The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the ''phonon-glass electron crystal'' (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg 3 Sb 2 and its derivative of isoelectronically Bi doped Bi; Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) alloys in Mg 3 Sb 2 . Single phase p-type Mg 3 Sb 2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg 3 Sb 2 . However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 6 10 20 /cm 3 for x ¢0.25. The substantial increase in ZT in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

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We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

The optical transmission of LiNbO3 single crystals has been measured in the wavelength range 200–900 nm, for different degrees of reduction, to study the effect of reduction on the optical characteristics of LiNbO3 near the fundamental absorption edge. The optical transitions in LiNbO3 were found to be indirect and the band gap decreased with increasing degree of reduction. The band observed at 2.48 eV in the absorption spectrum in heavily reduced samples has been attributed to the formation of polarons, and the theoretical model of Reik and Heese [J. Chem. Solids 28, 581 (1967)] for small polarons is used to correlate the optical and electrical properties.

We report the use of Boltzmann transport theory to investigate the electrical properties of thermoelectric Mg 2 Si, Mg 2 Sn, and a supercell model of the 50-50 alloy. The results are based on first-principles electronic structure calculations with the modified Becke-Johnson potential of Tran and Blaha, which yields band gaps in good accord with experiment. The calculated transport coefficients are discussed in relation to the thermoelectric performance of these materials. The results imply roughly symmetric behavior with respect to carrier type and the possibility of improvements in ZT , especially for p-type and lower temperatures.