Thermoelectric figure of merit, ZT, exceeding 2.6 at 850 K and copper electromigration inhibition have been demonstrated in indium modified Cu2Se.

Pb(7)Bi(4)Se(13) crystallizes in the monoclinic space group C2/m (No. 12) with a = 13.991(3) Å, b = 4.262(2) Å, c = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses N(1) = 5 and N(2) = 4 [N = number of edge-sharing (Pb/Bi)Se6 octahedra along the central diagonal] are arranged along the c axis in such a way that the bridging monocapped trigonal prisms, PbSe7, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m(-1) K(-1) at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb(7)Bi(4)Se(13) is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb(7)Bi(4)Se(13) is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼-160 μV K(-1) at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb(7)Bi(4)Se(13) and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples.

Abstract:Previous studies have indicated that the figure of merit (ZT) of half-Heusler (HH) alloys with composition MNiSn (M = Ti, Zr, or Hf) is greatly enhanced when the alloys contain a nano-scale full-Heusler (FH) MNi 2 Sn second phase. However, the formation mechanism of the FH nano-structures in the HH matrix and their vibrational properties are still not well understood. We report on first principles studies of thermodynamic phase equilibria in the MNiSn-MNi 2 Sn pseudo-binary system as well as HH and FH vibrational properties. Thermodynamic phase diagrams as functions of temperature and Ni concentration were developed using density functional theory (DFT) combined with a cluster expansion and Monte Carlo simulations. The phase diagrams show very low excess Ni solubility in HH even at high temperatures, which indicates that any Ni excess will decompose into a two-phase mixture of HH and FH. Vibrational properties of HH and FH alloys are compared. Imaginary vibrational modes in the calculated phonon dispersion diagram of TiNi 2 Sn indicate a dynamical instability with respect to cubic [001] transverse acoustic modulations. Displacing atoms along unstable vibrational modes in cubic TiNi 2 Sn reveal lower energy structures with monoclinic symmetry. The energy of the monoclinic structures are found to depend strongly on the lattice parameter. The origin of the instability in cubic TiNi 2 Sn and its absence in cubic ZrNi 2 Sn and HfNi 2 Sn is attributed to the small size of the Ti 3d shells compared to Zr and Hf atoms. Lattice constants and heat capacities calculated by DFT agree well with experiment.