Electronic structure and thermoelectric properties of n- and p-type SnSe from first principles calculations

We present results of electronic band structure, Fermi surface and electron transport properties calculations in orthorhombic $n$- and $p$-type SnSe, applying Korringa-Kohn-Rostoker method and Boltzmann transport approach. The analysis accounted for temperature effect on crystallographic parameters in $Pnma$ structure as well as the phase transition to $CmCm$ structure at $T_csim 807 $K. Remarkable modifications of conduction and valence bands were notified upon varying crystallographic parameters within the structure before $T_c$, while the phase transition mostly leads to jump in the band gap value. The diagonal components of kinetic parameter tensors (velocity, effective mass) and resulting transport quantity tensors (electrical conductivity $sigma$, thermopower $S$ and power factor PF) were computed in wide range of temperature ($15-900 $K) and, hole ($p-$type) and electron ($n-$type) concentration ($10^{17}-10^{21}$ cm$^{-3}$). SnSe is shown to have strong anisotropy of the electron transport properties for both types of charge conductivity, as expected for the layered structure. In general, $p$-type effective masses are larger than $n$-type ones. Interestingly, $p$-type SnSe has strongly non-parabolic dispersion relations, with the pudding-mold-like shape of the highest valence band. The analysis of $sigma$, $S$ and PF tensors indicates, that the inter-layer electron transport is beneficial for thermoelectric performance in $n$-type SnSe, while this direction is blocked in $p$-type SnSe, where in-plane transport is preferred. Our results predict, that $n$-type SnSe is potentially even better thermoelectric material than $p$-type one. Theoretical results are compared with single crystal $p$-SnSe measurements, and good agreement is found.

Pressure effects on the superconductivity of HfPd2Al Heusler compound: Experimental and theoretical study

Polycrystalline HfPd2Al has been synthesized using the arc-melting method and studied under ambient pressure conditions by x-ray diffraction from room temperature up to 450^oC. High pressure x-ray diffraction up to 23 GPa was also performed using Diacell-type membrane diamond anvil cells. The estimated linear thermal expansion coefficient was found to be {alpha} = 1.40(3)x10^{-5} K^{-1}, and the bulk modulus derived from the fit to the 3rd order Birch-Murnaghan EOS (BMEOS) is B0 = 97(2) GPa. Resistivity studies under applied pressure (p < 7.49 GPa) showed a linear decrease of superconducting critical temperature with increasing pressure and the slope dTc/dp = -0.13(1) K GPa^{-1}. The same behavior is observed for the electron-phonon coupling constant {lambda_{ep}}(p) that changes from 0.67 to 0.6, estimated for p = 0.05 GPa and 7.49 GPa, respectively. First principles electronic structure and phonon calculation results are presented and used to estimate the magnitude of electron-phonon interaction {lambda_{ep}} and its evolution with pressure. Theoretical results explain the experimentally observed decrease in Tc due to considerable lattice stiffening.

Importance of relativistic effects in electronic structure and thermopower calculations for Mg2Si, Mg2Ge and Mg2Sn

We present a theoretical study of the influence of the relativistic effects on electronic band structure and thermopower of Mg2X(X= Si, Ge, Sn) semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used, and the detailed comparison between the fully relativistic and semi-relativistic electronic structure features is done. We show that the spin-orbit (S-O) interaction splits the valence band structure at Gamma point in good agreement with the experimental data, and this effect strongly depends on X atom. The S-O modifications of the topology of the Gamma-centered hole-like Fermi surface pockets lead to a change in electron transport properties, which are investigated using the Boltzmann approach. In addition, the simple and efficient method is presented for the calculation of density of states effective mass m*, and then used to examine the impact of relativistic effects on m*. It is found that S-O coupling of the valence bands reduces effective mass and therefore significantly lowers the thermopower, primarily in Mg2Sn, but also in Mg2Ge. A detrimental influence of the S-O interaction on thermoelectric performance of p-type Mg2X is analyzed in function of temperature (10-900 K) and carrier concentration (10^18-10^22 cm-3). Interestingly, similar calculations in n-type Mg2X, show negligible effect of the S-O interaction on lowest conduction bands and consequently also on the Seebeck coefficient.