The AZn2Sb2 (Pm1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT∼ 1) and is an appealing system for the development of Zintl structure–property relationships. High temperature transport measurements have previously been conducted for all known compositions except for SrZn2Sb2; here we characterize polycrystalline SrZn2Sb2 to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn2Sb2 compounds, SrZn2Sb2 is found to be a hole-doped semiconductor with a thermal band gap ∼ 0.27 eV. The Seebeck coefficients of the AZn2Sb2 compounds are found to be described by similar effective mass (m* ∼ 0.6 me). Electronic structure calculations reveal similar m* is due to antimony p states at the valence band edge which are largely unaffected by the choice of A-site species. However, the choice of A-site element has a dramatic effect on the hole mobility, with the room temperature mobility of the rare earth-based compositions approximately double that found for Ca and Sr on the A site. This difference in mobility is examined in the context of electronic structure calculations.

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Electronic structure and transport in thermoelectric compounds AZn2Sb2 (A = Sr, Ca, Yb, Eu).

Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca_5Al_2Sb_6 is a charge-balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ∼2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca_5Al_2Sb_6 possesses an extremely low lattice thermal conductivity (0.6 W mK^(−1) at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_(4.75)Na_(0.25)Al_2Sb_6 possesses a near-optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca_5Al_2Sb_6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.

The Zintl Compound Ca5Al2Sb6 for Low‐Cost Thermoelectric Power Generation

Density-functional methods are used to analyze the scaling of discrete oligomeric π-electron conducting molecules towards idealized isolated polymer chains, treated in periodic boundary conditions. The band gaps of a series of conjugated oligomers of incrementally increasing lengths exactly fit a nearly-free-electron molecular-orbital picture and exhibit a smooth deviation from the classical empirical "I/N" trend for long oligomers and infinite polymers. The calculations also show a smooth convergence of bond lengths. The full band structures and densities of states of a polyacetylene, polypyrrole, polyfuran, and polythiophene show that band crossing, localized bands, and other effects cannot be accurately determined from simple extrapolation of oligomer electronic structures. Systematic comparisons of the electronic structure variations of the polymers investigated indicate that the electron affinity, rather than the electronegativity of the heteroatom or the bond-length alternation of the conjugated backbone, significantly affects the band gap of the resulting polymer as indicated by the presence of heteroatom states in the partial density of states of the conduction band, requiring revision of previous semiempirical analyses. Consequences for doping processes are also studied, along with a comparison of valence bandwidths, conduction bandwidths, and carrier effective masses as a function of heteroatom.

Electronic structure of conducting polymers: Limitations of oligomer extrapolation approximations and effects of heteroatoms

First-principles studies of Al–Ni intermetallic compounds

The DMol methodology is reviewed with special emphasis on recent developments such as gradient dependent, or so-called nonlocal, density functionals. Applications to crystalline silicon structures in the nanometer size imply a discussion of excited states. This is also a first application using periodic boundary conditions. The application to carbonyl complexes is a case study of the effect of some recent gradient dependent density functionals on the energy surface.

DMol, a standard tool for density functional calculations: Review and advances

We report ab initio density-functional calculations of the structural, electronic, and optical properties of ${\mathrm{NiAl}}_{3}$, using the full-potential linearized augmented plane wave method within the generalized gradient approximation to the exchange-correlation potential. The $D{0}_{11}$ structure is found to be energetically favorable over both the cubic $L{1}_{2}$ and $A15$ phases. The density of states around the Fermi energy, including a pseudogap just above it, is dominated by strongly hybridized $\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}d$ and $\mathrm{Al}\phantom{\rule{0.3em}{0ex}}p$ states. We further present a fully first principles study of the optical properties of ${\mathrm{NiAl}}_{3}$, using the long wavelength random phase approximation expression for the dielectric function obtained within linear response theory, with full matrix elements. Our calculations cover a large frequency range, extending previous theoretical studies to energies going up to and beyond the effective plasma frequency, which we calculate to be $\ensuremath{\sim}16.84\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Our results are analyzed in the light of the calculated electronic band structure and density of states, and compared with experimental findings. In the low energy range $(\ensuremath{\lesssim}5\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, where data from different experimental techniques coincide, the calculated reflectivity reproduces very well the main features observed. At higher frequencies, we find a Drude-like behavior, dressed by interband transitions. Our findings invite future measurements that will allow a fuller comparison between theory and experiments.

