Electronic and optical properties of CdTe under hydrostatic pressure effect

We have determined the pressure dependence of the direct bandgap, ionicity, transverse effective charge and refractive index for CdTe. The study is performed using the sp³s ^* semi-empirical tight-binding method with the scaling law and Murnaghan`s equation of state. All these properties were found to have a sublinearity behaviour with the pressure. The results are compared with other works.     

Calculation of core-level-shifts from an alternative ionicity scale

Shevchik et al.^1,2 proposed an electrostatic model relating charge transfer between ions and chemical shifts in ionic compounds. The charge transfer can be calculated from the ionicity. If one uses Phillips ionicities one obtains the wrong sign for the calculated core-level-shifts of copper halides as it is pointed out by Shevchik et al.^1,2. We show that the shifts can be computed fairly successfully if one uses an alternative definition of ionicity recently proposed⁴. Using this definition it is further shown that the charge transfer is identical with the longitudinal (Callen) charge of a crystal.     

XPS study on silica-bismuthate glasses and glass ceramics

X-ray photoelectron spectroscopy (XPS) was used to evidence the effect of the Bi₂O₃ to SiO₂ ratio and of partial crystallisation on the electronic charge density around the atoms entering silica-bismuthate glasses of nominal composition 0.01Fe₂O₃⋅0.99[xSiO₂⋅(100−x)Bi₂O₃] with . The core level spectra show significant composition dependent changes in binding energy, and the full width at half maximum of photoelectron peaks both of cations and of oxygen atoms. The analysis reveals changes in electron density correlated with the ionic and covalent character of the samples. The shift in binding energy suggests charge transfer from silicon and oxygen atoms to bismuth atoms. Contrary to the expected behaviour in conventional silicate oxide systems, the results indicate an increase of ionicity for silicon and of covalency for bismuth atoms. The same evolution of ionicity/covalency is observed after partial crystallisation.     

Photoelectron-spectroscopic determination of lattice self-potentials in lanthanide trifluorides

Since the photoelectron spectroscopically measured binding energies probe the potentials at the cation and anion sites separately in solids, they should be a direct determination of the lattice self-potentials at these sites through comparisons with the respective ionization potentials of the gaseous ions. In a study of the photoionization of the 1s orbital electrons on the fluoride ion and of the outer 5p or 4f orbital electrons on the cations of the lanthanide trifluorides, it is found that the so determined lattice self-potentials at the anion sites deviate from those calculated from the electrostatic interactions in the point charge model by an amount which can be explained in terms of ionicity. For the cations, however, the deviations for all except La³⁺ and perhaps Gd³⁺ exceed those expected for ionicity in the point charge model by as much as 2.3 eV in the case of the maximum at Pr³⁺. These deviations are discussed in terms of crystal field interactions, covalency, polarization, and complication caused by final state relaxation during photoionization.     

The bond ionicity in ANB8−N compounds from maximally localized Wannier functions

The bond ionicity in seventy two A^NB^8−N compounds is investigated according to the recently introduced first-principles ionicity scale, based on the centers of the maximally localized Wannier functions, which has several interesting features. The obtained bond ionicities (q_i) are found to exhibit the expected trends, according to electronegativity arguments. In particular, the bond ionicity in the alkaline-earth oxides increases by going from MgO to BaO. A strong crystal structure dependence of q_i is observed. A critical value of q_i (of 0.91) that separates between the tetrahedrally and octahedrally coordinated systems is inferred directly from the calculated values of q_i. The volume dependence of q_i is investigated for all the considered compounds and found to reduce by volume decrease for most of the studied systems. The adopted ionicity scale is established as a very strong competitor to the most widely accepted Phillips and Pauling ionicity measures.     

A mixed bonding band structure calculation for GaAs and AlAs using the APW-k · p method

The Augmented Plane Wave (APW) method and the k.p expansion are used to obtain the band structure of GaAs in a mixed covalent and ionic bonding. The energy levels at the Γ point are also obtained for AlAs. The ionicity is introduced as a parameter into the crystalline potential. We explore the dependence of the energy levels and the momentum matrix elements on the values of the ionicity. The value of the ionicity which gave the direct gap Eg = 1.52eV for GaAs (the accepted experimental value) was found to be 0.1, and for AlAs Eg = 2.50 eV was obtained for an ionicity of 0.2.     

