K. Colakoglu

Electronic, elastic, thermodynamical, and dynamical properties of the rock-salt compounds LaAs and LaP

We have studied the structural, elastic, electronic, thermodynamical and vibrational properties of LaAs and LaP in the rock-salt (B1) structure by performing ab initio calculations within the local-density approximation (LDA). Some basic physical properties, such as the lattice constant, bulk modulus, cohesive energy, second-order elastic constants (Cij), the electronic band structures, and phonon frequencies, are calculated and compared with the available experimental and other theoretical values. In order to gain further information, we have, also, predicted the Youngs modulus, Poissons ratio (ν), anisotropy factor (A), sound velocities, and Debye temperature (θD), and reasonable values have been found. We also present the temperature-dependent behaviour of some thermodynamical properties such as entropy and heat capacity for these compounds in the B1 phase.

First-Principles Calculations on Structure, Elastic and Thermodynamic Properties of Al2X（X=Sc,Y） under Pressure

To understand deeply the structural, elastic and thermodynamic characteristics of Al 2 X ( X = Sc, Y) compounds in C15 type (space number 227) Cu 2 Mg structure, we have performed ab-initio density functional theory within the local density approximation (LDA) and the generalized gradient approximation (GGA). The thermodynamic properties of the considered structures are obtained through the quasi-harmonic Debye model. We have presented the results on the basic physical parameters, such as the lattice constant, bulk modulus, pressure derivative of bulk modulus, second-order elastic constants, Zener anisotropy factor, Poissons ratio, Youngs modulus, and isotropic shear modulus. In order to gain further information, the pressure- and temperature-dependent behaviour of the volume, bulk modulus, thermal expansion coefficient, heat capacity, entropy, Debye temperature and Gruneisen parameter were also evaluated over a pressure range of 0–20 GPa for Al 2 Sc and 0–17 GPa for Al 2 Y compound and a wide temperature range of 0–2000 K for both compounds. The obtained results were compared with the other reported values.

Theoretical predictions of the structural, mechanical and lattice dynamical properties of XW2 (X = Zr, Hf) Laves phases

We have investigated the structural, mechanical and lattice dynamical properties of ZrW2 and HfW2 compounds in cubic C15 (space group Fd-3m), hexagonal C14 (space group P63/mmc) and C36 (space group P63/mmc) phases using generalized gradient approximation within the plane-wave pseudo-potential density functional theory. We have found that ZrW2 and HfW2 in cubic C15 phase are the most stable among the considered phases. From calculated elastic constants, it is shown that all phases are mechanically stable according to the elastic stability criteria. The related mechanical properties, such as bulk, shear and Young moduli, Poisson’s ratio, Debye temperature and hardness have been also calculated. The results show that ZrW2 and HfW2 compounds are ductile in nature with respect to the B/G and Cauchy pressure analysis. The phonon dispersion curves, phonon density of states and some thermodynamic properties are computed and discussed exhaustively for considered phases.

Structural, electronic, elastic and optical properties of CdxZn1-xTe mixed crystals

The structural, electronic, elastic and optical properties of Cd(x)Zn(1-x)Te ternary mixed crystals are investigated by the first-principles plane-wave pseudopotential method within the LDA approximations. Basic physical properties, such as lattice constant, bulk modulus, second-order elastic constants (C(ij)) and the electronic band structures, are calculated. We also predict the shear modulus, Youngs modulus and Poissons ratio. Moreover, we have calculated the optical properties (dielectric functions, refractive index, extinction coefficient and energy loss function) of these ternary mixed crystals. Our results agree well with the available data in the literature.

Lattice dynamical and thermodynamical properties of ReB2, RuB2, and OsB2 compounds in the ReB2 structure

Structural and lattice dynamical properties of ReB2, RuB2, and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation. The present results show that these compounds are dynamically stable for the considered structure. The temperature-dependent behaviors of thermodynamical properties such as internal energy, free energy, entropy, and heat capacity are also presented. The obtained results are in good agreement with the available experimental and theoretical data.

