Static Dielectric Tensor Research Articles

Second-harmonic generation tensors from high-throughput density-functional perturbation theory

Optical materials play a key role in enabling modern optoelectronic technologies in a wide variety of domains such as the medical or the energy sector. Among them, nonlinear optical crystals are of primary importance to achieve a broader range of electromagnetic waves in the devices. However, numerous and contradicting requirements significantly limit the discovery of new potential candidates, which, in turn, hinders the technological development. In the present work, the static nonlinear susceptibility and dielectric tensor are computed via density-functional perturbation theory for a set of 579 inorganic semiconductors. The computational methodology is discussed and the provided database is described with respect to both its data distribution and its format. Several comparisons with both experimental and ab initio results from literature allow to confirm the reliability of our data. The aim of this work is to provide a relevant dataset to foster the identification of promising nonlinear optical crystals in order to motivate their subsequent experimental investigation.

Automated all-functionals infrared and Raman spectra

AbstractInfrared and Raman spectroscopies are ubiquitous techniques employed in many experimental laboratories, thanks to their fast and non-destructive nature able to capture materials’ features as spectroscopic fingerprints. Nevertheless, these measurements frequently need theoretical and computational support in order to unambiguously decipher and assign complex spectra. Linear-response theory provides an effective way to obtain the higher-order derivatives needed, but its applicability to modern exchange-correlation functionals and pseudopotential formalism remains limited. Here, we devise an automated, open-source, user-friendly approach based on density-functional theory and the electric-enthalpy functional to allow seamless calculation from first principles of infrared absorption and reflectivity, together with zone-center phonons, static dielectric tensor, and Raman spectra. By employing a finite-displacement and finite-field approach, we allow for the use of any functional, as well as an efficient treatment of large low-symmetry structures. Additionally, we propose a simple scheme for efficiently sampling the Brillouin zone at different electric fields. To demonstrate the capabilities of the present approach, we study ferroelectric LiNbO3 crystal as a paradigmatic example, and predict infrared and Raman spectra using various (semi)local, Hubbard corrected, and hybrid functionals. Our results also show how PBE0 and extended Hubbard functionals (PBEsol+U+V) yield for this case the best match in term of peak positions and intensities, respectively.

High Dielectric Constants in BaTiO3 Due to Phonon Mode Softening Induced by Lattice Strains: First Principles Calculations

AbstractHigh‐dielectric‐constant materials attract much attention due to their broad applications in modern electronics. Barium titanate (BTO) is an established material possessing an ultrahigh dielectric constant; however, a complete understanding of the responsible underlying physical mechanism remains elusive. Here a set of density‐functional‐theory calculations for the static dielectric tensors of barium titanate under strain has been performed. The dielectric constant increases to ≈7300 under strain. The analysis of the computed vibrational modes shows that transverse vibrational mode softening (the appearance of low‐frequency modes) is responsible for this significant increase as driven by the relationship between lattice contribution for the static dielectric constant (k) and vibrational frequency (ω), i.e., . The relevant vibrational mode indicates a large counter‐displacement of Ti ions and O anions, which greatly enhances electrical dipoles to screen the electric field. The calculations not only interpreted experimental data on the high dielectric constants of BTO, where the lattice deformation due to the strains from the grain nanostructure plays an important role, but also pointed to exploring high‐throughput calculations to facilitate the discovery of the advanced dielectric materials. Moreover, the calculations can prove useful for doped BTO, for which local strains fields can be achieved using defect engineering.

Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB.

