Tailoring the structure and thermoelectric properties of BaTiO₃via Eu²⁺ substitution

Abstract

A series of BaEuTiO (0.1 ≤ x ≤ 0.9) phases with ∼40 nm particle size were synthesized via a Pechini method followed by annealing and sintering under a reducing atmosphere. The effects of Eu substitution on the BaTiO crystal structure and the thermoelectric transport properties were systematically investigated. According to synchrotron X-ray diffraction data only cubic perovskite structures were observed. On the local scale below about 20 Å (equal to ∼5 unit cells) deviations from the cubic structure model (Pm3[combining macron]m) were detected by evaluation of the pair distribution function (PDF). These deviations cannot be explained by a simple symmetry breaking model like in EuTiO. The best fit was achieved in the space group Amm2 allowing for a movement of Ti and Ba/Eu along 〈110〉 of the parent unit cell as observed for BaTiO. Density functional calculations delivered an insight into the electronic structure of BaEuTiO. From the obtained density of states a significant reduction of the band gap by the presence of filled Eu 4f states at the top of the valence band was observed. The physical property measurements revealed that barium-europium titanates exhibit n-type semiconducting behavior and at high temperature the electrical conductivity strongly depended on the Eu content. Activation energies calculated from the electrical conductivity and Seebeck coefficient data indicate that at high temperatures (800 K < T < 1123 K) the conduction mechanism of BaEuTiO (0.1 ≤ x ≤ 0.9) is a polaron hopping when 0 < x ≤ 0.6 and is a thermally activated process when 0.6 < x < 1. Besides, the thermal conductivity increases with increasing Eu concentration. Due to a remarkable improvement of the power factor, BaEuTiO showed a ZT value of 0.24 at 1123 K.

Evaluation of Polaron Transport in Solids from First‐principles

“…Reports on TMOs are available for many systems, such as CaMnO 3 , (Eu,Ba)TiO 3 , (La,Pb)MnO 3 , (La,Sr,Ag)MnO 3 , Ca 4 Mn 3 O 10 , LaMnO 3 , (La,Ca)MnO 3 , LaCrO 3 , (Ca/Sr/Pb,Bi)MnO 3 , (Sr,La)FeO 4 , (Pr,Ca,Sr)MnO 3 , (Ca,Nd/Pr/Sm)MnO 3 , (Sm,Ca)MnO 3 , TiO 2 , BaCoS 2 , La 2‐x Sr x NiO 4 , CeO 2 , (Ca,Gd)(Mn,Nb)O 3 , (Ca,Yb/Lu)(Mn,Nb)O 3 , (Bi/La/Mo,Ca)MnO 3 , and Fe 1‐x Zn x Cr 2 S 4 . These compounds exhibit fascinatingly rich physics originating from intricate electron‐electron and electron‐phonon interactions, e. g. double exchange and superexchange interactions, and Jahn‐Teller distortions .…”

“…TMOs are of particular interest for thermoelectric power generation; great efforts have been made to improve their heat‐to‐electricity conversion efficiency . Thanks to their low cost, natural abundance, low toxicity, structural and chemical stability, as well as property richness, these materials are promising candidates for waste heat harvesting at elevated temperatures .…”

“…In addition to fine tuning, such simulations can be used to guide experimental procedures. For instance, evaluation of polaron hopping energies from first‐principles and their dependence on the concentration and chemical identity of dopants is beneficial for enhancement of the electrical conductivity in oxides …”

Disorder in La1−x Ba1+x GaO4−x/2 ionic conductor: resolving the pair distribution function through insight from first-principles modeling

“…Empty circles: experimental data; red solid line: calculated PDF; green line: residual. (b) Residual of the fit of the calculated DFT models against experimental PDF curves for x = 0.2 (full circles) and x = 0.3 (empty circles), compared with the average model (dashed and dotted lines for x = 0.2 and x = 0.3, respectively) within 6.3 < r < nanometres (Jeong et al, 2005;Xiao et al, 2017;Checchia et al, 2016), symmetry-breaking effects play a key role in macroscopic physical properties of different classes of materials. In the present investigation, a short-range atomic rearrangement maintains, on average, a crystallographic motif suitable for ionic conduction (channels and cages wide enough to allow the passage of ions).…”

Effects of Nanodomains on Local and Long-Range Phase Transitions in Perovskite-Type Eu0.8Ca0.2TiO3–δ

“…This method has been applied to various perovskite-type oxide electroceramics to study their local structures [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. We recently used it to analyze deviations between local and long-range structures of the thermoelectric materials Eu 1-x Ba x TiO 3-δ [26] and Eu 1-x Ca x TiO 3-δ [6] at low temperatures. In solid materials, increasing the system temperature past the critical temperature T c , typically leads to a polymorph with higher symmetry [27,28].…”