Thermoelectric phase diagram of the SrTiO3–SrNbO3 solid solution system

Here, we present a thermoelectric phase diagram for the SrTi1−xNbxO3 (0.05 ≤ x ≤ 1) solid solution system, which we derived from the characterization of epitaxial films. We observed two thermoelectric phase boundaries in the system, which originate from the step-like decrease in carrier effective mass at x ∼ 0.3 and from a local minimum in carrier relaxation time at x ∼ 0.5. The origins of these phase boundaries are considered to be related to isovalent/heterovalent B-site substitution: parabolic Ti 3d orbitals dominate electron conduction for compositions with x < 0.3, whereas the Nb 4 d orbital dominates when x > 0.3. At x ∼ 0.5, a tetragonal distortion of the lattice, in which the B-site is composed of Ti4+ and Nb4+ ions, leads to the formation of tail-like impurity bands, which maximizes the electron scattering. These results provide a foundation for further research into improving the thermoelectric performance of SrTi1−xNbxO3.