We report on the temperature dependence of birefringence and of the static dielectric permittivity tensor in a series of binary mixtures between the symmetric, bent-shaped, 1'',9''-bis(4-cyanobiphenyl-4'-yl)nonane (CB9CB) dimer and the monomeric nematogen 5CB. In the studied composition range the mixtures exhibit two nematic phases with distinct birefringence and dielectric features. Birefringence measurements are used to estimate the temperature dependence of the tilt between the axis defining the nanoscale helical modulation of the low temperature nematic phase with the (local) direction of the maximal alignment of the cyanobiphenyl units. Planar as well as magnetically and/or electrically aligned samples are used to measure the perpendicular and parallel components of the dielectric permittivity in both nematic phases. A self-consistent molecular field theory that takes into account flexibility and symmetry of the constituent mesogens is introduced for the calculation of order parameters and intra-molecular orientational dipolar correlations of the flexible dimers as a function of temperature/concentration. Utilising the tilt angle, as calculated from the birefringence measurements, and the predictions of the molecular theory, dielectric permittivity is modelled in the framework of the anisotropic version of the Kirkwood-Fröhlich theory. Using the inter-molecular Kirkwood correlation factors as adjustable parameters, excellent agreement between theory and permittivity measurements across the whole temperature range and composition of the mixtures is obtained. The importance of the orientational, intra- and inter-molecular, dipolar correlations, their relative impact on the static dielectric properties, as well as their connection with the local structure of the nematic phases of bent-shaped bimesogens, is discussed.

Ab initio моделирование диэлектрических и оптических свойств льдов I-=SUB=-h-=/SUB=-, I-=SUB=-II-=/SUB=- и решеток гидратов sI, sH

The results of calculating the dielectric and optical characteristics of solid polymorphic phases of water—ices Ih, III and lattices of hydrates sI, sH—are presented. Static dielectric tensors εik and complex frequency-dependent tensors εik(ω) are calculated for these materials. It is shown that, in terms of optical properties, the crystal lattices Ih, III, and sH are uniaxial, the tensor components εxx(ω) and εyy(ω) coincide for them, and the hydrate lattice sI is isotropic. Based on the calculated frequency-dependent dielectric functions εik ′ (ω) and εik ′′ (ω), important optical characteristics were obtained: reflection R(ω), absorption a(ω), loss function L(ω), refractive indices n(ω) and k(ω). Comparison of the dielectric and optical spectra of the sI and sH gratings with the known spectra for methane hydrate sI revealed a broadening of the spectra in the high-energy direction. For the unfilled hydrate sI, a reflection peak was found at an energy of 17.3 eV, the appearance of which is associated with a change in the electronic structure of the crystal in the absence of a methane molecule. Qualitative agreement is obtained between the reflection spectra R(ω) and the functions εik ′ (ω), εik ′′(ω), calculated by quantum mechanical simulation, with experimental spectroscopy data for hexagonal and amorphous ices.

The Optoelectronic Property Analysis of P doped SrGeN2: A First Principle Calculation for Solar Cell Compound

The first principle investigation of P doped SrGeN2 - a wideband semiconductor is undertaken to prove the beneficial usage of the compound in a particular optoelectronic application field. Wideband semiconductor is the semiconductor compound that contains a larger energy gap difference in comparison to the traditional semiconductors. But to utilize the compound in the form of a particular application like solar cell, bandgap engineering for the compound is followed to reduce the bandgap as per required value range. For this purpose, doping feature is introduced in the bandgap engineering procedure and can be done at a particular site to gain the desired energy gap. Thus the investigation of the properties to learn about the optical, electronic and structural features of the P doped SrGeN2 compound is done using the Tran Blaha modified Becke Johnson (TB-mBJ) exchange correlation. The exchange correlation is rooted in the framework of DFT based on the first principle-linear augmented plane wave methodology. This paper deals with varied property analysis schemes like structural, optical and electronic behaviour deployed in DFT framework stating the value of absorption curve and dielectric tensor which is analysed for the optical property study with integrated absorption curve value being 33.681 and static dielectric tensor value ε (ω) being 4.28 in case of P doped SrGeN2 gained through mBJ potential.

Isotropic or anisotropic screening in black phosphorous: Can doping tip the balance?

Black phosphorus (BP), a layered van der Waals (vdW) crystal, has unique in-plane band anisotropy and many resulting anisotropy properties such as the effective mass, electron mobility, optical absorption, thermal conductivity and plasmonic dispersion. However, whether anisotropic or isotropic charge screening exist in BP remains a controversial issue. Based on first-principles calculations, we study the screening properties in both of single-layer and bulk BP, especially concerning the role of doping. Without charge doping, the single-layer and bulk-phase BP show slight anisotropic screening. Electron and hole doping can increase the charge screening of BP and significantly change the relative static dielectric tensor elements along two different in-plane directions. We further study the charge density change induced by potassium (K) adatom near the BP surface, under different levels of charge doping. The calculated two-dimensional (2D) charge redistribution patterns also confirm that doping can greatly affect the screening feature and tip the balance between isotropic and anisotropic screening. We corroborate that screening in BP exhibits slight intrinsic anisotropy and doping has significant influence on its screening property.

