Thermoelectric studies of IV–VI semiconductors for renewable energy resources

In the present study we explore the structural, electronic and thermoelectric properties of IV-VI semiconductors using first principle calculations based on the density functional theory. Generalized gradient approximation (GGA-PBEsol) is used as an exchange-correlation functional for the structural properties of the different crystal phases of these semiconductors. The compounds SnS, SnSe, GeS and GeSe favor orthorhombic structure, lead chalcogenides (PbS, PbSe and PbTe) are stable in the cubic rocksalt structure, while SnS₂ and SnSe₂ exist in hexagonal structure. For band structures additional to GGA, modified Becke and Johnson (mBJ) exchange potential is used. These compounds are narrow band gap semiconductors in the energy range 0.2–2.8 eV. These binary IV-VI chalcogenides have direct band gap nature in cubic crystals while indirect in the orthorhombic phases. The post-DFT (BoltzTraP) calculations are performed to estimate Seebeck coefficient, electric and thermal conductivities, power factor and figure of merit of these compounds to check their applicability in thermoelectric devices. Spin orbit coupling effect influences the electronic and thermoelectric properties of these compounds due to the large size of the constituent elements. Further, the total thermal conductivity is computed using ad-hoc calculation with the experimental results. The calculated values of figure of merit for PbS, PbSe, PbTe and SnTe are in the range 0.68–0.77 at room temperature.     

First-principles investigations of structural,electronic and magnetic properties of cubic LaMnO3

Using first-principle density functional calculations, the structural, electronic and magnetic properties of cubic perovskite LaMnO₃ were studied by means of the full-potential linear muffin-tin orbital method. Calculations were performed within the local spin density approximation (LSDA) to the exchange correlation potential. The magnetic phase stability was determined from the total energy calculations for both ferromagnetic (FM) and non-magnetic (NM) phases. Our calculations show that the magnetic phase is more stable than the non-magnetic phase. To our knowledge the elastic constants of this compound have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Additionally, the band structure, the density of state and the magnetic moments were analyzed.     

TiNx系统的电子结构及所关联的光学性质

本文使用LMTO－ASA能带方法和超原胞方法计算了Ti-N系统中三个稳定相（α-Ti，ε-Ti2N和δ-TiN)的电子结构，然后通过将这三个稳定相作为样本并借助于统计超原胞方法。本文全面计算了TiNx系统的电子结构。在这基础上，我们又进一步计算了TiNx系统的联合态密谋，并利用所得结果初步探讨了TiNx系统的光学性质。通过与金的联合态密度的比较，我们得到了当组分x在0.4至0.6范围内，TiNx镀膜表观上可呈现金黄色。     

Electronic structure,optical and dielectric constant of compounds Indium-based: InAlP2, and InGaP2 in its chalcopyrite,CuPt and CuAu-I structures

First principles calculations within the density functional theory framework were carried out to calculate electronic structures and dielectric constant predictions of InGaP₂ and InAlP₂ compounds. We use three arrangements of these compounds: CuAu-I, CuPt and chalcopyrite ones. Different approximations have been dealt with in order to predict valuable bands gaps energy using DFT calculations. Electronics structure results are promising, due to the good agreement with a number of observable physical-chemistry properties. On the other hand, electron localization function and atom in molecule formalisms have been done to give more insight on the bonding properties. Capabilities that exhibit the InAlP₂ in its CuAu-I structure, such as the anisotropy and second harmonic generation, make it promising for an intensive optoelectronic application.     

Structural,mechanical and electronic properties of sodium based fluoroperovskites NaXF3 (X=Mg,Zn) from first-principle calculations

The structural stability, mechanical, electronic and thermodynamic properties of the cubic sodium based fluoro-perovskite NaXF₃ (X=Mg, Zn) have been studied using density functional theory (DFT). The generalized gradient approximation of Perdew–Burke and Ernzerhof (GGA-PBE) is used for modeling exchange-correlation effects. In addition, the alternative form of the GGA proposed by Engel and Vosko (GGA-EV) is also used to improve the electronic band structure calculations. The results show that both compounds are stable in the cubic Pm3m structure. From Poisson׳s ratio, it is inferred that cubic anti-perovskite NaXF₃ are ductile in nature and that bonding is predominantly of ionic in nature. The electronic band structure calculations and bonding properties show that anti-perovskites have an indirect energy band gap (M–Г) with a dominated ionic character. The thermal effects on thermal expansion coefficient, Debye temperature and Grüneisen parameter were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The calculations are found to be in good agreement with other results.     

