  Thermoelectric energy conversion - the exploitation of the Seebeck effect to
convert waste heat into electricity - has attracted an increasing amount of
research attention for energy harvesting technology. Niobium-doped strontium
titanate (SrTi1-xNbxO3) is one of the most promising thermoelectric material
candidates, particularly as it poses a much lesser environmental risk in
comparison to materials based on heavy metal elements. Two-dimensional electron
confinement, e.g. through the formation of superlattices or two-dimensional
electron gases, is recognized as an effective strategy to improve the
thermoelectric performance of SrTi1-xNbxO3. Although electron confinement is
closely related to the electronic structure, the fundamental electronic phase
behavior of the SrTi1-xNbxO3 solid solution system has yet to be
comprehensively investigated. 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 4d 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.