Theoretical and Experimental Overview of Structural, Dielectric, Crystallographic, Electronic, Optical, and Physical Tensors of α-DIPAB and Iodine-Doped α-DIPAB Molecular Ferroelectric Crystals

Self-consistent ab initio calculations of the structural, crystallographic, electronic, optical, and physical properties as well as spontaneous polarization of P21 (α)-ferroelectric phase of diisopropylammonium bromide (α-DIPAB) molecular crystal are carried out. Examination of the total density of states as obtained using the generalized gradient approximation and hybrid exchange–correlation functional (HSE06) methods yields an optical bandgap of ∼ 5 eV and 6 eV, respectively. Furthermore, by using the Berry phase approach, we found that polar DIPAB exhibits a spontaneous polarization of 22.64 μC/cm2, indicating that this remarkable ferroelectric molecule could be used as an alternative to replace several known ferroelectrics for piezoelectric and optoelectronic applications such as capacitors, sensors, and data storage as well as microelectromechanical system (MEMS) devices. Using the vdW + DF2 approximation, we identify the majority of the vibrational modes in the Raman spectra and analyze the modes generated by the bromine (Br) deficiency. We find that the Br deficiency strongly affects the electric and elastic properties of α-DIPAB. Furthermore, the elastic, dielectric, and piezoelectric tensors of α-DIPAB are calculated and interpreted. α-DIPAB is found to exhibit a static dielectric tensor of ∼ 2.5, i.e., slightly smaller than that of typical perovskite-based ferroelectrics. In addition, the clamped-ion piezoelectric tensor is calculated. We found that the piezoelectric coefficient e21 corresponding to applying uniaxial strain in the direction perpendicular to the N–C bonds is smaller than the e22 principle coefficient. Moreover, the components e15 = 0.220 C/m2 and e35 = −0.2032 C/m2 correspond to applying shear strain η31, indicating a reasonable piezoelectric response of this polar crystal and making it a cheap attractive candidate for piezoelectric applications. The components of the elastic moduli tensor are calculated and interpreted. The α-DIPAB exhibits Young’s modulus of up to 50 GPa along specific crystalline directions despite its relatively low hardness. We attribute such a large value to the presence of a dense H–Br bond network between the Br and DIPAB molecule. The Poisson ratio is also strongly anisotropic, with values ranging from 0.5 to 0.2. The DIPAB systems are brittle based on the ratio between the bulk and shear elastic constants, suggesting that this material has potential for use in flexible electronic applications. A real-part dielectric anomaly at T = 140 K induced by iodine doping of α-DIPAB exhibits switchable behavior. This abnormality can be attributed to suppression of ferroelectricity induced by trapping of vortices at dislocation defects generated by iodine. Another dielectric anomaly found in the vicinity of the Curie temperature of α-DIPAB can be explained in terms of a structural phase-induced ferroelectric-to-paraelectric transition.

Physical properties of prop-2-en-1-one based single crystals using molecular mechanics

A study of mechanical properties of prop-2-en-1-one based compounds using molecular dimension simulations was investigated. Several mechanical properties have been calculated such as elastic constant tensors, shear modulus, bulk modulus, Young’s modulus and Poisson’s ratio. The s-wave and p-wave velocities as well as static dielectric tensor were also calculated. A comparison of these properties with the number of carbon atoms have been carried out to obtain a correlation between molecular structure and mechanical properties and all calculations were performed using GULP program. Correlation between one of the physical parameter and molecular weight has been shown using Functional analysis method. The present study provides a concrete basis to discuss the dominant role played by elastic constants and the critical values associated with these lead to an extent of elastic displacement in the form of 3-dimensional shape which is novel in the context of single crystal analysis. Validation of these results is done in relation to number of carbon atoms and their experimental elastic constants measurements.

