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Instruments / Technical Area	Publication Details	Published
I11-High Resolution Powder Diffraction	Thermal properties of TiNiSn and VFeSb half-Heusler thermoelectrics from synchrotron X-ray powder diffraction Daniella Ferluccio , Blair Fitzgerald Kennedy , Sonia Barczak , Srinivas Popuri , Claire Murray , Michael Pollet , Jan-Willem Bos Journal Of Physics: Energy DOI: 10.1088/2515-7655/abf41a Diamond Proposal Number(s): [17825] Open Access Show Abstract ▼ Abstract: Half-Heusler alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron X-ray diffraction study of several TiNiSn- and VFeSb-based half-Heuslers. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn based samples was found to be independent of alloying or presence of Cu interstitials with αav = 10.1 × 10-6 K-1 between 400-850 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a much reduced αav = 8.9 × 10-6 K-1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport.	Mar 2021
I11-High Resolution Powder Diffraction	Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn Sonia A. Barczak , Jim Buckman , Ronald I. Smith , Annabelle R. Baker , Eric Don , Ian Forbes , Jan-Willem Bos Materials 11 DOI: 10.3390/ma11040536 Diamond Proposal Number(s): [14032] Open Access Show Abstract ▼ Abstract: TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5–3 mW m−1 K−2 for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m−1 K−1 at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn.	Mar 2018
I15-Extreme Conditions	Atomic ordering in cubic bismuth telluride alloy phases at high pressure I. Loa , J.-W. Bos , R. A. Downie , K. Syassen Physical Review B 93 DOI: 10.1103/PhysRevB.93.224109 Diamond Proposal Number(s): [8105] Show Abstract ▼ Abstract: Pressure-induced transitions from ordered intermetallic phases to substitutional alloys to semi-ordered phases were studied in a series of bismuth tellurides. By using angle-dispersive x-ray diffraction, the compounds Bi 4 Te 5 , BiTe, and Bi 2 Te were observed to form alloys with the disordered body-centered cubic (bcc) crystal structure upon compression to above 14–19 GPa at room temperature. The BiTe and Bi 2 Te alloys and the previously discovered high-pressure alloys of Bi 2 Te 3 and Bi 4 Te 3 were all found to show atomic ordering after gentle annealing at very moderate temperatures of ∼100 ∘ C . Upon annealing, BiTe transforms from bcc to the B2 (CsCl) crystal-structure type, and the other phases adopt semi-disordered variants thereof, featuring substitutional disorder on one of the two crystallographic sites. The transition pressures and atomic volumes of the alloy phases show systematic variations across the Bi m Te n series including the end members Bi and Te. First-principles calculations were performed to characterize the electronic structure and chemical bonding properties of B2-type BiTe and to identify the driving forces of the ordering transition. The calculated Fermi surface of B2-type BiTe has an intricate structure and is predicted to undergo three topological changes between 20 and 60 GPa.	Jun 2016

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