Exploring the Thermoelectric Performance of BaGd₂NiO₅ Haldane Gap Materials

Abstract

One-dimensional Haldane gap materials, such as the rare earth barium chain nickelates, have received great interest due to their vibrant one-dimensional spin antiferromagnetic character and unique structure. Herein we report how these 1D structural features can also be highly beneficial for thermoelectric applications by analysis of the system CaBaGdNiO 0 ≤ x ≤ 0.25. Attractive Seebeck coefficients of 140-280 μV K at 350-1300 K are retained even at high acceptor-substitution levels, provided by the interplay of low dimensionality and electronic correlations. Furthermore, the highly anisotropic crystal structure of Haldane gap materials allows very low thermal conductivities, reaching only 1.5 W m K at temperatures above 1000 K, one of the lowest values currently documented for prospective oxide thermoelectrics. Although calcium substitution in BaGdNiO increases the electrical conductivity up to 5-6 S cm at 1150 K < T < 1300 K, this level remains insufficient for thermoelectric applications. Hence, the combination of highly promising Seebeck coefficients and low thermal conductivities offered by this 1D material type underscores a potential new structure type for thermoelectric materials, where the main challenge will be to engineer the electronic band structure and, probably, microstructural features to further enhance the mobility of the charge carriers.

Unravelling the Effects of Calcium Substitution in BaGd2CoO5 Haldane Gap 1D Material and Its Thermoelectric Performance

“…The low activation energy values calculated for these materials confirm the thermally induced/activated polaron-hopping-type conduction mechanism, characteristic for these Haldane gap materials. A similar trend was observed previously by Nasani et al in BaY 2 NiO 5 and BaGd 2 NiO 5 materials.…”

“…While the amount of these charge carriers is enough to suppress the Seebeck coefficient to one of the lowest measured values, the conductivity of BGCC0 in air is still below the rest of the samples, suggesting the lowest mobility. This hypothesis also agrees with the increase in charge carrier mobility, observed for BaGd 2– x Ca x NiO 5 materials, upon substitution with calcium …”

“…It should be noticed that these power factor values are notably below those found/measured for state-of-the-art Ca 3 Co 4 O 9 and SrTiO 3 thermoelectric oxides. The highest power factor value measured for the BaGd 2 NiO 5 Haldane gap material is ∼21 μW/K·m 2 at 1179 K, as previously reported by our group . Thus, in this context, only a marginal enhancement in power factor values is achieved for calcium-substituted BaGd 2 CoO 5 compositions.…”

“…As compared to the Ca x BaGd 2– x NiO 5 materials, the substituted BaGd 2– x Ca x CoO 5 samples from this work present a rather significant fraction of oxidized transition metal cations, with expected positive impact on the electronic transport, as can be seen in Figure . Based on the previously proposed defect models for Haldane gap nickelates, , the corresponding defect chemistry reactions in this case can be described using the standard Kröger–Vink notation as where β represents the contribution provided by the formation of charge carriers Co 3+ (Co Co • ). Even minor substitutions ( x = 0.05) with Ca 2+ in BaGd 2– x Ca x CoO 5 result in a dramatic increase of [Co 3+ ]/[Co] tot fraction, which further decreases for x = 0.10–0.20.…”

“…The thermoelectric performance of stoichiometric and nonstoichiometric donor-substituted strontium titanates has been widely studied and achieved the dimensionless figure of merit of 0.35−0.40 at 1200 K. 10,14,21,22,24−26 Layered Ca 3 Co 4 O 9 -based materials have also attracted huge attention as promising ptype thermoelectrics, reaching a ZT of 0.3−0.7 at 1000 K. 17,27,28 In the quest for new oxide materials, our team has systematically studied for the first time the thermoelectric performance of one-dimensional (1D) BaGd 2 NiO 5 Haldane gap material. 12 The ubiquitous 1D structure, low thermal conductivity, and impressive Seebeck coefficient values of Haldane gap materials have convinced us to explore a similar family of cobalt-rich materials, i.e., BaGd 2 CoO 5 , for thermoelectric applications. The cobaltite-based materials were identified to be highly stable at high temperatures, ecofriendly, and widely available in nature.…”