Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

Structural,electronic and optical properties of Ilmenite ATiO3(A=Fe,Co, Ni)

Ilmenite-type ATiO₃ (A=Fe, Co, Ni) crystals have been investigated via Generalized Gradient Approximation (GGA) in the scheme of Revised Perdew-Burke-Ernzerhof (RPBE) using the first-principles method. The band structures, densities of states, bond orders and charge populations, optical properties including the dielectric function ε(ω), absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), electron energy loss function L(ω) and reflectivity function R(ω), are calculated. The results show that the GGA-optimized geometries agree well with the experimental data. FeTiO₃ has a direct band gap, but both CoTiO₃ and NiTiO₃ exhibit indirect band gap. The analysis for densities of states and atomic charge populations exhibits that TiO bonds possess the stronger covalent bonding strength than AO bonds. The calculated optical properties along [100], [010] and [001] as well as polycrystalline directions demonstrate the significant optical anisotropy parallel and perpendicular to c-axis for ATiO₃. Finally, the origins of main peaks for optical spectra are presented based on electron transitions. Theoretical insights into the microscopic intrinsic properties of ATiO₃ should provide fundamental investigations for further understanding the Ilmenite ATiO₃ materials and improving their practical applications.     

First principles study of structural,electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH_x (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH₂ and from hexagonal to cubic phase in GdH₃. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH₃. The calculated magnetic moment values of GdH_x (x=1, 2, 3) are in accord with the experimental values.     

Structural,electronic, mechanical and magnetic properties of rare-earth nitrides REN (RE=Ce,Pr, Nd): A first principles study

The structural stability of rare earth nitrides REN (RE=Ce,Pr,Nd) is investigated among three cubic structures, namely, NaCl (B1), CsCl(B2) and zinc blende (B3). It is found that NaCl structure is the most stable structure for all the three nitrides. On increasing the pressure, structural phase transition from NaCl (B1) to CsCl (B2) phase is predicted in CeN and NdN at the pressures of 88 GPa and 36.5 GPa while NaCl (B1) to zinc blende (B3) phase transition is observed in PrN at the pressure of 68 GPa. At normal pressure, all the three nitrides are stable in the ferromagnetic state (FM) with cubic NaCl (B1) structure. The calculated lattice parameters and bulk modulus values are in good agreement with experimental and other theoretical values. Electronic structure reveals that PrN and NdN are half metallic while CeN is metallic at normal pressure. Ferromagnetism is quenched in CeN and PrN at the pressures of 152 GPa and 121 GPa respectively. The positive values of elastic constants indicate that all the three nitrides are mechanically stable in NaCl Phase. It is found that all these nitrides are ductile and anisotropic in nature.     

Electronic,optical and thermoelectric properties of XNMg3 (X=P,As, Sb,Bi) compounds

Different exchange correlation potential approximations are used to examine electronic, optical, and thermoelectric properties of XNMg₃(X=P, As, Sb, and Bi) antiperovskite compounds. Band structures of the compounds are direct in nature. Within a high-energy range (2–6 eV), these materials exhibit maximum levels of optical conductivity, and these materials may therefore be used in radiation detectors and solar cells. Optical properties such as dielectric function, optical conductivity, reflectivity, refractive indices and absorption coefficients vary in transitions from P to Bi. Furthermore, calculated peaks of conductivity and absorption coefficient values decrease with increasing photon energy. With respect to thermoelectric properties, electrical conductivity, Seebeck coefficient and thermal conductivity levels vary with increase in temperature, thus resulting in the formation of thermoelectric materials.     