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Structural, electronic, and optical properties of NiAl3: First-principles calculations

We report the synthesis, structure, spectroscopic properties, charge and thermal transport, and electronic structure of a new member of the Zintl family, Yb5Al2Sb6 The compound crystallizes in the Ba5Al2Bi6 structure type and requires the addition of Ge or Si in the synthesis, which appears to act as a catalyst Yb5Al2Sb6 has an anisotropic structure with infinite anionic double chains cross-linked by Yb2+ ions Polycrystalline ingots of Yb5Al2Sb6 prepared in the presence of 05 mol equiv of Ge showed room-temperature conductivity, thermopower, and thermal conductivity of ∼1100 S/cm, ∼20 μV/K, and ∼38 W/m·K, respectively Investigations of other solid solutions of Yb5Al2Sb6, doping effects, and chemical modifications are discussed Sr only partially replaces Yb in the structure leading to Sr085Yb415Al2Sb6 Electronic structure calculations performed using a highly precise full-potential linearized augmented plane wave method within the density functional theory scheme show the presence of a negative b

Synthesis, Structure and Charge Transport Properties of Yb5Al2Sb6: A Zintl Phase with Incomplete Electron Transfer

Results of first principles local density total energy and atomic force calculations carried out for free C60 and XC60 (X=K, Rb, Cs) molecular clusters are reported. The optimization of the geometry results in the bond lengths between adjacent carbon atoms being 1.387 and 1.445 A, which are in very good agreement with the latest X-ray diffraction values. Energy levels, charge distributions, and wavefunction characteristics are obtained and discussed. The results for C60 are in very good agreement with recently measured photoemission energy distribution curves (EDC) for the valence band states. The highest occupied molecular orbitals (HOMO) are found to be fully occupied Hu states and are 1.7 eV below the lowest unoccupied molecular orbitals (LUMO) which are of T1u symmetry. Similar results obtained for the XC60 clusters show that rigid-band-like behavior is found in the electronic structures after putting an alkali atom at the center of a C60 ball. In each case, the alkali atom is almost fully ionized with the transferred electron distributed over the surface shell of C60; the center region of the ball has very low charge density.

Local density functional study of the structural and electronic properties of C60 and XC60 (X=K, Rb, Cs)

The structural and electronic properties of H and ${\mathrm{NH}}_{2}$ adsorbed onto the Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface are studied using first-principles local-density total-energy and atomic force calculations carried out for cluster models. The results show that different adsorption sites, i.e., the center adatom site, the corner adatom site, and the first-layer ``rest-atom'' site, exhibit different chemisorption properties. The results obtained from energy minimization for H and ${\mathrm{NH}}_{2}$ on these possible sites deduced from the Auger electron spectroscopy and photoemission yield spectroscopy studies confirm the model of a two-step adsorption process. The electronic structures of the adsorption systems were determined and a spectrum of orbital overlap population ${\mathit{P}}_{\mathit{O}}$(\ensuremath{\varepsilon}) versus energy \ensuremath{\varepsilon} was calculated that is found to bear close relationship with images obtained from scanning tunneling microscope (STM) studies. We conclude from the \ensuremath{\varepsilon} properties of ${\mathit{P}}_{\mathit{O}}$(\ensuremath{\varepsilon}) that H and ${\mathrm{NH}}_{2}$ adsorbed on the rest-atom site may not be detected by the STM but are still visible when absorbed on the adatom sites.

L. Ye

Papers

Structural, electronic, and optical properties of NiAl3: First-principles calculations

Synthesis, Structure and Charge Transport Properties of Yb5Al2Sb6: A Zintl Phase with Incomplete Electron Transfer

Local density functional study of the structural and electronic properties of C60 and XC60 (X=K, Rb, Cs)

Surface properties of Si(111)7×7 upon H and NH2 adsorption: A local-density-functional study

Electronic structure and possible mechanism of potassium induced promotion of oxidation of Si(001)2×1