Measurement and ab initio Investigation of Structural, Electronic, Optical, and Mechanical Properties of Sputtered Aluminum Nitride Thin Films

Aluminum Nitrides (AlN) thin films have been deposited ‎on glass substrates using DC-magnetron sputtering technique for different back pressure. The piezoelectric and related properties of highly c-axis oriented AlN films fabricated by dc planar magnetron sputtering have been calculated. Experimental results show that highly c-axis oriented AlN films can be fabricated by dc planar magnetron sputtering. X-ray powder diffraction (XRD) technique shows that AlN thin films exhibit a hexagonal structure. Vienna Ab initio Simulation Package (VASP) within the framework of density functional theory (DFT) and generalized gradient approximation (GGA) was used to investigate the structural and electronic properties of hexagonal AlN structures. The experimental lattice parameters of the as prepared thin films are found to be in good agreement with ab-initio calculated parameters. UV–Vis spectrophotometer measurements are performed to investigate the optical properties of AlN thin films. We found that the refractive index of AlN thin films exhibits values ranging between 2.1 and 2.2. Furthermore, the elastic, piezoelectric and dielectric tensors of AlN crystal are calculated using VASP. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of AlN. The value of the principle component of electronic contribution to the static dielectric tensor of AlN is found to be ≈ 4.68 that is in good agreement with the experimental static dielectric constant. In addition, clamped-ion piezoelectric tensor is calculated. The diagonal components of the piezoelectric tensor are found to be e_33=1.784 C/m^2 and e_31=-0.8 C/m^2. The large values of the piezoelectric coefficients show that polar AlN crystal exhibits a strong microwave piezoelectric effect. Furthermore, the components of the elastic moduli tensor are calculated. The extraordinary electronic, optical, piezoelectric and elastic properties make AlN thin films potential candidates for several opto-electronic, elastic, dielectric and piezoelectric applications.

First-Principles Calculation of Physical Tensors of α-Diisopropylammonium Bromide (α-DIPAB) Molecular Ferroelectric Crystal

We report accurate calculations of tonsorial elements of α-Diisopropylammonium bromide (α-DIPAB) molecular ferroelectric crystal. In particular, elastic, piezoelectric and dielectric tensors were calculated using Vienna ab initio simulation package (VASP) within the framework of density functional (DFT) theory. The determination of above parameters allows an accurate description of the energy landscape for modeling of realistic devices at finite temperatures. We determine the major physical tensors in energy expansion of total energy per volume of un-deformed crystal to provide experimentalists with valuable information for designing and fabrication of pyroelectric detectors, capacitors, piezoelectric ferroelectric memories, display devices, electro-optic devices, and ultrasound transducers based on α-DIPAB. The spontaneous polarization P_s was calculated using Berry phase approach and found to be 22.64 μC⁄〖cm〗^2 in agreement with reported theoretical value. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of α-DIPAB to get a deeper insight into the bonding network. The neighboring layers of DIPA molecules were found to be strongly crenelated due to the strong short-ranged electrostatic repulsion between Br sites in the DIPAB crystal structure. The organization of species in DIPA molecular layer as well as in the bromine “stitching” layer is essential for accurate calculation of DIPAB elastic properties. Having understood the actual network bonding in α-DIPAB, we calculated the components of the elastic moduli tensor. Our results indicate that a Young’s modulus of 10-180 GPa and a shear modulus of 4-26 GPa were found. Thus, α-DIPAB phase has a great potential to be a terrific candidate for flexible electronic device applications. The value of the principle component of electronic contribution to the static dielectric tensor of α-DIPAB is found to be ≈ 2.5, i.e. 50% smaller than that in typical perovskite-based ferroelectrics. Therefore, α-DIPAB is anticipated to exhibit creative materials' innovations. It offers highly customized features for specific applications, including insulation on polymer thick film conductive inks in the manufacture of membrane switches and flex circuits. Furthermore, clamped-ion piezoelectric tensor is calculated. Our results indicate a reasonable piezoelectric response of this polar crystal making it a low cost attractive candidate for piezoelectric applications.

Field-deformational effects in GPI ferroelectric materials

ABSTRACTThe effects appearing under the simultaneously applied mechanical stress and electric field are studied. We use a model of glycinium phosphite ferroelectric, modified by taking into account of piezoelectric coupling of the ordering structure elements with the lattice strains . Within the framework of two-particle cluster approximation, the components of polarization vector and the static dielectric permittivity tensor as well as piezoelectric and thermal characteristics are calculated. The separate and simultaneous effects of shear stress and electric field on the ferroelectric phase transition in the crystal are studied and mutually compared.

