Electronic properties of tin dichalcogenides (SnS2 and SnSe2)

Using Phillips and Van Vechten's theory the homopolar gap (E_h), ionic gap (E_c) and Penn gap (E_p) have been derived for the layered compound semiconductors SnS₂ and SnSe₂. The values of E_p so derived have been found to be in agreement with the values obtained from the Grimes-Cowley modification of the Penn model, and also with the reflectance spectra of the respective materials. Using the above data the Phillips ionicity has been calculated and the octahedral coordination of SnS₂ and SnSe₂ has been discussed on the basis of the calculated value of ionicity. The bond electronic polarizability has been evaluated using Chemla's approach and the values so obtained have been found in agreement with the values obtained from the Clausius-Mossottirelation. The Varshni formula has been shown to explain fairly well the temperature dependence of energy gap and the constants involved have been derived. The calculated isobaric temperature gradient of E_g[($\text{?E}{}_{\mathrm{g}}\text{?T)}{}_{\mathrm{p}}$] has been found to be in accordance with the reported values, and the electron lattice interaction has been evaluated.     

Evaluation of the second order strain derivatives of the electronic dielectric constant of alkali halides

We have evaluated the second order strain derivatives of the electronic dielectric constant of alkali halides employing two different theories viz. the Clausius—Mossotti theory of polarizability and the Penn model of energy gap. The results obtained from the two theories are in fair agreement with each other. It has been emphasised that the present calculations can be used to separate the electronic and ionic contributions to the second order strain derivative of the static dielectric constant.     

Semiconducting properties of pyrite

It is well known that Pyrite associated with Black Ore induces the severest difficulty among other constituents in the flotation process in mineral dressing. One of the possible reasons for this difficulty is related to its semiconducting properties which may affect surface conditions. We present here a study of the semiconducting properties of pyrite. Our calculations of the energy gap at various temperatures are shown to be in very good agreement with the experimental data. The Penn gap calculated using the Grimes-Cowley modification [1] of the Penn model has been shown to agree very well with the reflectivity data of Horita [2]. Using the Clausius-Mossotti relation, we calculate the electronic polarizability for pyrite. Also evaluated are the pressure and lattice parameter dependence of the energy gap. We predict the temperature of decomposition of natural pyrite in inert atmosphere from Mössbauer studies.     

Optoelectronic properties of rutile (TiO2)

We present here a study of the electronic and optical properties of rutile (TiO₂). An investigation into the energy, pressure and temperature dependence of n_eff and ε_{∞ eff} is presented. Using the calculated values of n_eff and ε_{∞ effs}, we evaluate the Penn gap. The Penn gap thus obtained is shown to be in good agreement with the reflectivity data and also with the value obtained from band structure. In addition, we also show that essentially the Varshni formula explains the temperature dependence of the energy gap for rutile fairly well.     

Opto-electronic properties of gallium chalcogenides

A study of opto-electronic properties of gallium chalcogenides (GaS, GaSe and GaTe) is presented here. An investigation into the various effective charge parameters ($\text{Z}{}_{\mathrm{<mtext>eff</mtext>}}\text{, e}{}^1{}_{\mathrm{s}}\text{, e}{}^1{}_{\mathrm{T}}\text{and}\text{e}{}^1{}_{\mathrm{i}}$ and their correlation with ionicity have been done. The energy dependence of optical effective charge parameter (n_eff) have been discussed with respect to the band structure of respective chalcogenides. The low frequency optical dielectric constants (?₀₀) have been derived using reflectivity data and the obtained values of ?₀₀ have been found in agreement with the values obtained using Moss relation as also with the reported experimental data. The difference in the arrangement of fundamental structures of GaS and GaSe on one hand and GaTe on the other hand does not seem to effectively change the opto-eleetronic properties and the fundamental structure (which is common in all the three chalcogenides) is mainly responsible for determining these properties. The effect of surface conditions on the values of n_eff and ?∞ have been evaluated for GaTe. Using derived values of n_eff and ?_∞ the value of Penn gap have been determined. The Penn gap thus obtained is shown to be in agreement with the reflectivity data. In the passing we also show that essentially the Varshni formula explains fairly well the temperature dependence of the energy gap for gallium chalcogenides and the constants involved are evaluated.     

