Thermoelectric Responses in Layered Strontium‐Niobates Via Two Ways of Charge Carrier Control Techniques

We have systematically examined the thermoelectric properties of the perovskite-related layered strontium-niobates SrnNbnO3n+2 (n = 4, 4.33, 4.5, and 5) and Sr2−xLaxNb2O7, prepared using two charge carrier control techniques. Both have a similar anisotropic crystal structure that gives all the samples prepared a relatively low thermal conductivity. However, the two systems show very different behaviors with respect to Seebeck coefficient and resistivity. In SrnNbnO3n+2, an n-driven semiconductor-metal transition occurs between n = 4.33 and 4.5, with highly metallic properties observed in their magnitudes and temperature dependence at higher values of n. We also observed the microstructures to gain a further understanding of their properties, and elucidated the unconventional charge distribution in the intermediate n = 4.5 material. By contrast, in Sr2−xLaxNb2O7, thermally activated behaviors were displayed even in heavily La-doped materials. Strong electron lattice interactions due to low symmetric NbO6 octahedra appeared to be present over the whole doping range, with Sr2−xLaxNb2O7 showing a lower electrical conductivity than metallic SrnNbnO3n+2. However, the ZT of Sr2−xLaxNb2O7 is one order of magnitude larger than for SrnNbnO3n+2 due to the much larger Seebeck coefficient, and increased up to 0.048 at room temperature on increasing the La dopant level.