Theoretical study of the adsorption of 3d- and 4d-metals on a WC(0001) surface

The interaction of 3d- and 4d-metals with a WC(0001) surface has been studied theoretically by density-functional theory methods depending on surface termination and adsorbate position. The most stable sites of metal adsorption on the surface have been determined. The binding energy of d-metals with the surface is shown to be higher in the case of carbon terminated surface. This is explained by the predominant ionic-covalent contribution to the chemical bond at the interface, with the bond ionicity being determined by charge transfer from the metals to the electronegative carbon. Analysis of the electronic and structural characteristics has revealed the factors affecting the bonding energetics at the metal-carbide interface depending on the metal d-shell filling with electrons.     

Evolution of the bonding mechanism of ZnO under isotropic compression: A first-principles study

The electronic structure and the bonding mechanism of ZnO under isotropic pressure have been studied by using the full-potential linear augmented plane wave (FP-LAPW) method within the density-functional theory (DFT) based on LDA+U exchange correlation (EXC) potential. We used the theory of Atoms in Molecules (AIM) method to analyze the change of the charge transfer and the bonding strength under isotropic pressure. The results of the theoretical analysis show that charge transfer between Zn and O atomic basins nearly linearly increases with the increasing pressure. Charge density along the Zn-O bond increases under the high pressure. The bonding strength and the ionicity of Zn-O bond also increase with the increasing pressure. The linear evolution process of the bonding mechanism under isotropic pressure was shown clearly in the present paper.     

Ionicities of boron-boron bonds in B(12) icosahedra

First-principles calculations are used to investigate ionicities of boron-boron bonds in B(12) icosahedra. It is observed that the geometrical symmetry breaking of B(12) icosahedra results in the spatial asymmetry of charge density on each boron-boron bond, and further in the ionicity of B(12) icosahedra. The results calculated by a new ionicity scale, a population ionicity scale, indicate that the maximum ionicity among those boron-boron bonds is larger than that of boron-nitrogen bonds in the III-V compound cubic BN. It is of great importance that such an ionicity concept can be extended to boron-rich solids and identical atom clusters.     

Temperature dependent phonon modes and ionicity of LiGaO2 single crystal

This paper reports that polarized far-infrared reflectivity measurements have been done on LiGaO2 single crystal along two crystalline taxes at different temperatures. The temperature dependent frequencies of the longitudinal and transverse optical phonon have been obtained from the real part of optical conductivity and the loss function respectively. A small Drude component is observed at frequency below 300 cm^-1 which could arise from Li ions or oxygen deficiencies. The ionicity of LiGaO2 has been studied from the analysis of the Born effective charge of different ions.     

Energy gaps,charge distribution and optical properties of AlxIn1−xSb ternary alloys

The electronic and optical properties of Al_xIn_1−xSb ternary alloys have been investigated using a pseudopotential approach within the virtual crystal approximation. The effect of alloy disorder on the studied properties has been examined and found to be weak. The extent of the direct-to-indirect band gap transition is found to occur at x = 0.73. Our results agree well with those reported in the literature. Trends in bonding and ionicity are discussed by means of the electron charge distribution. The present study may be a useful information for mid-infrared inter band cascade lasers applications and other antimonide device structures.     

Quantum chemical study of Li-, Na- and K-faujasites

The physico-chemical properties of Li-, Na- and K-zeolites modelled by T₆O₆(OH)₁₂ clusters were studied by the CNDO/2 method. It was shown that the physical characteristics of the zeolite skeleton (charge density, bond strength, electron structure) are practically independent of the type of cation coordinated in its S_II and S_I' cation positions. The studied zeolites differ in the cation charge, which has values of about 0.0, 0.3 and 0.6 for Li, Na and K, respectively, and in the character of the cation-skeleton bonds in the zeolite, whose ionicity decreases in the order, K>Na>62Li. The calculated characteristics of zeolites are employed in the discussion of their interactions with H₂O.     

Calculation of the optical and electronic properties of the zinc-blende alloy Zn1 − xMgxSe

In order to clarify the electronic and optical properties of wide-energy gap zinc-blende structures ZnSe, MgSe and their alloys (ZnSe)_1 − x(MgSe)_x, a simple pseudo-potential scheme (EPM) within an effective potential, the virtual crystal approximation (VCA) which incorporates compositional disorder as an effective potential, is presented. Various quantities, including the fundamental band gap, the energies of several optical gaps, charge densities, ionicity character, transverse effective charge, and refractive index are obtained for this alloy.     