Structural,elastic, electronic and thermodynamic properties of uranium filled skutterudites UFe4P12: First principle method

We present a theoretical study of structural, elastic, thermodynamic, and electronic properties of the uranium filled skutterudite UFe₄P₁₂. We use the full-potential linear muffin–tin orbital (FP-LMTO) method in which the local density approximation (LDA) is used for the exchange-correlation (XC) potential. The lattice parameter at equilibrium, the bulk modulus, its pressure derivative, the elastic constants and the band structure energy of the filled skutterudite UFe₄P₁₂ are calculated and systematically compared to available theoretical and experimental data. Herein, we use the total energy variation as function of strain technique to determine independent elastic constants and their pressure dependence. Furthermore, using quasi-harmonic Debye model with phonon effects, the effect of pressure P and temperature T on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature and the heat capacity of UFe₄P₁₂ are investigated for the first time. Band structure of UFe₄P₁₂ indicates a tendency of forming a pseudo-gap that appears above the Fermi level at Γ point. This is a unique characteristic of skutterudite, especially when a single phosphorous p-band crosses the Fermi level. The crossing band is, indeed, pushed down by the repulsion of U f-resonance states.     

Elastic and thermodynamic properties of the SiB2O4 (B=Mg,Zn and Cd) cubic spinels: An ab initio FP-LAPW study

The structural, elastic and thermodynamic properties of the SiB₂O₄ (B=Mg, Zn and Cd) cubic spinels have been investigated through ab initio full-potential linearized augmented plane wave calculations. The calculated structural parameters are in good agreement with the available experimental and theoretical data. The single crystal elastic constants are numerically estimated using total energy-strain approach with two different sets of distortions. The polycrystalline aggregate elastic parameters are calculated from the single crystal elastic constants via the Voigt–Reuss–Hill approximations. Mechanical stability, sound velocities, ductility/brittleness, elastic anisotropy, Debye temperature and pressure dependence of the elastic constants of the title compounds are also assessed. The temperature dependence of the lattice parameter, bulk modulus, volume thermal expansion coefficient, isochoric and isobaric heat capacity and Debye temperature in a wide temperature interval at some different fixed pressures is predicted through the quasi-harmonic Debye model.     

Theoretical investigation of the electronic structure,optical, elastic,hardness and thermodynamics properties of jadeite

A detailed theoretical study of the electronic structure, optical, elastic and thermodynamics properties of jadeite have been performed by means of the first principles based on the state-of-the-art of density functional theory within the generalized gradient approximation. The optimized lattice constants and the atomic positions are in good agreement with experimental data. The total density of states and partial density of states of jadeite have been discussed. The energy gap has been calculated along the Γ direction found to be 5.338 eV, which shows that jadeite has wide direct band gap. The optical properties, such as the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, loss function and absorption coefficient for [100] and [001] directions have been described for the first time in the energy range 0–40 eV. The elastic constants, bulk modulus, Young׳s modulus, anisotropic factor and Poisson׳s ratio have been calculated. Furthermore, the Vickers hardness and Debye temperature of jadeite have been predicted. The calculated values of all above parameters are compared with the available experimental values.     

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.     

Structural,electronic and mechanical properties of RuO2 from first-principles calculations

The structural, electronic and mechanical properties of ruthenium oxide (IV) (RuO₂) in various space groups have been calculated using full-potential linear muffin-tin orbital method. The exchange and correlation potential is treated by local density approximation. The calculated ground state properties, including, lattice constants, internal parameters, bulk modulus and the pressure derivative of the bulk modulus are in good agreement with the available data. This compound is found to undergo a series of structural phase transitions under high pressure. The sequence of the structural phase transition is: rutile→marcasite→pyrite→fluorite that occurs at around 4.92, 22.9 and 100.6 GPa, respectively. The elastic constants C_ij for RuO₂ in its different structures are calculated using the total energy variation with strain technique. The polycrystalline elastic moduli, namely; shear modulus, Young's modulus, Poisson's ratio, sound velocities and Debye temperature were derived from the obtained single-crystal elastic constants. Band structure calculations show that this compound is a narrow band gap semiconductor with a gap of 0.47 eV in its fluorite structure. While for rutile, marcasite and pyrite structures, this compound exhibits metallic properties.     

