The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.

The full potential augmented plane wave plus local orbital method using the local density approximation within the framework of density functional theory is applied to investigate structural, electronic, and thermodynamic properties of SnxGe1−x alloys for small Sn compositions (x=0.0625, 0.125, 0.1875, and 0.25). For the structural properties, we found strong deviation from Vegard’s law for the variation in the lattice parameter, moreover, this deviation is found positive as found experimentally. This feature is in direct contrast with conventional IV-IV alloys, were the deviation of the variation in the lattice parameter from Vegard’s law is generally weak and negative. The calculated bond lengths of Sn–Ge, also show significant departures of bond lengths from the virtual crystal approximation (VCA). The calculations confirm a strong band gap reduction in Ge. For small Sn incorporation, the calculated optical band gap bowing (i.e., bowing of the direct band gap) is found strongly composition dependent. For small Sn composition (x=0.0625), we found a strong optical band gap bowing of 2.9 eV, in very good agreement with the measured values at low Sn composition of 2.8 eV of [He and Atwater, Phys. Rev. Lett. 79, 1937 (1997)] and 2.84 eV of Pérez Ladrón de Guevara et al. [Appl. Phys. Lett. 91, 161909 (2007)]. For small composition regime (0<x<0.1875) we found an optical band gap bowing of 1.9 eV again in good agreement with the measured value of 1.94 eV at room temperature. For the indirect band gap at L point, a bowing of 0.90 eV is found in agreement with the measured value of 1.23 eV. Regarding the local environment of the Sn atoms, we notice that the clustering has a strong influence on the direct band gap; the maximal (minimal) Sn-clustered configurations have the highest (lowest) band gap. From a detailed analysis of the physical origin of the optical band gap bowing, we found that the relative contribution of the three components [volume deformation (VD), charge exchange, and strain] show that the most significant effect is the structural one (relaxation and VD) due to the large mismatch of the lattice constants of Sn and Ge (∼15%). Our results show that the change from indirect to direct band gap occurs at ∼0.105 in perfect agreement with the measured value of 0.1. Our value for the critical composition is found lower than the value predicted by the VCA (∼0.20). Finally, we found that the instability of SnGe alloys is basically dominated by a strong charge transfer between Sn and Ge, leading a positive chemical energy, this instability is amplified by positive strain energy. The relatively weak negative structural energy is not enough to stabilize the SnGe alloys.

Using the electronic charge densities obtained by the empirical pseudopotential method for tetrahedrally bonded semiconductors, an ionicity scale is established that is in good agreement with the Phillips ionicity scale.

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Ba 8 Ga 10 Si 36 and Ba8Ga16Si30 clathrates were prepared by the arc melting method and characterized for their structure, transport, and thermal properties by x-ray diffraction, electron probe microanalysis, electrical resistivity, and thermal conductivity measurements. The Rietveld refinement results gave the total occupancy of Ga/Si equal to 3.49/2.01, 0.99/14.01, and 5.51/17.99 for Ba8Ga10Si36 and 3.99/2.01, 1.99/14.01, and 10.01/13.99 for Ba8Ga16Si30, respectively, for 6c, 16i, and 24k sites. From the refined isotropic thermal displacement parameters, Debye temperature, average velocity of sound, lattice thermal conductivity, and Einstein temperature values were estimated. The calculated lattice thermal conductivity values were in agreement with room temperature experimental values of 1.128 and 1.071 W m−1 K−1, respectively, for two clathrates. The temperature variation of (300–850 K) electrical resistivity of Ba8Ga16Si30 showed metallic like behavior, whereas that of Ba8Ga10Si36 showed semiconducting behavior. The semiconducting nature was due to vacancies in the framework sites. High temperature thermal conductivity above Debye temperature was explained with anharmonic approximation.

The electronic band structure and density of states for zinc-blende nitrides GaN, AIN, and BN are reported using an sp3s* tight-binding method. The large band gap found for these compounds is analyzed in terms of chemical and volume deformation effects. The calculated ionicity agrees with the Phillips scale and shows a strongly ionic character for GaN and AlN, while BN is found more covalent.') Es-