Spectral signatures of critical charge and spin fluctuations in cuprates

We discuss how Raman spectra of high temperature superconducting cuprates are affected by nearly critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We find that specific fingerprints of nearly critical collective modes can be observed and that the selectivity of Raman spectroscopy in momentum space may be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover spin modes have larger masses and are more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuating with high frequency.     

Volume of formation and electronic configuration in dilute alloys

A method is presented to relate the volume of formation to the electronic configuration of impurities in metals. The electronic configuration determines the effective size of the impurity through a modified ThomasFermi-Dirac model combined with elasticity theory. The size of the impurity determines the macroscopic volume change in the crystal, and the electronic configuration can be varied to fit the experimental volume of formation. Alloys of mainly transition metals are studied. The general conclusion of the work is that the charge transfer deduced from the changes in electronic configuration agrees with the expectations of the electronegativity scales, except for a few exceptions. For most of the alloys studied the changes consist of reoccupation of d and non-d levels at each site, maintaining the local neutrality. Only in a few cases were ionic configurations found, and the degree of ionicity was very small.     

Pressure-induced neutral-to-ionic phase transition in TTF-p-chloranil studied by infrared vibrational spectrocopy

Pressure-induced neutral-to-ionic (NI) phase transition in TTF-p-chloranil has been investigated by means of infrared vibrational spectroscopy. Below 10 Kbar, the ionicity of molecules increases continuously with increasing pressure, in contrast with the first-order-like behaviour at the thermally induced NI phase transition. At about 11 Kbar, however, the ionicity shows a sharp increase indicating an onset of the first-order transition to the more ionic phase. At the same time, the lattice undergoes a large dimeric distortion as evidenced by a strong enhancement of the normally infrared-inactive (a_g) molecular vibration.     

Dielectric permittivity and electric modulus of polyethylene oxide (PEO)-LiClO4 composite electrolytes

Dielectric permittivity and conductivity relaxation in polyethylene oxide (PEO)-LiClO₄ salt polymer electrolytes have been investigated for different lithium ion concentrations. We have observed that imaginary modulus spectra exhibit asymmetric maxima with peak-width much broader than that of the Debye peak and are skewed toward the high frequency sides of the maxima. The charge carriers for the electrolyte having higher lithium salt concentration relax much faster than that for other electrolytes and produces higher conductivity. The modulus data have been fitted using non-exponential Kohlrausch-Williams-Watts (KWW) function φ(t). We have observed that the value of the non-exponential parameter (β) is fairly low and nearly constant for different salt concentrations. The low value of β suggests a wide distribution of non-exponential relaxation times. Using the scaling of modulus data we have observed that the relaxation dynamics of charge carriers in these PEO-Li salt based electrolytes is independent of temperature and salt concentration.     

Interatomic bonding in oxygen-stabilized Ti2 Ni-type compounds

X-ray photoelectron-spectroscopic studies of oxygen-stabilized Ti₂ Ni-type compounds Ti₄M₂O_x (M = Fe, Co and Ni;x = 0.5, 1) have revealed some charge transfer from the titanium to the oxygen atoms with no direct involvement of the second metallic constituent M. Limited partial ionicity is deduced from the measured core-electron binding energies, which are intermediate between the respective values for TiO₂ and for the pure elements (Ti⁰, O₂). In particular, approximate estimations of the partial charge residing on Ti are consistent with the charges derived for the oxygen atoms, and the results correlate with the TiO distance in Ti₄M₂O, as compared with corresponding data for other systems. The implications of the interatomic bonding for the hydrogen absorption properties of the oxygen-stabilized compounds are discussed.     

Chemical and physical insight on the local properties of the phosphides XSiP2 (X = Be, Mg, Cd, Zn and Hg) under pressure: from first principles calculations

Local properties of the XSiP₂ (X = Be, Mg, Cd, Zn and Hg) compounds are revisited through the partition of static thermodynamic properties under pressure. We pay attention to the metallization that occurs when the investigated compounds undergo a phase transition from chalcopyrite to the NaCl structure. Electron localization function analysis shows that the local valence basin attractors values decrease as a function of pressure. As the pressure increases, the tetragonal distortion (c/a) diminishes while the degree of ionicity enhances. In addition, by means of atom in molecule approach, atomic-like local compressibility and pressures are analyzed. We found that the basins volumes of the investigated compounds in the NaCl phase have lower compressibilities than those in the chalcopyrite phase. According to the predicted core-valence basins, the phosphorus cation is found to be the more affected by the hydrostatic pressure.