Gregory Pomrehn

Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu2Zn1–xFexGeSe4

Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu(2)Zn(1-x)Fe(x)GeSe(4). Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials.

Defect‐Controlled Electronic Properties in AZn2Sb2 Zintl Phases

Experimentally, AZn2Sb2 samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A(1-x)Zn2Sb2 compounds.

Dopants effect on the band structure of PbTe thermoelectric material

PbTe is a promising thermoelectric material and its dimensionless figure of merit, zT, can be enhanced by optimizing the band structure near the Fermi level via chemical doping. This letter describes the dopants effect on bandgap, E_g, and effective mass, m*, for disordered La- and I-doping, based on theoretical calculations. E_g increases with increasing La and decreases with increasing I concentration. While m* increases upon La-doping, I-doping does not change m* noticeably. The calculated results are qualitatively consistent with the experimental results and explain the higher zT, up to 1.4 at 800 K, observed in I-doping PbTe compared to La-doping.

Entropic stabilization and retrograde solubility in Zn_4Sb_3

Zn_4Sb_3 is shown to be entropically stabilized versus decomposition to Zn and ZnSb through the effects of configurational disorder and phonon free energy. Single-phase stability is predicted for a range of compositions and temperatures. Retrograde solubility of Zn is predicted on the two-phase boundary region between Zn_4Sb_3 and Zn. The complex temperature-dependent solubility can be used to explain the variety of nanoparticle formation observed in the system: formation of ZnSb on the Sb-rich side, Zn on the far Zn-rich side, and nano-void formation due to Zn precipitates being reabsorbed at lower temperatures.

Predicted electronic and thermodynamic properties of a newly discovered Zn8Sb7 phase.

A new binary compound, Zn(8)Sb(7), has recently been prepared in nanoparticulate form via solution synthesis. No such phase is known in the bulk phase diagram; instead, one would expect phase separation to the good thermoelectric semiconductors ZnSb and Zn(4)Sb(3). Here, density functional calculations are employed to determine the free energies of formation, including effects from vibrations and configurational disorder, of the relevant phases, yielding insight into the phase stability of Zn(8)Sb(7). Band structure calculations predict Zn(8)Sb(7), much like ZnSb and Zn(4)Sb(3), to be an intermetallic semiconductor with similar thermoelectric properties. If sufficient entropy or surface energy exists to stabilize the bulk material, it would be stable in a limited temperature window at high temperature.

Validity of rigid band approximation of PbTe thermoelectric materials

The tuning of carrier concentration through chemical doping is very important for the optimization of thermoelectric materials. Traditionally, a rigid band model is used to understand and guide doping in such semiconductors, but it is not clear whether such an approximation is valid. This letter focuses on the changes in the electronic density of states (DOS) near the valence band maximum for different p-type dopants (Na, K, Tl, or vacancy on Pb site) maintaining the high symmetry of the NaCl structure. Na- and K-doped, and vacancy-introduced PbTe show a clear rigid-band like change in DOS unlike that concluded from supercell based calculations.

Thermoelectric properties and electronic structure of the zintl-phase Sr3AlSb3

The Zintl-phase Sr3 AlSb3 , which contains relatively earth-abundant and nontoxic elements, has many of the features that are necessary for good thermoelectric performance. The structure of Sr3 AlSb3 is characterized by isolated anionic units formed from pairs of edge-sharing tetrahedra. Its structure is distinct from previously studied chain-forming structures, Ca3 AlSb3 and Sr3 GaSb3 , both of which are known to be good thermoelectric materials. DFT predicts a relatively large band gap in Sr3 AlSb3 (Eg ≈1 eV) and a heavier band mass than that found in other chain-forming A3 MSb3 phases (A=Sr, Ca; M=Al, Ga). High-temperature transport measurements reveal both high resistivity and high Seebeck coefficients in Sr3 AlSb3 , which is consistent with the large calculated band gap. The thermal conductivity of Sr3 AlSb3 is found to be extremely low (≈ 0.55 W mK(-1) at 1000 K) due to the large, complex unit cell (56 atoms per primitive cell). Although the figure of merit (zT) has not been optimized in the current study, a single parabolic band model suggests that, when successfully doped, zT≈ 0.3 may be obtained at 600 K; this makes Sr3 AlSb3 promising for waste-heat recovery applications. Doping with Zn(2+) on the Al(3+) site has been attempted, but does not lead to the expected increase in carrier concentration.

Thermoelectric properties and electronic structure of the Zintl phase Sr5In2Sb6 and the Ca5−xSrxIn2Sb6 solid solution

The Zintl phase Sr5In2Sb6 is isostructural with Ca5In2Sb6-a promising thermoelectric material with a peak zT of 0.7 when the carrier concentration is optimized by doping. Density functional calculations for Sr5In2Sb6 reveal a decreased energy gap and decreased valence band effective mass relative to the Ca analog. Chemical bonding analysis using the electron localizability indicator was found to support the Zintl bonding scheme for this structure type. High temperature transport measurements of the complete Ca(5-x)Sr(x)In2Sb6 solid solution were used to investigate the influence of the cation site on the electronic and thermal properties of A5In2Sb6 compounds. Sr was shown to be fully miscible on the Ca site. The higher density of the Sr analog leads to a slight reduction in lattice thermal conductivity relative to Ca5In2Sb6, and, as expected, the solid solution samples have significantly reduced lattice thermal conductivities relative to the end member compounds.

Enhanced thermoelectric properties of Sr5In2Sb6via Zn-doping

Zintl phases exhibit inherently low thermal conductivity and adjustable electronic properties, which are integral to designing high-efficiency thermoelectric materials. Inspired by the promising thermoelectric figure of merit of optimized A5M2Sb6 Zintl phases (A = Ca or Sr, M = Al, Ga, In), Zn-doped Sr5In2−xZnxSb6 (x = 0, 0.025, 0.05, 0.1) compounds were investigated. Optical absorption measurements combined with band structure calculations indicate two distinct energy transitions for Sr5In2Sb6, one direct (Eg ∼ 0.3 eV) and the other from a lower valence band manifold to the conduction band edge (Eg ∼ 0.55 eV). Sr5In2Sb6 exhibits nondegenerate p-type semiconducting behavior with low carrier concentration (∼4 × 1018 h+ cm−3 at 300 K). Charge carrier tuning was achieved by Zn2+ substitution on the In3+ site, increasing carrier concentrations to up to 1020 h+ cm−3. All samples displayed relatively low thermal conductivities (∼0.7 W m−1 K−1 at 700 K). The Zn-doped samples exhibited significantly higher zT values compared to the undoped sample, reaching a value of ∼0.4 at 750 K for Sr5In1.9Zn0.1Sb6.