Structural distortion dependence of thermoelectric properties in CoFeZrSi Heusler material

https://doi.org/10.1016/j.jallcom.2019.153492Get rights and content

Highlights

  • The thermoelectric properties of CoFeZrSi were theoretically investigated.

  • Evidence for p-type half-metallic Heusler compound in cubic structure was provided.

  • Estimated 480 μV/K at 700K when a = b = c = 5.97 Å and α = β = γ = 90°.

  • Estimated ZT = 0.94 in optimum cubic structure at 350K

Abstract

The effect of potential tetragonal and triclinic distortions of the energetic favorable cubic crystalline structure in CoFeZrSi Heusler compound, was investigated, using semiclassic Boltzmann theory on the thermoelectric functionalities. Chemical potential dependence of the conductivity integral σ/τ for relaxed cubic and tetragonal structures confirms p-type thermoelectric characteristics. When triclinic deformation is investigated, the electrical conductivity response indicates that the material’s ability to conduct electric current decreases. The calculated Seebeck coefficients exhibit positive values for the crystalline structures whose angles are equal to 90o (cubic and tetragonal), over the 300–1200K temperature range. The figures of merit ZT, for relaxed cubic and tetragonal structures, at optimum unit cell volume or higher, present around room temperature, promising features as a potential thermoelectric material (i.e. ZT = 0.94 for 350K in optimum cubic structure).

Introduction

Direct converting the waste heat generated by industrial furnace, engines or gas pipes into electricity via the Seebeck effect, actively counteract global warming, as one of the clean energy conversion techniques, that can significantly reduce the CO2 emissions and the fossil-fuel consumption.

The efficiency of thermoelectric converters for power generation depends strongly on carrier concentration and electronic structure [1]. Therefore, relentless efforts have been focused on discovery new and promising materials, such are Heusler compounds [[2], [3], [4], [5], [6], [7]] or Zintls [8], with characteristics enabling convenient combinations of large Seebeck coefficient, high electrical conductivity and low thermal conductivity to enhance energy conversion efficiency which depends on compounds’ dimensionless figure-of-merit (ZT).

State-of the art thermoelectric converters, generally developed on the Edisonian trial and error approach, present a combination of expensive elementary constituents and low efficiency values, which is time-consuming and economic inefficient. As a result, the theoretical studies could provide valuable guidance for a more efficient rational approaches, to balance the cost and reliability in identifying new promising materials with thermoelectric performance, that may be experimentally identified as stable.

Heusler materials recently attracted the technological interest as potential thermoelectric compounds, due to their unique physical properties: at the Fermi energy, the minority spin density of Heusler systems vanishes, while the majority spin density displays electronic states, a property referred in literature as half-metallic ferromagnetism (HMF) [9], that imply a high spin polarization. Moreover, the possibility to optimize the thermoelectric properties by individually doping of each one of the three fcc sublattices, turned out to be a great advantage in favor of detailed investigation of the Heusler compounds.

Kübler et al. [10] reported from theoretical calculations that Co2-based Heusler compounds exhibit exceptional transport properties and the first principle calculations were confirmed experimentally, as in Co2TiAl case [11].

The list of promising materials for thermoelectric applications is long and the compounds were designed to tailor the energy band gap from one spin channel, to be located around the Fermi level [[12], [13], [14], [15], [16], [17], [18], [19], [20]] Nevertheless, the thermoelectric effect in two-dimensional (2D) materials is attracting a wide appeal, due to the anticipated superiority of converting heat energy to useful electricity comparing with their bulk counterparts. Additionally, in 2D materials the degree of disorder observed is remarkably low for as cast samples [11], this interesting feature being useful in thin film devices, where high annealing temperatures are not allowed. However, in contrast to the bulk structures that generally exhibit high symmetry crystalline configurations, 2D structures may have crystalline configuration, whose symmetry decreases due to distortions and stresses.

In order to substantially reduce the experimental efforts required to identify the influence of symmetry decrease (from cubic to tetragonal/triclinic structure) on the target thermoelectric functionalities in CoFeZrSi Heusler compound, (reported by Kanbe et all [21]), theoretical investigation were performed on the typical thermoelectric parameters: electrical conductivities, electronic thermal conductivities, Seebeck coefficients and figures of merit.

Section snippets

Method of calculation

Theoretical investigations on CoFeZrSi quaternary Heusler compound were performed within the Density Functional Theory framework, using the Full Potential Linearized Augmented Plane Wave (FPLAPW) method, as implemented in Wien2k code [22] with the exchange and correlation interactions described by the Perdew-Burke-Ernzehof (PBE) functional [23,24]. The technical details regarding first principle calculations have been reported in Ref. [25]. The calculated bandstructure were the starting point

Results and discussions

The electronic structures and magnetic properties of CoFeZrSi Heusler compound, investigated via self-consistent procedure, having the cubic structure (reported as the energetic most favorable one) and the tetragonal/triclinic distortions were published in our previous study [25]. The result regarding the electronic configuration of CoFeZrSi, with F-43 m symmetry and Si(4a) Fe(4c) Zr(4b) Co(4d) atomic arrangement along the unit cell diagonal, presents half-metallic properties and a calculated

Conclusions

Self-consistent investigations on cubic structure, the energetic favorable one, and on tetragonal/triclinic distortions, which may appear in experimental condition, at the interface between the deposited layer of CoFeZrSi Heusler material and substrate or other layer, due to lattice mismatch, provide information regarding thermoelectric characteristics.

In cubic structure, the material exhibits p-type thermoelectric characteristics for relaxed unit cell (optimum or with a volume increased

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author would like to thank Dr. P. Palade for computational support and Prof Dr. V. Kuncser for helpful discussions. The author acknowledge the financial support provided by the Romanian National Authority for Scientific Research through the Core Program PN21 N, PN–III–P1-1.2-PCCDI-2017-0871.

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