  The Seebeck effect describes the generation of an electric potential in a
conducting solid exposed to a temperature gradient. Besides fundamental
relevance in solid state physics, it serves as a key quantity to determine the
performance of functional thermoelectric materials. In most cases, it is
dominated by an energy-dependent electronic density of states at the Fermi
level, in line with the prevalent efforts toward superior thermoelectrics
through the engineering of electronic structure. Here, we demonstrate an
alternative source for the Seebeck effect based on charge-carrier relaxation: A
charge mobility that changes rapidly with temperature can result in a sizeable
addition to the Seebeck coefficient. This new Seebeck source is demonstrated
explicitly for Ni-doped CoSb3, where a dramatic mobility change occurs due to
the crossover between two different charge-relaxation regimes. Our findings
unveil the origin of pronounced features in the Seebeck coefficient of many
other elusive materials characterized by a significant mobility mismatch. As
the physical origin for the latter can vary greatly, our proposal provides a
unifying framework for the understanding of a large panoply of thermoelectric
phenomena. When utilized appropriately, this effect can also provide a novel
route to the design of improved thermoelectric materials for applications in
solid-state cooling or power generation.
