Atomic-Level Characterization of Thermoelectric La1/3NbO3

There are limited numbers of ceramic compositions suitable for high-temperature thermoelectric (TE) applications. [1] The most promising candidates include SrTiO3 based perovskites, due to their high Seebeck coefficient and a simple perovskite structure, which easily lends itself to doping and thus to tailoring of its electron- and thermal-transport properties.[2] However, one their main the drawbacks of is their high thermal conductivity which has a strong temperature dependence. In the search for new oxides with low thermal conductivities, which could be used on their own as new materials or in conjunction with STO as binary systems, we have identified A-site deficient perovskites as systems with great potential in TE applications and in particular the La1/3NbO3 ceramic his presents the highest vacancy content among the A-site-deficient perovskite family (whereby 2/3 of the A sites are vacant), a feature which may promote glasslike low thermal conductivity [3].
Here, we use aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to validate at the atomic scale structure of LNO and determine precisely the cation distribution in the structure, which has an important impact on the macroscopic properties of the ceramic. Crystallographic data from X-ray and electron diffraction confirmed that the room temperature structure is orthorhombic with Cmmm as a space group. Atomically resolved imaging and analysis showed that there are two distinct A sites: one (A1) is occupied with La and vacancies, and the second site (A2) is fully unoccupied (Figure 1 &2). Further imaging and EELS analysis reveals that the A1-site vacancies are not uniformly distributed across the A1 site but pair in lines appear to be lying in lines parallel to the [1̅11] and [111̅] directions.
First-principles electronic structure calculations are used to link the temperature dependence of the Seebeck coefficient measured experimentally to the evolution of the density of states with temperature and indicate possible avenues for further optimization through electron doping and control of the A-site occupancies. The calculations informed by structural characterization eveal that the La1/3NbO3 ceramic possesses a desirable band structure for n-type TE materials. Moreover, lattice thermal conductivity calculations give insights into the asymmetric dependence of the thermal conductivity on specific crystallographic directions of the material, which could be exploited via nanostructuring to create highefficiency compound thermoelectrics.