2020同步年報

Neutron Science 081 DFT calculations revealed the origin of the thermoelectric properties of a record high zT of Ge 0.92 Sb 0.08 Te single crys- tals. The work on Ge 0.92 Sb 0.08 Te highlights the importance of phonon engineering approaches to improve the thermo- electric (TE) performance for future applications. (Reported by Shinichiro Yano) This report features the work of Yang-Yuan Chen and his collaborators published in Adv. Sci. 7 , 2002494 (2020). This paper was chosen as the inside back cover of the Nov.-Dec. 2020 issue. ANSTO SIKA – Cold Neutron Triple-axis Spectrometer • INS, Phonon • Materials Science, Chemistry, Condensed-matter Physics, Environmental Science Fig. 2 : Phonon density of states and dispersion relations. Theoretical calculation of partial density of states for (a) GeTe and (b) Ge 0.92 Sb 0.08 Te crystals. The red trace in (b) indicates the contribution from Sb. (c) The VCA method based on calculations to examine the effects of partially doped Sb into GeTe on the phonon dispersion and -PDOS. Two Sb-doping levels, 0.02 (blue traces) and 0.08 (orange traces), are shown for a comparison with that of pristine GeTe (green traces). The right panel shows the supercell used in the DFT calculation of GeTe. Similar DFT calculations were performed for GST with varied con- centrations of Sb; the overall phonon frequency softens because of the Sb doping. In pristine GeTe, no phonon mode is present between 0 and 10 meV at the Γ point and 12–13 at the W point. [Reproduced from Ref. 1] Reference 1. R. K. Vankayala, T.-W. Lan, P. Parajuli, F. Liu, R. Rao, S. H. Yu, T.-L. Hung, C.-H. Lee, S.-I. Yano, C.-R. Hsing, D.-L. Nguyen, C.-L. Chen, S. Bhattacharya, K.-H. Chen, M.-N. Ou, O. Rancu, A. M. Rao, Y.-Y. Chen, Adv. Sci. 7 , 2002494 (2020). Yang-Yuan Chen’s collabora- tors Lotus Hung and Min-Nan Ou (Academia Sinica) have performed several neutron inelastic-scattering experiments on the SIKA spectrometer in ACNS ANSTO (user programs P6776 and P7553). Using 14 days of beam time on SIKA , the team successfully recorded phonon-dispersion data for both GeTe and Ge 0.92 Sb 0.08 Te samples. The samples were mounted on the sample stage of SIKA at room temperature. The measurement of phonon dispersion for a thermoelectric material is becoming popular around the world. This work also proved that SIKA is fully capable of measurements of outstand- ing results for publication in a high-impact journal. The group also conducted the theoretical calculations (DFT) to support their findings; this theoretical support is important to com- plete the study. In summary, the work here has reported a recent study of GeTe and Ge 0.92 Sb 0.08 Te single crystals on the cold triple-axis spectrom- eter SIKA . The phonon disper- sion has been measured at SIKA ;