First-principles calculations on elastic,electronic and optical properties for the alkaline platinum hydrides A2PtH6 (A=K,Rb and Cs)

The alkaline platinum hydrides are considered the most promising as hydrogen storage materials. The alloying ability of crystal, elastic constants and related parameters, electronic and optical properties have been studied using pseudo-potential plane–wave method based on the density functional theory. The investigated compounds show a weaker resistance to compression along the principal a-axis and their resistance to shear deformation is lower than the resistance to the unidirectional compression. The band structure indicates that A₂PtH₆ (A=K, Rb and Cs) are X–X direct gap semiconductors. The effective electron mass at equilibrium has been predicted towards X–Γ, X–W and L–W directions. The strong hybridization between Pt-d and H-s states in the upper valence band translates the existence of covalent bonding character in these compounds. The static optical dielectric constant is inverse proportional to the fundamental gap.     

A density functional study of structural,electronic and optical properties of titanium dioxide: Characterization of rutile,anatase and brookite polymorphs

Study of fundamental physical properties of titanium dioxide (TiO₂) is crucial to determine its potential for different applications, such as study of electronic band gap energy is essential to exploit it for optoelectronics and solar cell technology. We present here investigations pertaining to structural, electronic and optical properties of rutile, anatase and brookite polymorphs of TiO₂ by employing state of the art full potential (FP) linearized (L) augmented plane wave plus local orbitals (APW+lo) approach realized in WIEN2k package and framed within density functional theory (DFT). To incorporate exchange correlation(XC) energy functional/potential part into total energy, these calculations were carried out at the level of PW–LDA, PBE–GGA, WC–GGA, EV–GGA, and mBJ–GGA which are exploited as the manipulated variables in this work. From our computations, the obtained structural parameters results were found to be consistent with the available experimental results. The analysis of electronic band gap structure calculations point to TiO₂ as a semiconducting material in all three phases, whereas band gap character around Fermi level was found to be indirect for anatase, and direct for rutile and brookite phases. Density of state (DOS) profiles showed a substantial degree of hybridation between O 2p and Ti 3d in conduction and valence band regions, illustrating a strong interaction between Ti and O atoms in TiO₂ compund. In addition, our investigations of the optical properties also endorse the interband transitions from O 2p in valence band to Ti 3d in conduction band.     

Electronic,optical and bonding properties of MgYZ2 (Y=Si,Ge; Z=N,P) chalcopyrites from first principles

The electronic and optical properties of MgYZ₂ (Y=Si, Ge; Z=N, P) compounds are carried out using first-principle calculations within the density functional theory. The calculations show close correspondence to the available experimental data compared to the previous theoretical calculations. Band gap decreases by changing the cations Y from Si to Ge as well as Z from N to P in MgYZ₂. The N/P p-states contribute majorly in the density of states. Bonding nature of the herein studied compounds is predicted from the electron density plots. Optical response of these compounds is noted from the complex refractive index, reflectivity and optical conductivity. The direct band gap and the high reflectivity of these compounds in the visible and ultraviolet regions of electromagnetic energy spectrum ensure their applications in optoelectronic and photonic domains.     

Quantum‐Chemical Studies on the Geometric and Electronic Structures of Bertholloide Cobalt Oxynitrides

We report electronic structure calculations using density‐functional theory (local density approximation (LDA) and generalized gradient approximation (GGA); plane waves and muffin‐tin orbitals; pseudopotentials and all‐electron approaches) on non‐stoichiometric CoN_xO_1–x oxynitride phases. The preference of the experimentally suggested zinc‐blende structure type over the rock‐salt type is confirmed and explained, on the basis of COHP (crystal orbital Hamilton population) chemical bonding analyses, by reduced Co–Co antibonding interactions in the ZnS structural alternative. A pressure‐induced phase transition into the NaCl type, however, is predicted at approximately 30 GPa. Supercell calculations touching upon the exact composition and local structure of CoN_xO_1–x provide evidence for a broad range of energetically metastable compositions with respect to the zinc‐blende‐type boundary phases CoN and CoO, especially for the more oxygen‐rich phases. All non‐stoichiometric compounds are predicted to be metallic materials which do not exhibit significant magnetic moments. Likewise, there is no indication for anionic ordering such that random anion arrangements are preferred.     