Orientational order parameters of nematogenic trans-4-propyl cyclohexyl-4 (trans-4-alkyl cyclohexyl) benzoates

The refractive indices and densities of three members of trans-4-propyl cyclohexyl-4 (trans-4-alkyl cyclohexyl) benzoates are reported as functions of temperature in the nematic and isotropic phases. The principal molecular polarizabilities and order parameters (S) have been evaluated using the anisotropic internal field model (Neugebauer’s approach). Also order parameters have been estimated by studying the dipole-dipole splittings in the wide line proton magnetic resonance (PMR) spectra. The results obtained are in close agreement with those obtained from refractive index measurements.     

Revisiting the Phillips ionicity of conductors and the quantitative determination of the hardness of carbides and nitrides of transition metals using the LDA + U technique

The Phillips ionicity is modified, simulated and calculated for conductors. The results show that the percentage of metallic bonding in multiplex chemical bonds of transition metal (TM) carbides and nitrides is large; this affects the Phillips ionicity. The redefinition of Phillips ionicity has been applied to estimate the hardness of TM carbides and nitrides; the values obtained are in agreement with experimental and theoretical evidence. In addition, materials with the zinc blende structure are harder than those with rock salt structure.     

Optical birefringence and order parameter of three nematogens

Results of the measurement of refractive indices and densities of three nematic liquid crystals at different temperatures are reported. The molecular polarizabilities have been calculated from refractive indices using both the Vuks’ and the Neugebauer’s relations. The orientational order parameters are determined from the polarizability values. The variation of order parameter with temperature for these compounds shows a reasonably good agreement with Maier and Saupe theory except near the clearing points where the experimental values are less than those obtained from the theory. The possible reasons for this have been discussed.     

Differential oscillator strengths and dipole polarizabilities for transitions of the helium sequence

The dipole radial integral for an initial discrete 1s state and a final continuum state has been calculated under the screened hydrogenic model. In this model, single-electron hydrogenic wave functions are employed and the initial and the final states are treated by two different effective-charge parameters. Numerical values of differential oscillator strengths for transitions from 1s 21S to the continuum for the helium sequence ions are obtained. Also calculated are the dipole polarizabilities, which are found to be in excellent agreement with the results of other authors.     

Central cell effects on the polarizabilities of shallow donors in silicon

With the Topp and Hopfield form for the ionic pseudopotentials, the screened impurity pseudopotentials of the shallow donors P, As and Sb in silicon are obtained in closed form in the linearized Thomas-Fermi model. The results are compared with the screening using a k-dependent dielectric function. Using these impurity pseudopotentials in the multivalley effective mass equation, the binding energies of these donors in Si are estimated variationally. Adopting the Hasse variational method, the polarizabilities of the P, As and Sb donors in Si are computed and are found to be in excellent agreement with the polarizability values of isolated P and Sb in Si, deduced from recent piezocapacitance measurements.     

Effect of the crystal potential on electronic polarizabilities of ions in alkaline earth chalcogenides

The electronic polarizabilities of ions in alkaline earth chalcogenides are estimated by taking account of the effect of the crystalline potential. The polarizabilities thus obtained are found to present a good agreement with experimental data. It has been shown that the polarizabilities and radii of alkaline earth and chalcogenide ions follow the polarizability-radius cube relation approximately well.     

Modified Fues potential for many-electron atoms

A modification of the model Fues potential based on taking into account the effects of Coulomb screening and polarization of the core in the ground and lower excited states of many-electron atoms in the parameters of this potential is proposed. Numerical values of parameters of the modified model potential are obtained for the ground states of alkali metal, alkaline earth, and inert gas atoms and for the metastable triplet state of helium. The numerical values of polarizabilities and hyperpolarizabilities calculated with the use of the modified potential are in significantly better agreement (as compared to the results obtained by using the traditional approach) with the most accurate literature data.     

