Glass-like thermal conductivity in SrTiO₃ thermoelectrics induced by A-site vacancies

Abstract

The introduction of A-site vacancies in SrTiO 3 results in a glass-like thermal conductivity while Nb substituted samples maintain good electrical conductivity. This unexpected result brings SrTiO 3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.Thermoelectric waste heat recovery is widely expected to be an important component of a sustainable energy future. 1 However, affordable and relatively high-performance materials are lacking. Metal oxides are good candidates because of their abundance, low toxicity, and stability at high temperatures. 2 In addition, metal oxides are used in multilayer capacitors and the infrastructure for large scale device production exists. 3 SrTiO 3 is among the most promising n-type materials because it has the unusual combination (for an oxide) of good electrical conduction (s) and a high Seebeck coefficient (S), yielding power factors S 2 s # 3.5 mW m À1 K À2 in single crystals and epitaxial lms. 4,5 Unfortunately, the overall performance is compromised by a large thermal conductivity k ¼ 12-6 W m À1 K À1 for undoped SrTiO 3 (Fig. 1), 6 which reduces the thermoelectric gure of merit, ZT ¼ (S 2 s/k)T, where T is the absolute temperature. Outstanding thermoelectric materials generally have phononglass and electron-crystal (PGEC) properties, 7 meaning that the electronic transport is characteristic of a crystalline solid, while the k is low and resembles that of a glass. The main focus in the optimisation of SrTiO 3 has therefore been on the reduction of k, which consists of a large lattice contribution (k lat ) and a small electronic component (k el ¼ sLT; L is the Lorenz number). One approach to reduce k lat is to introduce point defects within the perovskite structure. This is for example used in Sr 1Àx La x TiO 3Àd (0 # x # 0.15) and SrTi 1Ày Nb y O 3Àd (0 # y # 0.2) and results in k lat z 3 W m À1 K À1 at 1073 K and a maximum ZT ¼ 0.35 at 1073 K. 5 However, to achieve ZT ¼ 1, a much more substantial reduction to k ¼ 1-2 W m À1 K À1 is needed. One drawback of La 3+ and Nb 4+ substitution is that the conicting requirements of charge carrier doping limit x, y # 0.2. However, even for Sr 1Àx Eu x Ti 0.8 Nb 0.2 O 3Àd with Sr 2+ /Eu 2+ mixtures (0 # x # 1), k was not reduced below $3 W m À1 K À1 for x ¼ 0.5 at 1000 K. 8 Recently, perovskites with A-site vacancies have started to generate attention. Promising S 2 s values and low k were reported in A-site and oxygen decient Sr 1Àx Pr 0.67x TiO 3Àd and Sr 1Àx Ti 0.8 Nb 0.2 O 3Àd , 9 while reductions in k were also observed in Ca 1Àx Nd 0.67x MnO 3Àd perovskites. 10 Here, we report a systematic investigation of the A-site decient Sr 1Àx La 0.67x , 0.33x Ti 1Ày Nb y O 3Àd perovskites. In order to separate the impact of A-site vacancies from oxygen defects we rst prepared the electrically insulating oxygen stoichiometric Sr 1Àx La 0.67x , 0.33x TiO 3 series and measured its thermal conductivity. ‡ Compositions with x ¼ 0, 0.4 and 0.8 were synthesised using standard solid state reactions...

Phonon-glass electron-crystal behaviour by A site disorder in n-type thermoelectric oxides

“…S9, ESI †) are comparable to many of the titanates reported previously, which were doped on either the A or B sites. 27,28,36,37 The observation of high S and σ suggests that the unique band structure of STO, consisting of heavy and light electrons, that allows for high power factors is retained in La0.5Na0.5Ti1-xNbxO3 despite the slight distortion of the structure away from cubic symmetry, consistent with the DFT calculations. A maximum power factor of 0.620.07 mW m -1 K -2 was obtained for La0.5Na0.5Ti0.8Nb0.2O3 at ≈700 K (Fig.…”

“…The observation of a transition from PC to PG behaviour in the Sr1-xLa0.67x0.33xTiO3 system when the A site cation vacancy () content exceeded 20% (x=0.6), significantly increasing MF (Table 1) and resulting in a κ close to κmin from Cahill's model, was rather encouraging. 27 However, the complex defect chemistry of these materials did not allow for electronic doping whilst simultaneously retaining the PG behaviour, as compositions with carrier concentrations sufficient for good thermoelectric response displayed PC thermal conductivity. In order to realise PGEC behaviour in titanates, a strategy is required that combines an equivalent  to those achieved by cation vacancycontaining materials with a structural chemistry that permits electronic doping.…”

Thermoelectric properties of n-type SrTiO3

“…2(d), with a peak ZT value up to 0.7 at 1400 K and 0.45 at 1000 K with similar doping level (∼3-4 × 10 20 cm −3 ). Though a little higher in the latter case, our calculated ZT with both experimental range κ agree reasonably well with existing experiments (∼0.2-0.4 at 900 K-1000 K [21][22][23][24]70 ). Complex isoenergy surfaces are favorable for electronic performance as mentioned above.…”

“…A-site vacancies also can lower κ. Particularly, Popuri et al has shown a glass-like thermal conductivity through A-site vacancies and κ = 2.5 W m −1 K −1 can be realized at 1000 K. 23 Recently, Lu and co-workers found high ZT about 0.4 at 973 K with similar low κ = 2.5 W m −1 K −1 from their La-doped and A-site-deficient samples. 24 Importantly, STO has been identified as a material with a corrugated band structure of a form that is particularly beneficial for the electronic part of ZT, but which at the same time is not reasonably described by standard parabolic band models.…”

“…There have been a number of studies of the thermal conductivity measurements on both the bulk and doped STO at elevated temperatures. [20][21][22][23][24][69][70][71][72] Due to the significant difference of κ between bulk and heavily doped materials, we consider two scenarios with one taking the bulk κ at 300 K (∼11 W m −1 K −1 ) 21,71 and the other using the doped κ at the same T (∼8 W m −1 K −1 , based on moderately doped samples 21,24 ). With the second scenario, we are able to get low κ from 2.3 to 2.5 W m −1 K −1 at 1000 K. These values are in better accord with experiments at high temperatures (∼1000 K).…”