Predicted Thermoelectric Properties of the Layered XBi<Subscript>4</Subscript>S<Subscript>7</Subscript> (X = Mn,Fe) Based Materials: First Principles Calculations

We report a theoretical investigation of electronic structures, optical and thermoelectric properties of two ternary-layered chalcogenides, MnBi₄S₇ and FeBi₄S₇ , by combining the first principles density functional calculations and semi-local Boltzmann transport theory. The calculated electronic band structure have demonstrated that both compounds exhibit indirect band gaps. The optical transitions are explored via the dielectric function (real and imaginary parts) along with other related optical constants including refractive index, reflectivity, and energy loss spectrum. These chalcogenides have exhibited interesting thermoelectric properties such as Seebeck’s coefficient, electrical and thermal conductivity, and power factor as function of temperatures.     

Structural and electronic properties of zirconia phases: A FP-LAPW investigations

The structural and electronic properties of ZrO₂ polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation (GGA) in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, transition pressures, bulk modulus and its pressure derivative as well as the structural phase stability were calculated. The results were compared with previous calculations and experimental data when available. The FP-LAPW method correctly orders the zero temperature energies of all zirconia polymorphs. We have also studied the effect of distortion from the cubic to the tetragonal structure on the basis of charge density calculations. On the other hand, band structure and density of states (DOS), which allow us to discuss the features of orbital mixing, are also given. Our results suggest that the cotunnite structure should be better than the other zirconia phases as gate dielectric material.     

Band Profile Comparison of the Cubic Perovskites CaCoO3 and SrCoO3

Structural geometries, electronic band structures, spin densities, and magnetic properties of the cubic perovskites CaCoO₃ and SrCoO₃ are studied using the highly accurate spin-polarized density functional theory. It is found that the structural parameters and geometry of SrCoO₃ are consistent with experimental results. Careful analysis of the band profiles reveals that the overall electronic band structure of CaCoO₃ is similar to the electronic band structure of SrCoO₃, with a small difference in details. The total and partial densities of states show that CaCoO₃ and SrCoO₃ compounds are ferromagnetic metals. The calculated magnetic moments of these compounds also reveal that they are ferromagnets. Furthermore, the comparison of the calculated magnetic moments for Co and SrCoO₃ is consistent with experimental results, confirming the validity of our theoretical results. On the basis of the presented electronic structure and magnetic properties, it is expected that CaCoO₃ is also a colossal magnetoresistive material like SrCoO₃.     

First principles study of Mg2X (X=Si,Ge, Sn,Pb): Elastic,optoelectronic and thermoelectric properties

Full potential linearized augmented plane wave method within the framework of density functional theory was applied to calculate the structural, elastic, electronic, thermoelectric and optical properties of Mg₂X (X=Si, Ge, Sn, Pb) compounds. Exchange-correlation effects were treated using generalized gradient approximation and modified Becke–Johnson technique. Calculated structural parameters were found in good agreement to the experimental data. With the pressure application, the lattice constant decreased while the bulk modulus increased. Brittleness and ductility of these compounds were interpreted via the calculated elastic constants. The optical properties like complex dielectric function, refractive index, reflectivity, and optical conductivity were investigated in the pressure range 0–10 GPa. Very high reflectivity in a wide energy range indicates the usefulness of these materials as a shield from high energy radiations. In addition, the thermopower of the materials was calculated as a function of the chemical potential at various temperatures. These materials are suitable for applications in optoelectronic and thermoelectric devices due to their high thermopower and narrow bandgap.     

First principles calculations of the electronic structure and optical properties of (001), (011) and (111) Ga0.5Al0.5As surfaces

Using the first-principles plane-wave pseudo-potential method based on density function theory (DFT), the electronic structure and optical properties of Ga_0.5Al_0.5As (001), (011) and (111) surfaces are calculated. Result shows that (001) surface is reconstructed, (011) surface is not reconstructed but wrinkled, (111) surface is only relaxed. (111) is the most stable surface. (001) surface owns the lowest work function. Absorption coefficient and reflectivity of these surfaces are smaller than bulk, the transmittance of the surfaces are larger than the bulk, which is helpful for the incident light to excite photoelectrons. The decrease of the absorption coefficient and reflectivity at (001) surface are the largest. Calculation of electronic structure and optical properties predict that the (001) surface should have the strongest photoemission.     

First-Principles Study of Electronic Structure,Mechanical, and Thermoelectric Properties of Ternary Palladates CdPd<Subscript>3</Subscript>O<Subscript>4</Subscript> and TlPd<Subscript>3</Subscript>O<Subscript>4</Subscript>

Ternary palladates CdPd₃O₄ and TlPd₃O₄ have been studied theoretically using the generalized gradient approximation (GGA), modified Becke–Johnson, and spin–orbit coupling (GGA–SOC) exchange–correlation functionals in the density functional theory (DFT) framework. From the calculated ground-state properties, it is found that SOC effects are dominant in these palladates. Mechanical properties reveal that both compounds are ductile in nature. The electronic band structures show that CdPd₃O₄ is metallic, whereas TlPd₃O₄ is an indirect-bandgap semiconductor with energy gap of 1.1 eV. The optical properties show that TlPd₃O₄ is a good dielectric material. The dense electronic states, narrow-gap semiconductor nature, and Seebeck coefficient of TlPd₃O₄ suggest that it could be used as a good thermoelectric material. The magnetic susceptibility calculated by post-DFT treatment confirmed the paramagnetic behavior of these compounds.     

