Thermoelectric materials are able to directly convert a difference of temperature into electricity and vice-versa. Hence, they can be employed either in energy harvesting from waste heat or in refrigeration. The potential use of thermoelectric ceramics for direct conversion of waste heat into electricity is gaining momentum and oxides are envisaged as the most promising materials. Nevertheless, prior to the commercial deploying of this technology, the efficiency of thermoelectric oxides needs to be improved. Basically, they need to show high electrical conductivity and thermopower combined with low thermal conductivity. Reducing the thermal conductivity of oxides is currently the main challenge. Thermal conductivity in oxides is ultimately dependent on phonon propagation. This proposal addresses the impact of chemical modifications on the phonon spectra of a promising thermoelectric.