Phonon-glass and heterogeneous electrical transport in A-site-deficient SrTiO3
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The phonon-glass electron crystal concept is one of the key guiding principles for the development of efficient thermoelectric materials. Here, we confirm that SrTiO 3 becomes a phonon-glass for large numbers of A-site vacancies in the Sr 1-x La 0.67x → 0.33x TiO 3 series and show that its electron crystal properties are stymied by the presence of a core-shell grain structure. Thermal conductivity, heat capacity, and neutron powder diffraction, complemented by representational analysis and phonon calculations, were used to investigate the thermal transport. This reveals that the heat carrying modes are dominated by Sr motions and that these become more localized upon the introduction of the A-site vacancies, consistent with the observed phonon-glass state. Impedance spectroscopy and direct current electrical measurements were used to probe the electrical properties of insulating and conducting samples. This reveals the coring of grains due to oxidation on cooling from sintering temperatures. The resultant insulating shell limits the thermoelectric power factor to S 2 /ρ = 0.45 mW m -1 K -2 and the figure-of merit to ZT = 0.15 at 900 K for Sr 0.20 La 0.53 → 0.27 Ti 0.95 Nb 0.05 O 3?δ . The thermal properties of these materials are, therefore, controlled by an intrinsic feature of the microstructure (i.e., the A-site vacancies), whereas the electrical properties are grain boundary limited, which in principle can be controlled independently to raise S 2 /ρ and ZT.
Popuri , S R , Decourt , R , McNulty , J A , Pollet , M , Fortes , A D , Morrison , F D , Senn , M S & Bos , J W G 2019 , ' Phonon-glass and heterogeneous electrical transport in A-site-deficient SrTiO 3 ' , Journal of Physical Chemistry C , vol. 123 , no. 9 , pp. 5198-5208 . https://doi.org/10.1021/acs.jpcc.8b10520
Journal of Physical Chemistry C
Copyright © 2019 American Chemical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1021/acs.jpcc.8b10520
DescriptionS.R.P. and J.W.G.B. acknowledge the EPSRC (grant EP/N01717X/1) and Leverhulme Trust (grant RPG-2012-576) for financial support and the STFC for the provision of beam time at ISIS. F.D.M. acknowledges the EPSRC for financial support (grant EP/P024637/1). M.S.S. acknowledges the Royal Society for a University Research Fellowship (grant UF160265).
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