Electronic structure and magnetism in ternary gadolinium-based cubic inverse perovskites

The electronic structure of cubic inverse perovskites Gd₃MX (M=Al, Ga and In; X=B, C, N and O) are investigated with the first-principles method. The ground-state structure, equilibrium structural parameters, and magnetic structure are calculated; a comparison between these compounds is made based upon follow-on analysis. The electronic structures reveal that all these compounds have a metallic character. We report a detailed analysis of the different physical properties and their changes induced by varying the cation and/or the anion.     

Development of Electronic and Electrical Materials from Indian Ilmenite

Some new complex electronic materials have been prepared by mixing bismuth oxide (Bi₂O₃) and ilmenite in different proportions by weight, using a mixed-oxide technique. Room-temperature x-ray diffraction analysis confirms the formation of a new compound with trigonal (rhombohedral) crystal structure with some secondary phases. Studies of dielectric parameters (ε _r and tan δ) of these compounds as a function of temperature at different frequencies show that they are almost temperature independent in the low-temperature range. They possess high dielectric constant and relatively small tangent loss even in the high-temperature range. Detailed studies of impedance and related parameters show that the electrical properties of these materials are strongly dependent on temperature, showing good correlation with their microstructures. The bulk resistance, evaluated from complex impedance spectra, is found to decrease with increasing temperature. Thus, these materials show negative temperature coefficient of resistance (NTCR)-type behavior similar to that of semiconductors. The same has also been observed from their I–V characteristics. Complex electric modulus analysis indicates the possibility of a hopping conduction mechanism in these systems with nonexponential-type conductivity relaxation. The nature of the variation of the direct-current (dc) conductivity with temperature confirms the Arrhenius behavior of these materials. The alternating-current (ac) conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law.     

Structural,elastic, electronic and thermodynamic properties of the filled skutterudite CeOs4Sb12 determined by density functional theory

Structural, elastic, electronic and thermodynamic properties of the ternary cubic filled skutterudite CeOs₄Sb₁₂ compound were calculated using the full-potential linear muffin-tin orbital implementation of density functional theory. The exchange-correlation potential was treated with the local density approximation. The calculated ground state quantities such as the lattice parameter, atomic position parameters of Sb atoms, bulk modulus and its pressure derivative are compared to the available experimental data. We have computed the elastic moduli and their pressure dependence, which have not been calculated or measured yet. The Debye temperature is estimated from the average sound velocity. From the elastic parameter behavior, it is inferred that this compound is elastically stable and brittle in nature. The electronic band structure calculations revealed metallic behavior for the herein studied compound at zero pressure, but under pressure effect, the metallic character disappears and the compound becomes a narrow indirect band gap semiconductor. Through the quasi-harmonic Debye model, in which phononic effects are considered, the effect of pressure P and temperature T on the lattice constant, bulk modulus, heat capacity, thermal expansion coefficient and Debye temperature are investigated.     

Multiple Quantum States Induced in 1T-TaSe2 by Controlling the Stacking Order of Charge Density Waves

Van der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken-symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T-TaSe₂ with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T-TaSe₂ can host various electronic phases including quasi-1D and 3D metals and band insulators. Particularly, the ground-state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal-insulator transition. In ultrathin-layer 1T-TaSe₂, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four-layer emerges as an insulator or a semimetal dependent on its stacking order. The wide-tunable electronic properties of 1T-TaSe₂ CDW compound will open a new pathway for designing novel quantum devices.     

Pseudopotential study of PrO2 and HfO2 in fluorite phase

Praseodymium and hafnium oxides are prospective candidates to subsitute SiO₂ in decanano MOSFET transistors. We report first ab initio pseudopotential band structure calculations for these materials. We find that fluorite phases of PrO₂ and HfO₂ have similar electronic structures. The important difference is a narrow sub-band forming the conduction band bottom in PrO₂ but absent in HfO₂. Electrons in this f-type sub-band have large masses. This explains why ultrathin epitaxial Pr oxide films have low leakage in spite of a relatively small conduction band offset (1 eV) between the oxide and the Si substrate.