Linker conformation effects on the band gap in metal-organic frameworks.

In this work, we investigate how torsion in the middle aromatic ring on the terphenyldicarboxylate linker in UiO-68 affects the band gap. Furthermore, we incorporate the effect of monosubstitution on the linker (UiO-68-R; R = H, F, I, NH2, and NO2) in order to shed light on a possible route to tune the band gap by changing the torsional angle by substitutions. Our computations show that both the torsional angle and band gaps depend on the choice of the substituent, and it is, in fact, possible to tune the band gap through the substitution's effect of locking down the middle aromatic ring at different torsional angles, in combination with the substituents' electronic effects.

Electronic structure of thermoelectric Zn-Sb.

The electronic structures of the two main compounds of the binary zinc antimonides that are stable at room temperature, Zn(1)Sb(1) and ß-Zn(4)Sb(3), were probed with x-ray photoelectron spectroscopy. Additionally, electron energy loss measurements and density functional theory calculations are presented. The compounds are found to share a very similar electronic structure. They both feature only small charge transfers and differ moderately in their screening potentials. These results are in line with recent theoretical works on the Zn-Sb system and are discussed in light of the reported thermoelectric performance of the materials.

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

The AZn(2)Sb(2) (P3m1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT approximately 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 SrZn(2)Sb(2); here we characterize polycrystalline SrZn(2)Sb(2) to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn(2)Sb(2) compounds, SrZn(2)Sb(2) is found to be a hole-doped semiconductor with a thermal band gap approximately 0.27 eV. The Seebeck coefficients of the AZn(2)Sb(2) compounds are found to be described by similar effective mass (m* approximately 0.6 m(e)). 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.