Thermoelectric Properties of Alkaline Earth Metal Substituted Europium Titanates

Abstract

EuTiO3 (ETO) is a perovskite-type oxide consisting of divalent Eu and tetravalent Ti. Due to Eu2+ possessing a half-filled 4f shell with a large spin magnetic moment (7 μB), ETO is a magnetic material with localized Eu 4f electrons, which are related to its giant Seebeck coefficient and considerable thermoelectric performance. Crystalline ETO is isostructural with SrTiO3 (cubic, Pm3m) on the long-range scale at ambient temperature, but its local structure has a high degree of disorder. In this dissertation, the magnetic Eu2+ was partly substituted by isovalent nonmagnetic alkaline-earth metal cations A (A = Ba2+, Sr2+, Ca2+, and Mg2+). The aims were to synthesize perovskite-type Eu1−xAxTiO3−δ solid solutions and investigate their crystal structures, local structures, electronic band structures, thermoelectric properties, and interrelations among them. All samples were synthesized by using a two-step process including the Pechini method for precursor synthesis and subsequent high-temperature annealing under reducing atmosphere. The combination of crystal structure and transport properties analysis revealed that polycrystalline Eu1−xAxTiO3−δ (A = Ba2+, Sr2+, Ca2+) solid solutions were successfully synthesized in the full compositional range of 0 ≤ x ≤ 1. An exception was the (partial) Mg2+ substitution. It was impossible within the used experimental parameters to obtain a single-phase Eu1–xMgxTiO3 solid solution, resulting from that the ionic radius of Mg2+ is too small to be bonded in an AO12 cuboctahedron. In the case of Sr2+ substitution, the long-range cubic symmetry was maintained in the entire compositional range due to the virtually equal ionic radii of Sr2+ and Eu2+. A minor effect on the electrical conductivity and the ZT values had been achieved by the Sr2+ substitution in a wide compositional range. Promising thermoelectric results were observed for the Ba2+ and Ca2+ substituted samples. The partial substitution of Eu2+ with Ba2+/Ca2+ led to an expansion/contraction of cubic sub-cell volume, resulting in different kinds of lattice defects and local structural distortions. Both Ba2+ and Ca2+ substitutions increased electrical conductivity, decreased lattice thermal conductivity, and hence improved the ZT values. In comparison with Ba2+ substitution, the enhanced thermoelectric performance by Ca2+ substitution was more pronounced at lower temperatures. Since the ionic radius of Ca2+ is smaller than that of Eu2+, the given smaller unit cell induced chemical pressure which facilitated lattice deformations and defect formations (e.g. Eu3+). The resulting impurity levels in the electronic band structure played an important role in the electronic conduction at low temperatures. The pristine ETO sample displayed a glass-like thermal conductivity (κ) at low temperatures, while the Ca2+-rich samples displayed ordinary crystalline κ behavior. The glass-feature of κ was one solid evidence for verifying the local structure disorder of ETO. Due to a large spin moment of Eu2+, magnons as additional heat carriers contributed with a large magnitude to the heat conduction in all Eu2+ containing samples at intermediate temperatures. The temperature-independent large Seebeck coefficient below ambient temperature was ascribed to the magnon-drag effect together with the influence of Eu 4f hybridization and the local structural disorder. The influence of Eu2+ 4f electrons on transport properties was strongly related to crystal structure variations, which was verified through isovalent chemical substitution by A-site cations with different ionic radii. The obtained results demonstrated how by invoking “lattice deformation and local disorder” one can manipulate the electronic transport properties and in parallel reduce intrinsic lattice κ of the perovskite-type ETO oxides. The substitution of Eu2+ with naturally abundant Ca2+ is a promising approach for improving thermoelectric performance and simultaneously reducing the usage of valuable europium.