Crystal Structure, Stability, and Physical Properties of Metastable Electron-Poor Narrow-Gap AlGe Semiconductor

We report for the first time the full crystal structure, the electronic structure, the lattice dynamics, and the elastic constants of metastable monoclinic AlGe. In addition to ultrarapid cooling techniques such as melt spinning, we show the possibility of obtaining monoclinic AlGe by water-quenching in a quartz tube. Monoclinic AlGe and rhombohedral Al₆Ge₅ are competing phases with similar stability since they both begin to decompose above 230 °C. The crystal structure and electronic bonding of monoclinic AlGe are similar to those of ZnSb and comply with its 3.5 valence electrons per atom: besides classical two electron–two center Al–Ge and Ge–Ge covalent bonds, Al₂Ge₂ parallelogram rings are formed by uncommon multicenter bonds. Monoclinic AlGe could be used in various applications since it is found theoretically to be an electron-poor semiconductor with a narrow indirect energy bandgap of about 0.5 eV. The lattice dynamics calculations show the presence of low energy optical phonons, which should lead to a low thermal conductivity

Improved Power Factor in Self-Substituted Fe₂VAl Thermoelectric Thin Films Prepared by Co-sputtering

We present a strong improvement of the electronic transport properties in the Fe2VAl Heusler alloy obtained in thin-film form by a co-sputtering process. The power factor is improved when deposition occurs at temperatures close to 873 K and when the composition is tuned using a co-sputtering process. High values up to 5.6 mW/K2m are obtained for n-type films deposited at 873 K, which is up to now a record for self-substituted Fe2VAl thermoelectric thin films. The influence of co-sputtering conditions on atomic composition and the substrate effect on electronic transport properties are also presented

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α‑As₂Te_3–<i>x</i>Se_<i>x</i> (0 ≤ <i>x</i> ≤ 1.5)

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5–523 K) of the solid solution α-As₂Te_3–<i>x</i>Se_<i>x</i> (0 ≤ <i>x</i> ≤ 1.5). All of the polycrystalline compounds α-As₂Te_3–<i>x</i>Se_<i>x</i> crystallize isostructurally in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12, <i>Z</i> = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing <i>x</i> up to <i>x</i> = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As₂Te₃. The electrical resistivity and thermopower gradually increase with <i>x</i> as a result of the progressive increase in the band gap energy. Despite the fact that α-As₂Te₃ exhibits very low lattice thermal conductivity κ_L, the substitution of Se for Te further lowers κ_L to 0.35 W m^–1 K^–1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering

Polymorphism in Thermoelectric As₂Te₃

Metastable β-As₂Te₃ (<i>R</i>3̅<i>m</i>, <i>a</i> = 4.047 Å and <i>c</i> = 29.492 Å at 300 K) is isostructural to layered Bi₂Te₃ and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As₂Te₃ leads to multiphase samples, while β-As₂Te₃ could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As₂Te₃ reconstructively transforms into stable α-As₂Te₃ (<i>C</i>2/<i>m</i>, <i>a</i> = 14.337 Å, <i>b</i> = 4.015 Å, <i>c</i> = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As₂Te₃ layers into the van der Waals bonding interspace. Upon cooling, β-As₂Te₃ displacively transforms in two steps below <i>T</i>_S1 = 205–210 K and <i>T</i>_S2 = 193–197 K into a new β′-As₂Te₃ allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β′-As₂Te₃ crystal structure (<i>P</i>2₁/<i>m</i>, <i>a</i> = 6.982 Å, <i>b</i> = 16.187 Å, <i>c</i> = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As₂Te₃ is accompanied by a 4-fold modulation along its <i>b</i> axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β′-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As₂Te₃ layers in all three structures