a Phonon-glass electron-crystal (PGEC) behaviour is realised in La0.5Na0.5Ti1-xNbxO3 thermoelectric oxides. The vibrational disorder imposed by the presence of both La 3+ and Na + cations on the A site of the ABO3 perovskite oxide La0.5Na0.5TiO3 produces a phonon-glass with a thermal conductivity, κ, 80% lower than that of SrTiO3 at room temperature. Unlike other state-of-the-art thermoelectric oxides, where there is strong coupling of κ to the electronic power factor, the electronic transport of these materials can be optimised independently of the thermal transport through cation substitution at the octahedral B site. The low κ of the phonon-glass parent is retained across the La0.5Na0.5Ti1-xNbxO3 series without disrupting the electronic conductivity, affording PGEC behaviour in oxides.Thermoelectric generators offer enhanced energy efficiency across the industrial and automotive sectors through the conversion of waste heat into electricity. Materials performance is assessed by the dimensionless figure of merit ZT = (S 2 σ/κ)T, combining the thermal conductivity (κ), absolute temperature (T), the Seebeck coefficient (S) and electrical conductivity (σ), which together define the power factor (S 2 σ). One strategy to optimise ZT is the phononglass electron-crystal (PGEC) concept proposed by Slack, 1 which aims to decouple the quantities governed by the Boltzmann transport equation in an "electron-crystal", by maximising S and σ, from the quantity governed by phonon transport processes in a "phonon-glass" (PG) by minimising the lattice contribution to the thermal conductivity (κlatt); essentially, a crystal that transports charge like a semiconductor, whilst hindering the transport of heat by the lattice like a glass. 2 Heat transport in solids consists of lattice and electronic (κelec) contributions through κ = κlatt + κelec, and κlatt=1/3Cv·lph·νs (Equation 1), where Cv is the isochoric heat capacity, lph the average phonon mean free path (MFP), and vs the mean velocity of sound. Different heat-carrying mechanisms lead to a broad distribution of MFPs, ranging from the atomic scale (<1 nm) for point defect scattering, to the mesoscale (>100 nm) for grain boundary scattering. In a "phonon-crystal" (PC), heat is transported by phonons of well-defined wavelength. The phonon scattering rate (ph -1 ), which is inversely proportional to κlatt (Equation S1, ESI †), corresponds to the sum of the inverse relaxation times of several phonon-scattering mechanisms that contribute separately to κ and is described quantitatively by the modified Debye-Callaway model (Equation S3, ESI †). 3 This produces a T -1 temperature dependence of κ above the Debye temperature (D) where heat conduction is by Oxide materials are of current interest as high-temperature energy-harvesting thermoelectrics in the automotive and manufacturing sectors due to their low cost, low toxicity and high chemical robustness. The phonon-glass electron crystal (PGEC) concept which has been successfully used to optimise the thermoelectric figure of merit (Z...
