Thermoelectric properties of lantanide doped Sr0.7Ba0.3Nb2O6 ceramics were investigated in the temperature range from 323 K to 1073 K. A better electrical conductivity is obtained in the sample with larger ionic radius of doping element. Thus, the highest PF value is achieved in La-doped sample. The NbO2 second phase is counterproductive to reduce the lattice thermal conductivity, so La-doped sample without impurity shows low thermal conductivity. Thus, La-doped sample shows an excellent thermoelectric performance ZT ∼ 0.35. The small average grain size and the nano-sized phases are observed in Gd and Dy doped samples, both of which contribute to scattering phonons, resulting in low thermal conductivities.

The thermoelectric properties of interstitial doped Sr0.7Ba0.3YbxNb2O6-δ ceramics were investigated in the temperature range from 323 K to 1073 K. The ytterbium interstitial doping contributes to increasing the power factor (PF) by improving the electrical conductivity. The PF values of Sr0.7Ba0.3Yb0.1Nb2O6-δ are comparable to those of the substitution doped sample with twice the doping content. This can be ascribed to the large electronic charge of the dopants relative to the occupied site in the interstitial case. The lattice thermal conductivity of Sr0.7Ba0.3YbxNb2O6-δshows a glass-like behavior due to lattice disorder. The disorder degree deepens as the doping content increases, resulting in plateau values of the lattice thermal conductivity at high temperatures in the Sr0.7Ba0.3Yb0.05Nb2O6-δ and Sr0.7Ba0.3Yb0.1Nb2O6-δ samples.

Thermoelectric properties of La doped Sr0.7Ba0.3Nb2O6-δ ceramics were investigated in the temperature range from 323 K to 1073 K. The electrical resistivity decreases due to the subsitution of La for Sr, leading to the enhancement of thermoelectric power factor in the heavily doped sample Sr0.50La0.20Ba0.30Nb2O6−δ. Slight La-doping promotes the growth of Sr0.7Ba0.3Nb2O6−δ grains, but it will be impeded in the heavily La doped samples. Thus, La heavily doped sample Sr0.50La0.20Ba0.30Nb2O6−δ shows low thermal conductivity because of the phonon scattering caused by grain boundaries. The highest thermoelectric performance was observed in the sample Sr0.50La0.20Ba0.30Nb2O6−δ, and the thermoelectric figure of merit reaches the maximum value ~0.22 at 1073 K.

•Magnetic susceptibility of Li1−xNaxCu2O2 single crystals increases with the increasing doping content.•The temperatures, corresponding to the maximum magnetic susceptibility and to the maximum probability of the magnetic transition, remain unchanged.•The magnetic interactions J1 and J2 decrease with doping content, while the frustration factor α keeps unchanged.•It suggests that the frustration factor is the main factor to determine the magnetic properties of this system.•At low temperatures, an upturn is obviously observed in magnetic susceptibility for the Na doped samples, due to the re-filling of the spin gap caused by the disorder.Li1−xNaxCu2O2 (x=0, 0.05, 0.10, and 0.20) single crystals were synthesized by the self-flux method, and the temperature dependent magnetic susceptibility χ(T) were measured along the c axis. Na doping at the Li sites induces almost no change in the temperature (about 40 K) corresponding to the maximum of magnetic susceptibility. However, it results in the upturn of the magnetic susceptibility in the low temperature region (T<14 K), which could be attributed to the filling of the spin-gap. The behaviors of magnetic susceptibility in the high temperature region (T>60 K) of all the samples were well fitted by the spin S=1/2 chain frustrated model. The exchange coupling constants J1 (ferromagnetic interaction) and J2 (antiferromagnetic interaction) both decrease smoothly with the increase of the doping level x, which is possibly related to the increase of lattice parameters. At the same time, the frustration parameter α=J2/J1 is unchanged, resulting in the unchanged temperatures corresponding to the maximum of magnetic susceptibility (TMax=40 K) and to the maximum probability of the magnetic transition (Th=24 K). It indicates th at α is the decisive factor for the frustrated systems.

Sr1−xDyxTiO3 (x = 0.02, 0.05, 0.10) ceramics were prepared by the reduced solid-state reaction method, and their thermoelectric properties were investigated from room temperature to 973 K. The resistivity increases with temperature, showing metallic behavior. The Seebeck coefficients tend to saturate at high temperatures, presenting narrow-band behavior, as proved by ab initio calculations of the electronic structure. The magnitudes of the Seebeck coefficient and the electrical resistivity decrease with increasing Dy content. At the same time, the thermal conductivity decreases because the lattice thermal conductivity is reduced by Dy substitution. The maximum value of the figure of merit reaches 0.25 at 973 K for the Sr0.9Dy0.1TiO3 sample.

Sr0.95La0.05TiO3−δ solid solutions, combined with 10 mol% TiO2, were prepared at different temperatures. When sintered at a suitable temperature, the Seebeck coefficient presents the same behavior at high temperatures as that of Sr0.95La0.05TiO3−δ ceramic. Otherwise, the Seebeck coefficient would be smaller. After introducing TiO2, both of the thermal and electrical conductivity are reduced. The sample sintered at 1350 °C exhibits the highest ZT of about 0.2, which slightly larger than that of Sr0.95La0.05TiO3−δ ceramic.