Structural, electronic, and elastic properties of K-As compounds: a first principles study.

First-principle calculations are performed to investigate the structural, elastic and electronic properties of K-As compounds (KAs in NaP, LiAs and AuCu-type structures, KAs2 in MgCu2-type structure, K3As in Na3As, Cu3P and Li3Bi-type structures, and K5As4 in A5B4-type structure). The lattice parameters, cohesive energy, formation energy, bulk modulus, and the first derivative of bulk modulus (to fit to the Murnaghan’s equation of state) of the considered structures are calculated and reasonable agreement is obtained, and the phase transition pressure is also predicted. The repeated calculations on the electronic band structures and the related partial density of states are also given. The calculated second-order elastic constants based on the stress-strain method and the other related quantities such as Young’s modulus, shear modulus, Poissons ratio, sound velocities, Debye temperature, and shear anisotropy factors for considered structures are presented, and trends are discussed.

The structural, thermodynamical and elastic properties of TiO

In this paper, we have studied the structural, elastic, electronic and thermodynamical properties of TiO by performing ab initio calculations within the local density approximation (LDA). In particular, the lattice constant, bulk modulus, cohesive energy, phase transition pressure (Pt) from NaCl (B1) to CsCl (B2) structure, second-order elastic constants (Cij) and electronic band structures are calculated and compared with available experimental and other theoretical values. In order to gain further information, we have also predicted Youngs modulus, Poissons ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θD) and their pressure-dependent behaviours in the B1 phase.

First principles study of the structural, mechanical, phonon, optical, and thermodynamic properties of half-Heusler (HH) compound NbFeSb

We studied the structural, mechanical, phonon, optical and thermodynamic properties of MgAgAs-based NbFeSb compound by means of first-principles based on the density functional theory. The calculated lattice constant is in good agreement with the available experimental data. The electronic structure and corresponding density of states (DOS) were also calculated to give insight into the bonding mechanism and an indirect band gap was observed as ~1.77 eV, which is significantly higher than the previously reported one (0.529 eV). The elastic properties such as shear modulus, Youngs modulus etc under pressure, up to 65 GPa, were analyzed. The phonon dispersion curves, one-phonon DOS, and optical properties were obtained. In addition, the thermodynamic properties such as heat capacity, Debye temperature etc of NbFeSb were calculated at various temperatures and pressures by applying the quasi-harmonic Debye model.

A first-principles studies on TlX (X=P, As)

We present an ab initio study of the structural, electronic and thermodynamic properties of TlX(X=P,As). The plane-wave pseudopotential approach to the density-functional theory within the LDA and GGA approximations implemented in VASP (Viena Ab-initio Simulation Package) is used. The calculated lattice parameter, elastic constants, and band structures are compared with other available theoretical results, and good agreement is obtained. In addition, we have calculated the transition pressure (Pt) from zinc-blende (ZB) to (rock-salt) NaCl structures, and have examined some thermodynamic properties.

A large emitting area UV light source with semiconductor cathode

Light emission in the ultraviolet (UV) and visible (blue) range (330-440 nm) generated by a planar gas discharge system and the possibility of locally increasing the discharge light emission for a given photosensitivity of a planar GaAs semiconductor cathode has been studied. The use of metallic patched concentrators with an area of S=5/spl times/10/sup -4/ cm/sup 2/ and a density of 400 cm/sup -2/ leads to a fivefold increase in the gas light density. In a system with metallic patched concentrators, the local density of gas discharge light exceeds the density of uniform gas discharge light in the ionization system as many times as the working area of the semiconductor cathode exceeds the total area of the current concentrators. By using the infrared light to excite the semiconductor cathode of the system, we have shown that the uniform discharge light emission of the device with N/sub 2/ in the gap can serve as a source of UV radiation with a large emitting area of GaAs cathode if gas pressure and electric field are sufficiently high. Moreover, the use of metallic patched concentrators prolongs the working time of the cathode. The filamentation was primarily due to the formation of a space charge of positive ions in the discharge gap, which changed the discharge from the Townsend to the glow type.