Optimum in the thermoelectric efficiency of nanostructured Nb-doped TiO2ceramics: from polarons to Nb-Nb dimers

Rutile is the most common and stable polymorph form of titanium oxide TiO(2)at all temperatures. The doping of rutile TiO(2)with a small amount of niobium is reknown for being responsible for a large increase of the electrical conductivity by several orders of magnitude, broadening its technological interest towards new emerging fields such as the thermoelectric conversion of waste heat. The electronic conduction has been found to be of a polaronic nature with strongly localized charges around the Ti(3+)centers while, on the other side, the relatively high value of the thermal conductivity implies the existence of lattice heat carriers,i.e.phonons, with large mean free paths which makes the nanostructuration relevant for optimizing the thermoelectric efficiency. Here, the use of a high-pressure and high-temperature sintering technique has allowed to vary the grain size in rutile TiO(2)pellets from 300 to 170 nm, leading to a significant reduction of the lattice thermal conductivity. The thermoelectric properties (electrical conductivity, Seebeck coefficient and thermal conductivity) of Nb-doped rutile nanostructured ceramics, namely Nb(x)Ti(1-x)O(2)withxvarying from 1 to 5%, are reported from room temperature to similar to 900 K. With the incorporation of Nb, an optimum in the thermoelectric properties together with an anomaly on the tetragonal lattice constantcare observed for a concentration of similar to 2.85%, which might be the fingerprint of the formation of short Nb dimers.