Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO₃: Experimental and Computational Evidence

Abstract

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO, we need to explore structural control at the atomic level. In SrLaTiO ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.

Enhancing the thermoelectric properties of Sr 1− x Pr 2 x /3 □ x /3 TiO 3± δ through control of crystal structure and microstructure

“…Finally, the appearance of M- and X-point reflections for samples with x ≥ 0.85, and the peak splitting of (200) p , indicates the samples have an orthorhombic crystal structure ( Cmmm ). These changes in crystal structure with Pr/vacancy concentration are consistent with earlier investigations [13,14,22] of La/vacancy doping of SrTiO 3 . Rietveld refinement of the present XRD spectra confirmed the space groups indicated above.…”

“…Moreover, the l ph values for x ≥ 0.70 compositions were similar to the l ph of crystalline Quartz glass (≈7.8 Å at room temperature [53]). Therefore, the introduction of a mass contrast (substituting Sr for the heavier Pr) led to the formation of a layered crystal structure upon Pr/vacancy doping; this, in addition to the non-uniform distortion of TiO 6 octahedra, and the presence of anti-phase and twin boundaries, is responsible for the effective creation of a glassy state in the crystalline material [14,54,55]. Finally, the variation in κ lattice did not follow that of oxygen deficiency (deduced from TGA), suggesting that the Pr/vacancy concentration had a more dramatic effect on thermal transport than oxygen vacancies.…”

Enhancing the thermoelectric power factor of Sr0.9Nd0.1TiO3 through control of the nanostructure and microstructure

“…Gong et al 37 showed that with increasing oxygen deficiency, SrTiO3-crystals developed a tetragonal crystal structure. high symmetry structures formed at high temperature to lower symmetry structure at room temperature, 38 are shown in Figure 2d. The width of these features typically are >1 m for all the samples irrespective of ZrO2 content, which is larger than phonon mean free path for SrTiO3 (typically <10 nm).…”

Enhancement of Thermoelectric Performance of Sr0.9‐xNd0.1Ti0.9Nb0.1O3 Ceramics by Introducing Sr Vacancies

“…So introducing Sr vacancies demonstrates a valid method for raising the TE performances of SrTiO 3 ‐based materials. In the literature, the contents of Sr vacancies were often related with the substitution contents which resulting in equivalent doping, and most reports were made on single‐doped samples . The physical mechanisms how Sr vacancies influence thermoelectric properties are not very clear until now.…”

“…Enhanced thermoelectric performances for oxygen‐deficient samples with lanthanide doping on the Sr site and niobium doping on the Ti site have been reported by a lot of researchers . Very recent studies have stressed a potential outlook for improving TE performance by introducing Sr vacancies into SrTiO 3 ‐based materials . For example, Lu et al found a large ZT value of 0.41 at 973 K for Sr 1‐3x/2 La x TiO 3 with x = 0.15 .…”