Structural, electronic and thermoelectric performance of narrow gap LuNiSb half Heusler compound: Potential thermoelectric material

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Abstract

We report the band structure calculations of rare earth based half Heusler LuNiSb compound for thermoelectric performance. A narrow energy gap of 0.227 eV is opened using TB-mBJ approximation. The value of Seebeck coefficient 288 (μVK −1) at room temperature using mBJ scheme is quite large as compared to the experiment as well as well known thermoelectric material PbTe at same temperature. The observed highest value (6.1 × 10 10 Wm −1 K −2 s −1) at 647 K is close to our calculated value 9.93 × 10 10 Wm −1 K −2 s −1 at 650 K using GGA + U approximations. We found the Lu-4 f flat bands lie in conduction band along Γ– X direction are mainly responsible for maximum peaks of Power factor. The high value 0.804 of ZT and value of other thermoelectric parameters indicate that LuNiSb would be a favourable material for room temperature thermo electric applications.

Introduction

Thermoelectric (TE) materials have the ability to directly converting heat into electricity [1,2]. Recently, to improve energy efficiency from waste-heat recovery is the main challenge and TE materials can play an important role in improving energy efficiency. The efficiency of TE materials depends on the dimensionless figure of merit ZT. It is given by ZT = S2σT/κ, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and the thermal conductivity of the material, respectively [3]. The material having a higher ZT value exhibits better thermoelectric performance. In order to have a larger ZT, a large value for S2σ (also known as power factor) and a small value for thermal conductivity is needed.

Recently, half Heusler (HH) compounds are considered for potential thermoelectric materials and applications [[4], [5], [6]]. Particularly, n-type stannides MNiSn and p-type antimonides MCoSb (M = Zr, Hf, Ti) have received remarkable interest in the last few years [[7], [8], [9], [10], [11]].

Rare earth based HH compounds draw attention due to the report [12] that a system has good TE response in which f-levels lie close to the Fermi level. Hartjes and Jeitschko [13] investigated the crystal structure and magnetic properties of RNiSb (R = La–Nd, Sm, Gd–Tm, Lu) compounds by Rietveld technique from X-ray powder data and magnetic properties were studied by SQUID magnetometer. Electronic structure of rare earth nickel pnictides MNiP (M = rare earth and P = pnicogen) as TE materials were studied by Larson et al. [14] and found that the gap narrows as go from As to Bi. TE properties of some Ni-based HH compounds RNiSb (R = Ho, Er, Tm, Yb and Y) were studied experimentally by Sportouch et al. [15] and observed that their thermopower values range from 21 μV/K to 160 μV/K while thermal conductivity is in a reasonable range from 3 W/mK to 7.5 W/mK. Skolozdra et al. [16] have studied the magnetic and transport properties of RNiSb compounds (R = Gd, Tb, Dy, Yb and Lu). They observed the thermo power S of low temperature LuNiSb is positive and increases linearly with the temperature. The largest value of S is 136 μV/K at 380 K. However, in the literature we find diverse values on the thermoelectric power of LuNiSb, like positive S = 70 μV/K at 380 K [17] and S = −48 μV/K at 300 K [18]. Recently Synoradzki et al. [19] reported the high temperature TE properties of ternary compounds LuNiSb and LuNiSn and their composite material and found that the maximum absolute S values are 66 μV/K at 607 K. While close to room temperature, the magnitude of S is close to that quoted in Ref. [18]. Winiarski et al. [20] have theoretically studied the electronic structures and transport properties of RNiSb and RPdSb (R = Y and Lu) compounds and predict that the Ni-based systems are expected to exhibit very high thermoelectric power factors in the range of 4–5 mWK−2m−1 at room temperature.

We found much experimental work for TE performance of HH LuNiSb compound; however diverse values of S have been reported in these studies, which would directly influence the TE performance of the material. We also found that LuNiSb is very less explored theoretically. Hence the main objective of this work is to investigate theoretically the TE properties of the LuNiSb compound. This work would also validate the experimental findings of LuNiSb, particularly for thermoelectric performance. The predicted TE response of rare earth based LuNiSb is also compared with recent experimental observations [19].

In this work, we have calculated the electronic structure and thermo electric properties of the LuNiSb compound. Moreover, the role of d – states of Lu on thermo electric properties are also investigated. We used the most efficient method to treat the rare earth metals, Coulomb corrected generalized gradient approximation (GGA + U) [21] and the Tran-Blaha modified Becke-Johnson approximation (TB-mBJ) [22] known to predicts the energy band gap more accurately [[23], [24], [25], [26]]. The obtained results suggest that the presence of a heavy rare earth atom enhances the TE performance of Ni based HH materials and calculated TE properties are in agreement with the recent experimental observations [19]. Our theoretical study of band structure and thermoelectric performance on LuNiSb compound would motivate the researcher towards the thermoelectric study of rare earth based HH compounds. The remaining part of the manuscript is organised as follows. In section 2, we described, briefly the computational methods used in this work. Results of electronic and thermoelectric properties are presented and discussed in section 3. Finally, the last section includes the summary and main conclusions from this work.

Section snippets

Computational details

Ternary half-Heusler compounds are intermetallic compounds in the chemical composition of XYZ where X and Y represent different rare earth or transition metals and Z is an element from the main group. The LuNiSb compound crystallizes in the cubic structure of MgAgAs type [27] with the space group is F43 m (No. 216) [28]. This structure can also be viewed as four interpenetrating fcc lattices which contain the lattice of Lu atoms, Ni atoms, Sb atoms, and a lattice of vacancies. The Lu and Sb

Structure optimization

We first focus on the volume optimization of the unit cell for the description of the ground state structural parameters such as lattice constant (a) bulk modulus (B0) and its pressure derivative (Bo) of the LuNiSb HH compound. We adopted the experimental lattice parameter [13] for the calculation. The optimization was done with 104 k-points in the IBZ. The calculated total energy of the system as a function of unit cell volume thus fitted to Murnaghan's equation of state [[40], (b)] and

Conclusions

Electronic structure and thermoelectric properties of the LuNiSb compound have been performed under the GGA + U and TB-mBJ approximations. The values of the energy gap obtained from the present work are in good agreement with the other works. In the band structures, a small energy gap of 0.227 eV using TB-mBJ is obtained to show the semiconducting nature of the compound. Band Structure shows that VB which spans from −4 eV to the EF is dominated by Ni-3d and Lu-4f energy bands. On the other

Credit author statement

I declare that I am the sole author of this manuscript entitled “Structural, electronic and thermoelectric performance of narrow gap LuNiSb half Heusler compound: Potential Thermoelectric Material and the whole work is only carried out by me.

Declaration of competing interest

Author declare no conflict of interest.

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