Influence of Longitudinal Electric Field on Thermodynamic Properties of NH3CH2COOH·H2PO3 Ferroelectric

Using a modified model of ferroelectric glycine phosphite by considering the piezoelectric coupling with strains e1 within the two-particle cluster approximation, the expressions for the polarization vector, static dielectric permittivity tensor, piezoelectric coefficients, and elastic constants of the crystal in the presence of a longitudinal electric field E2 are calculated. An analysis of the influence of this field on thermodynamic characteristics of the crystal is carried out. The dependence of effective dipole moments on order parameters is taken into account. This allowed us to agree the effective dipole moments in the ferro- and paraelectricphases and to describe the smearing of phase transition under the influence of an electric field. The satisfactory quantitative description of the available experimental data for these characteristics has been obtained at the proper choice of the model parameters.

Anisotropy and phonon modes from analysis of the dielectric function tensor and the inverse dielectric function tensor of monoclinic yttrium orthosilicate

We determine the frequency dependence of the four independent Cartesian tensor elements of the dielectric function for monoclinic symmetry Y$_2$SiO$_5$ using generalized spectroscopic ellipsometry from 40-1200 cm$^{-1}$. Three different crystal cuts, each perpendicular to a principle axis, are investigated. We apply our recently described augmentation of lattice anharmonicity onto the eigendielectric displacement vector summation approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)], and we present and demonstrate the application of an eigendielectric displacement loss vector summation approach with anharmonic broadening. We obtain excellent match between all measured and model calculated dielectric function tensor elements and all dielectric loss function tensor elements. We obtain 23 A$_{\mathrm{u}}$ and 22 B$_{\mathrm{u}}$ symmetry long wavelength active transverse and longitudinal optical mode parameters including their eigenvector orientation within the monoclinic lattice. We perform density functional theory calculations and obtain 23 A$_{\mathrm{u}}$ symmetry and 22 B$_{\mathrm{u}}$ transverse and longitudinal optical mode parameters and their orientation within the monoclincic lattice. We compare our results from ellipsometry and density functional theory and find excellent agreement. We also determine the static and above reststrahlen spectral range dielectric tensor values and find a recently derived generalization of the Lyddane-Sachs-Teller relation for polar phonons in monoclinic symmetry materials satisfied [M. Schubert, Phys. Rev. Lett. 117, 215502 (2016)].

Empirical optimization of DFT + U and HSE for the band structure of ZnO

ZnO is a well-known wide band gap semiconductor with promising potential for applications in optoelectronics, transparent electronics, and spintronics. Computational simulations based on the density functional theory (DFT) play an important role in the research of ZnO, but the standard functionals, like Perdew–Burke–Erzenhof, result in largely underestimated values of the band gap and the binding energies of the Zn3d electrons. Methods like DFT + U and hybrid functionals are meant to remedy the weaknesses of plain DFT. However, both methods are not parameter-free. Direct comparison with experimental data is the best way to optimize the computational parameters. X-ray photoemission spectroscopy (XPS) is commonly considered as a benchmark for the computed electronic densities of states. In this work, both DFT + U and HSE methods were parametrized to fit almost exactly the binding energies of electrons in ZnO obtained by XPS. The optimized parameterizations of DFT + U and HSE lead to significantly worse results in reproducing the ion-clamped static dielectric tensor, compared to standard high-level calculations, including GW, which in turn yield a perfect match for the dielectric tensor. The failure of our XPS-based optimization reveals the fact that XPS does not report the ground state electronic structure for ZnO and should not be used for benchmarking ground state electronic structure calculations.

The effect of hydrostatic pressure on thermodynamic characteristics of NH3CH2COOH·H2PO3 type ferroelectric materials

The model of NH$_3$CH$_2$COOH$\cdot$H$_2$PO$_3$, modified by taking into account the piezoelectric coupling between the ordering structure elements and the strains $\varepsilon_i$, $\varepsilon_j$, is used for investigation of the effects that appear under external pressures. Within two-particle cluster approximation, the components of polarization vector and static dielectric permittivity tensor of the mechanically clamped and free crystals, their piezoelectric and thermal characteristics are calculated. The effect of hydrostatic pressure on the phase transition and the calculated physical characteristics of the crystal is studied. A good quantitative description of experimental data for these crystals is obtained.

