Entropic Stabilization and Retrograde Solubility in Zn4Sb3

Zn4Sb3 is shown to be entropically stabilized versus decomposition to Zn and ZnSb though 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 Zn4Sb3 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.Comment: 5 pages, 5 figure

Predicted Electronic and Thermodynamic Properties of a Newly Discovered Zn_8Sb_7 Phase

A new binary compound, Zn_8Sb_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_4Sb_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_8Sb_7. Band structure calculations predict Zn_8Sb_7, much like ZnSb and Zn_4Sb_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

Doping of p-type ZnSb: Single parabolic band model and impurity band conduction

Even though the ZnSb compound has been known for decades and used in the earliest thermoelectric devices, the potential of the material as a modern thermoelectric may be underestimated. We synthesized p-type doped samples using ball-milling and hot-pressing and measured their thermoelectric properties including mobility and carrier concentration. Establishing a single parabolic band (SPB) model using these measurements on the Cu, Sn, and self-doped samples allows for predictions on the optimum thermoelectric efficiency. It is projected to reach zT = 0.75 at 700 K. Deviations from the SPB model at low carrier concentrations are discussed and impurity band conduction is brought in as a possible explanation

Influence of the Triel Elements (M = Al, Ga, In) on the Transport Properties of Ca_5M_2Sb_6 Zintl Compounds

The Zintl compound Ca_5Al_2Sb_6 has extremely low lattice thermal conductivity (<0.6 W/mK at 1000 K) and tunable electronic properties, making it a promising thermoelectric material for high temperature waste-heat recovery. The current study investigates trends in the chemical and transport properties of the Ca_5M_2Sb_6 compounds (M = Al, Ga, or In), revealing potential routes toward improved thermoelectric properties in this system. Here, we show that isoelectronic M-site substitutions can be used to “fine-tune” the electronic properties of the Ca_5M_2Sb_6 system, without inducing electronic doping effects. Electronic structure calculations reveal that the electronegativity of the M element is a good indicator for the energy level of M electronic states. The effects of M-site substitutions on the effective mass and band gap are reflected in measurements of the high temperature electronic properties of Ca_5M_2Sb_6 samples (M = Al, Ga, and In) which reveal increased hole mobility as well as a smaller thermal band gap in the Ga analogue, relative to Ca_5Al_2Sb_6 and Ca_5In_2Sb_6. Optical absorption measurements reveal a trend in the direct band gaps consistent with both calculations and transport measurements. Additionally, a direct benefit of substituting heavier elements on the Al site arises from the increased density and softer lattice, which leads to reduced sound velocity and lattice thermal conductivity

Defect-Controlled Electronic Properties in AZn_2Sb_2 Zintl Phases

Experimentally, AZn_2Sb_2 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 A1−xZn_2Sb_2 compounds

Ab initio study of intrinsic point defects in PbTe: an insight into phase stability

The stability of intrinsic point defects in PbTe, one of the most widely studied and efficient thermoelectric material, is explored by means of Density Functional Theory (DFT). The origin of n- and p-type conductivity in PbTe is attributed to particular intrinsic charged defects by calculating their formation energies. These DFT calculated defect formation energies are then used in the Gibbs free energy description of this phase as part of the Pb-Te thermodynamic model built using the CALPHAD method, and in the resulting phase diagram it is found that its solubility lines and non-stoichiometric range agree very well with experimental data. Such an approach of using DFT in conjunction with CALPHAD for compound semiconductor phases that exhibit very small ranges of non-stoichiometry does not only make the process of calculating phase diagrams for such systems more physical, but is necessary and critical for the assessment of unknown phase diagrams