This paper reviews ferroelectric oxides in the unusual condition where the concentration of electronic carriers is close to a metal–insulator transition; in certain structures and compositions these materials have properties of interest for oxide based thermoelectric applications. In relaxor ferroelectrics, nanopolar regions associated with intrinsic localized phonon modes provide glass-like phonon characteristics due to the large levels of phonon scattering. The (Sr1−xBax)Nb2O6−δ relaxor ferroelectric single crystals have a high thermoelectric power factor, S2σ ∼ 40 μW/cm K2 at 277 °C along the c-axis, which is competitive with the best thermoelectrics. In the heavily reduced, nonstoichiometric n-type perovskite BaTiO3−δ and tungsten bronze (Sr1−xBax)Nb2O6−δ, it is shown that metallic-like conductivity occurs in the paraelectric phase and the onset of ferroelectricity stabilizes semiconducting character. Both the phase transition temperature dependence on the carrier concentration and evidence for polarization coupling to the conductivity mechanism will be discussed.

https://www.mri.psu.edu/sites/default/files/stm/publications/Soonil%20Lee%20Ferroelectric.pdf

Ferroelectric-thermoelectricity and Mott transition of ferroelectric oxides with high electronic conductivity

This paper reviews ferroelectric oxides in the unusual condition where the concentration of electronic carriers is close to a metal–insulator transition; in certain structures and compositions these materials have properties of interest for oxide based thermoelectric applications. In relaxor ferroelectrics, nanopolar regions associated with intrinsic localized phonon modes provide glass-like phonon characteristics due to the large levels of phonon scattering. The (Sr1−xBax)Nb2O6−δ relaxor ferroelectric single crystals have a high thermoelectric power factor, S 2 σ ∼ 40 W/cm K 2 at 277 ◦ C along the c-axis, which is competitive with the best thermoelectrics. In the heavily reduced, nonstoichiometric n-type perovskite BaTiO3−δ and tungsten bronze (Sr1−xBax)Nb2O6−δ, it is shown that metallic-like conductivity occurs in the paraelectric phase and the onset of ferroelectricity stabilizes semiconducting character. Both the phase transition temperature dependence on the carrier concentration and evidence for polarization coupling to the conductivity mechanism will be discussed. © 2012 Elsevier Ltd. All rights reserved.

Feature article Ferroelectric-thermoelectricity and Mott transition of ferroelectric oxides with high electronic conductivity

It is found that, far above the ferroelectric transition temperature, Tc, certain ferroelectric systems show a temperature dependent index of refraction, n(T), that is qualitatively different from ordinary ferroelectrics. We review measurements in these systems and show how some aspects of this n(T) data can be quantitatively understood. This is done by considering the polarization in these materials to arise from very small localized clusters of a few unit cells far above Tc. The strongly broken long range symmetry allows these effects to occur. Furthermore, we find a one-to-one correspondence between ferroelectrics that show this unusual high temperature behavior and those that show glass-like excitations at very low temperatures; this clarifies the ambiguities in the low temperature heat capacity and thermal conductivity results.

Crystalline ferroelectrics with a glassy polarization phase

Environmental friendly, non-toxic double perovskite BaxSr2-xTiCoO6 compositions with 0 ≤ x ≤ 0.2 were synthesized using solid-state reaction route for high temperature thermoelectric (TE) applications. XRD and SEM studies confirmed the presence of single-phase solid solution with highly dense microstructure for all the oxide compositions. Temperature dependent electrical conductivity measurement showed semiconductor to metal (M-S) transition in these double perovskites. Incorporation of barium in Sr2TiCoO6 pushed M-S transition to higher temperature making it a potential candidate for high temperature TE applications. Conductivity behaviors of these oxides were explained by small polaron model. Furthermore, these oxides exhibit a glass like behavior resulting in low thermal conductivity. Low temperature dielectric measurement revealed relaxor ferroelectric behavior in these oxides below room temperature. Transition of these relaxors into a glassy state beyond Burns temperature (TD) was found responsible f...

Enhanced thermoelectric figure-of-merit in environmentally benign BaxSr2-xTiCoO6 double perovskites

Amorphous materials display a universality of non-Debye properties at low temperatures: (1) A “plateau” in the thermal conductivity K at ∼10 K; (2) K ∝T2 below 1 K; (3) A linear contribution to the specific heat C below ∼ 2 K; (4) A broad maximum in C/T3 at ∼10 K describable by an Einstein term; and (5) A frequency-dependent minimum in the dielectric constant at ∼0.1 K. These properties are generally explained by a two-level tunnelling model, the central feature of which is a constant density of states due to the topological disorder.Measurements on some ferroelectrics, 0.2-30 K, have revealed these same glasslike properties. In particular, Pb(Mg1/3Nb2/3)O3 has been thoroughly measured and found to display all of the glasslike properties. Ferroelectric Cd2Nb2O7 has a broad maximum in C/T3 at ∼15 K and a K ∝ T2 below 4 K, whereas non-ferroelectric CdNb2O6 behaves as a Debye solid. Dielectric data on PZT 65/35 and BaTiO3 reveal minima at ∼50-100 mK. Although much work remains in this new area, no ferroelect...

W.N. Lawless

Papers

New ceramic material for thermal and dielectric isolation at low temperatures