Theoretical study of the structural,elastic and thermodynamic properties of chalcopyrite ZnGeP2

The structural, elastic, and thermodynamic properties of ZnGeP₂ with chalcopyrite structure are investigated using the pseudo-potentials plane wave method based on the density functional theory with the generalized gradient approximation. The lattice parameters (a, c and u) are directly calculated and agree well with previous experimental and theoretical results. The obtained negative formation enthalpy shows that ZnGeP₂ crystal has strong structural stability. We have also calculated the bulk modulus B and the elastic parameters (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, and C₆₆) which have not been measured yet. The accuracy and reliability of the calculated elastic constants of ZnGeP₂ crystal are discussed. In addition, the pressure and temperature dependencies of the lattice parameters, bulk modulus, Debye temperature, Grüneisen parameter, entropy, volume thermal expansion coefficient, and specific heat capacity are obtained in the ranges of 0–20 GPa and 0–1200 K using the quasi-harmonic Debye model. To our knowledge this is the first quantitative theoretical prediction of the thermodynamic properties for ZnGeP₂ compound and still awaits experimental confirmations.     

(Zn,Mg)TiO_3微波介质陶瓷性能的改善

采用固相反应法制备了(Zn,Mg)TiO3(ZMT)微波介质陶瓷,研究分别添加CaTiO3和BaLiBSi对ZMT陶瓷介电性能的影响。结果表明:CaTiO3和BaLiBSi均能调节ZMT陶瓷的温度系数τε值;BaLiBSi能有效降低ZMT陶瓷的烧结温度,抑制Zn2TiO4相的产生,提高所制陶瓷的微波介电性能。当添加质量分数10%的CaTiO3时,950℃烧结的(Zn1.06Mg0.12)TiO3陶瓷的τε值接近零:–6×10–6/℃。加入质量分数1.2%的BaLiBSi时,900℃烧结的(Zn1.13Mg0.048)Ti1.29O3陶瓷具有最佳的微波介电性能:εr≈24.8,Q.f=10 898 GHz,τε=17×10–6/℃。     

Structural and optoelectronic properties of hybrid halide perovskites for solar cells

Trihalide perovskites are an emerging class of materials, which have shown excellent performance so far in solution-processed optoelectronic devices such as perovskite solar cells (PSCs) and light emitting diodes (LEDs). The energy band gap (E_gap) of this class of materials is tunable and can be varied from 1.5 eV to 2.3 eV by changing its chemical composition, exhibiting a promising character to design versatile optoelectronic devices. It is thus, imperative to understand the relation between structural and optoelectronic properties of the perovskite-based materials offering intrinsic complexity. Hence, different interactions, defects as well as structural disorder have a defining role in the material properties. The intrinsic properties have been shown to have a significant impact on the performance of these perovskite materials. These properties include high dielectric constants, ambipolar transport features of long range, low exciton binding energies, and ferroelectric polarizations. In the current review, we briefly explore the crystal structure of the perovskite materials at atomistic-level and draw a comparison of the basic optical and electrical properties originating from particular atomic compositions together with their arrangements therein, and moreover, their applications in future optoelectronic devices are elaborated upon.     

Density functional theory study of tin and titanium dioxides: Structural and mechanical properties in the tetragonal rutile phase

Structural and mechanical properties in rutile (tetragonal) phases of SnO₂ and TiO₂ are investigated by performing first-principle density functional theory (DFT) calculations. Generalized Gradient Approximation (GGA) potentials of electronic exchange and correlation part parameterized by Perdew–Burke–Ernzerhof (PBE) are used. Second order elastic stiffness constants, bulk modulus, first-derivative of bulk modulus, and pressure behavior of these mechanical properties are studied up to pressure of 10 GPa. Structural properties and elastic constants of SnO₂ and TiO₂ calculated in this study are compatible with experimental and other available theoretical studies. Electronic band gap energies of these semiconductors are also calculated. As expected, the calculated values by standard DFT calculations are underestimated in comparison to experimental values.     

Pressure-dependent electronic,optical,and mechanical properties of antiperovskite X3NP(X=Ca,Mg):A first-principles study

Hydrostatic pressure provides an efficient way to tune and optimize the properties of solid materials without changing their composition. In this work, we investigate the electronic, optical, and mechanical properties of antiperovskite X₃NP(X²⁺ = Ca, Mg) upon compression by first-principles calculations. Our results reveal that the system is anisotropic, and the lattice constant a of X₃NP exhibits the fastest rate of decrease upon compression among the three dire...     

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.