Excess SrO and CaO were added to the Sr(Ti0.8Nb0.2)O3 thermoelectric material, which was structurally compensated by the formation of Ruddlesden–Popper-type planar faults with the compositions SrO and/or (Sr, Ca)O. Both types of doping significantly changed the original isotropic Sr(Ti0.8Nb0.2)O3 microstructure and resulted in the formation of lamellar Ruddlesden–Popper-type phases within the Sr(Ti0.8Nb0.2)O3 grains. Three-dimensional networks of single Ruddlesden–Popper-type faults were also observed in the Sr(Ti0.8Nb0.2)O3 for both types of doping. The combination of both structural features significantly lowered the thermal conductivity in comparison with Sr(Ti0.8Nb0.2)O3 due to the enhanced phonon scattering observed at the planar faults, which proves that introducing such defects is a promising method for lowering the thermal conductivity of the Sr(Ti0.8Nb0.2)O3 thermoelectric material. The highest figure of merit (ZT = 0.08) was achieved with CaO doping, since the significantly reduced thermal conductivity was accompanied by an increased power factor