Measurements of the generalized electric and magnetic polarizabilities of the proton at low Q2 using the virtual-compton-scattering reaction

The mean square polarizability radii of the proton have been measured for the first time in a virtual-Compton-scattering experiment performed at the MIT-Bates out-of-plane scattering facility. Response functions and polarizabilities obtained from a dispersion analysis of the data at Q2 = 0.057 GeV2/c2 are in agreement with O(p3) heavy baryon chiral perturbation theory. The data support the dominance of mesonic effects in the polarizabilities.     

Coupled cluster calculations of the ground state potential and interaction induced electric properties of the mixed dimers of helium,neon and argon

Using augmented polarized correlation consistent basis sets extended with midbond functions, we evaluate the ground state interaction potential and the induced electric dipole polarizabilities and first and second hyperpolarizabilities of the He–Ar, Ne–Ar and He–Ne van der Waals complexes. For the calculation of the potential we resort to the coupled cluster singles and doubles (CCSD) model corrected for triple excitations, CCSD(T), whereas properties are evaluated with CCSD response theory. As a check of the quality of the potential, the rovibrational spectrum and the gas second virial coefficients are evaluated. The rovibrational spectra improve previously available theoretical results, although the dissociation energies are probably still slightly underestimated. For the gas second virial coefficients the agreement with experiment is satisfactory. The frequency dependence of the interaction (hyper)polarizabilities is analysed and a comparison with previous results on the mixed dimers and the pure gases is made.     

Relation between orbital binding energies and ionicities in alkali and alkaline earth flourides

The differences between the ionization potentials and binding energies for M^q+ (np⁶), F^?(1 s²), andF^?(2p⁶) orbital electrons, adjusted for the electrostatic self-potentials, in alkali and alkaline earth fluorides have been correlated with ionicites derived from the indices of refraction through optical dispersion theory of Phillips and van Vechten. The differences are linear in ionicities and are related to the covalent energies and the polarizabilities. The gap between the valence band and the Fermi level, determined with X-ray photoelectron spectroscopy, has been compared with the energy in the Phillips-van Vechten model. Whereas the former is a measure of the thermal activation energy for conduction, the latter is determined essentially by the valence electron density, the molar volume, and the polarizability.     

A tight-binding approach to the electronic structure of the silver halides—II: The silver iodide polymorphs

We have applied the modified tight binding approach developed in the first paper of this series to calculate the electronic structure of the silver iodide polymorphs. The four phases which we have investigated are the α, β and γ modifications and the high pressure polymorph having the NaCl structure. The ionicity values obtained for these different phases agree well with the values of Phillips derived from the dielectric data and the computed bandstructures are in good agreement with independent pseudopotential calculations.     

Heat of formation of ternary chalcopyrite semiconductors

Heat of formation of I-III-VI₂ and II-IV-V₂ groups of ternary chalcopyrite semiconductors has been calculated using plasma oscillations theory of solids. Two simple relations between plasmon energy and heat of formation have been proposed. One is based on spectroscopic model of Phillips and Van Vechten and other is based on the best-fit data of heat of formation. The calculated values of heat of formation from both the equations are compared with the experimental values and the values reported by earlier workers. A fairly good agreement has been obtained between them.     

Dynamics of amplitude and phase mode and infrared absorption of linear conductors

The dynamics of the Amplitude and Phase mode in the incommensurate case is reduced to the dynamics of a diatomic molecule. Using this Hamiltonian, σ(ω) is given as a canonical average for a Penn model with fluctuating gap.     

Dipole polarizability of hydrogen atom at high pressures

Dipole polarizability of hydrogen atom at high pressures is investigated using the model of a trapped atom inside a spherical box with impenetrable surface. Both upper and lower bounds for the polarizability are obtained using accurate variational wavefunctions proposed recently. Buckingham polarizabilities calculated from the 1s state wavefunction are shown to be in good agreement with those calculated from the exact values of rⁿ for cage radii less than 2.5.