Exploring the electronic structure and optical properties of double perovskite Ba2RESbO6 (RE = Ho, Er) from first-principles calculations

ABSTRACTFirst principles density functional theory (DFT) calculations are performed to explore the electronic structure and optical properties of Ba2RESbO6 (RE = Ho, Er) in the cubic symmetry using the experimental lattice parameters. The calculations are performed in two different DFT approaches, FP-LAPW method and plane wave pseudo potential method. The electronic band structures of these compounds were analyzed using the generalized gradient approximation (GGA). A direct band gap (Γ-Γ for Ba2HoSbO6 and Ba2ErSbO6 was obtained. The calculated optical properties were analyzed from band structure calculations. The Born effective charges and the static dielectric tensor of these materials were also calculated.

Enhanced Energy-Storage Density and High Efficiency of Lead-Free CaTiO3-BiScO3 Linear Dielectric Ceramics.

A novel lead-free (1 - x)CaTiO3-xBiScO3 linear dielectric ceramic with enhanced energy-storage density was fabricated. With the composition of BiScO3 increasing, the dielectric constant of (1 - x)CaTiO3-xBiScO3 ceramics first increased and then decreased after the composition x > 0.1, while the dielectric loss decreased first and increased. For the composition x = 0.1, the polarization was increased into 12.36 μC/cm2, 4.6 times higher than that of the pure CaTiO3. The energy density of 0.9CaTiO3-0.1BiScO3 ceramic was 1.55 J/cm3 with the energy-storage efficiency of 90.4% at the breakdown strength of 270 kV/cm, and the power density was 1.79 MW/cm3. Comparison with other lead-free dielectric ceramics confirmed the superior potential of CaTiO3-BiScO3 ceramics for the design of ceramics capacitors for energy-storage applications. First-principles calculations revealed that Sc subsitution of Ti-site induced the atomic displacement of Ti ions in the whole crystal lattice, and lattice expansion was caused by variation of the bond angles and lenghths. Strong hybridization between O 2p and Ti 3d was observed in both valence band and conduction band; the hybridization between O 2p and Sc 3d at high conduction band was found to enlarge the band gap, and the static dielectric tensors were increased, which was the essential for the enhancement of polarization and dielectric properties.

Vibrational and dielectric properties of AlN: A first-principles study

The vibrational and dielectric properties of zinc-blende aluminum nitride (B3-AlN) were investigated within the framework of density functional perturbation theory. The phonon frequencies at the Brillouin zone center, the static dielectric constant, and the electronic dielectric constant were reported. The results of our studies were analyzed and discussed with respect to the available experimental and theoretical data. We found that the generalized gradient approximation potential resulted in more accurate phonon frequencies than those obtained using the local density approximation potential. The contributions to the static dielectric tensor due to purely electronic screening and infrared-active phonon modes were analyzed. The phonon, dielectric, refractive index, extinction coefficient and infrared reflectance spectra of B3-AlN were reported, and their values suggest that B3-AlN has good transmission properties in the spectral range above 1000cm−1 and below 400cm−1.

Vibrational properties ofLiNb1−xTaxO3mixed crystals

Congruent lithium niobate and lithium tantalate mixed crystals have been grown over the complete compositional range with the Czochralski method. The structural and vibrational properties of the mixed crystals are studied extensively by x-ray diffraction measurements, Raman spectroscopy, and density functional theory. The measured lattice parameters and vibrational frequencies are in good agreement with our theoretical predictions. The observed dependence of the Raman frequencies on the crystal composition is discussed on the basis of the calculated phonon displacement patterns. The phononic contribution to the static dielectric tensor is calculated by means of the generalized Lyddane-Sachs-Teller relation. Due to the pronounced dependence of the optical response on the Ta concentration, lithium niobate tantalate mixed crystals represent a perfect model system to study the properties of uniaxial mixed ferroelectric materials for application in integrated optics.