic crystal, and its cascaded and cross-cascaded χ⁽³⁾ nonlinear optical interactions are studied. Investigations revealed three SRS-active phonon modes of the crystal and identified all its recorded Stokes and anti-Stokes generation lines.     

Infrared reflectance spectrum of BN calculated from first principles

Using the linear response theory, vibrational and dielectric properties are calculated for c-BN, w-BN and h-BN. Calculations of the zone-center optical-mode frequencies (including LO-TO splittings) are reported. All optic modes are identified and excellent agreement is found between the theory and experimental results. The static dielectric tensor is decomposed into contributions arising from individual infrared-active phonon modes. It is found that all the structures have a smaller lattice dielectric constants than those of the electronic contributions. Finally, the infrared reflectance spectrums are presented. Our theoretical results indicate that w-BN shows a similar reflectivity spectrum as c-BN. It is difficult to tell apart the wurtzite structure from the zinc blende phase by IR spectroscopy.     

A comparative study of dielectric and Raman spectroscopy of Pb(Y b1/2Ta1/2)O3 and Pb(Y b1/2Nb1/2)O3

A comparative study of the properties of two highly ordered lead based complex perovskites Pb(Y b_1/2Ta_1/2)O₃ and Pb(Y b_1/2Nb_1/2)O₃ has been carried out through x-ray diffraction, dielectric and Raman scattering measurements. These two compounds differ significantly in their structure, dielectric response and phonon vibration although the ionic radii and valencies are same for Ta and Nb. The room temperature x-ray diffraction pattern and Raman spectra show that the symmetry of lead ytterbium tantalate is lower than that of lead ytterbium niobate. The Raman spectra of Pb(Y b_1/2Ta_1/2)O₃ also indicates the presence of local distortion in the lattice which may be one of the factors responsible for the existence of a secondary transition.     

CeO2的电子结构，光学和晶格动力学性质：第一性原理研究

基于考虑了Ce-4f电子间的库仑作用U和交换作用J的LDA+U方案,应用第一性原理计算系统研究了CeO₂的电子结构,光学和晶格动力学性质.电荷密度和电子局域函数的分布特征表明,CeO₂是属于共价键的绝缘体.介电常数、玻恩有效电荷张量和声子色散曲线的计算值和相应的实验结果符合得比较好. 关键词： 电子结构 光学性质 晶格动力学 第一性原理计算     

Elastic modulus,optical phonon modes and polaron properties in Al1−xBxN alloys

The results of first-principles plane-wave-based pseudopotential method calculations of the elastic modulus, optical phonon modes, and polaron properties of zinc-blende Al_1?xB_xN are presented. Good agreement is obtained with the exciting experimental data for AlN and BN compounds. Our results provide predictions for the remaining mixed crystals Al_1?xB_xN (0 < x < 1) for which no direct experimental or theoretical data are presently available. The compositional dependence of features such as elastic constants and their related mechanical parameters, phonon frequencies, dielectric constants, and polaron parameters was investigated. The study may be useful for the characteristic analysis of AlBN-based quantum well devices.     

Luminescence of La and Sc impurity centers in YAlO3 single-crystalline films

The luminescence of La_Y³⁺ and Sc_Y³⁺ and Sc_Al³⁺ centers created by lanthanum and scandium ions at Y³⁺ and Al³⁺ cation sites of YAlO₃ perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO₃:La and YAlO₃:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.     

Vibrational and dielectric properties of magnesium aluminate spinel: A first-principles study

The vibrational and dielectric properties of MgAl₂O₄ are investigated within the framework of density functional perturbation theory. Results of phonon frequencies at the Brillouin zone center, static dielectric constant, and electronic dielectric constant are reported. In comparison with experimental results, we find that the generalized gradient approximation potential results in more accurate phonon frequencies than local density approximation potential does. Dielectric, refractive index, extinction coefficient and infrared reflectance spectra of MgAl₂O₄ are given, and the figures suggest that MgAl₂O₄ presents good transmission properties in the spectrum range above 1000 cm−1 and below 300 cm−1.     

The pressure influence on the incommensurate-commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH₂C₅H₄NH] SbCl₄ (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure-temperature phase diagram was given. The paraelectric-ferroelectric phase transition (II→III) temperature (T_c) increases linearly with increasing pressure with a slope dT_c/dp=21×10⁻² K/MPa. The pressure dependence of Curie-Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C₊) was pressure dependent whereas the C₋ constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.     

A first-principles investigation of the properties of two predicted novel structures of Sn3P4

Two novel structures of Sn₃P₄ (t-Sn₃P₄ and o-Sn₃P₄) are presented in this study. For two novel phases, t-Sn₃P₄ and o-Sn₃P₄, the stabilities are verified based on the elastic constants and the phonon dispersion spectra. The phonon density of states (PHDOS), band structures, electronic density of states (DOS) and optical properties are investigated using density functional theory (DFT). o-Sn₃P₄ has better plasticity and stronger anisotropy than t-Sn₃P₄. The PHDOS in the lower frequency band is mainly derived from Sn atoms, and in the higher band it is mainly from P atoms. The band structure of t-Sn₃P₄ shows that it is a narrow indirect band gap semiconductor. And o-Sn₃P₄ presents a metallic characteristics. Below the Fermi level, the total DOS in the valence band originates mainly from P ‘p’ with the partial contribution from Sn ‘p’. The dielectric constants, conductivities, optical absorption, optical reflectivity, and energy loss functions of t-Sn₃P₄ and o-Sn₃P₄ are analyzed. For t-Sn₃P₄, the static dielectric constant is 12.66?F/m, the real part of conductivity reaches the maximum at 4.45?eV, and the peak in the loss function locates at about 15?eV. For o-Sn₃P₄, in order, they are 47.27?F/m, at 2.59?eV, and at 10.91?eV.     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.     

First-principles study of structural, elastic, electronic, and optical properties of hexagonal BiAlO3

The structural parameters, elastic, electronic, and optical properties of hexagonal BiAlO₃ were investigated by the density functional theory. The calculated structural parameters are in good agreement with previous calculation and experimental data. The structural stability of BiAlO₃ has been confirmed by calculation of the elastic constants. The energy band structure, density of states, and Mulliken charge populations were obtained. BiAlO₃ presents an indirect band gap of 3.28 eV. Furthermore, the optical properties were calculated and analyzed. It is shown that BiAlO₃ is a promising dielectric material.