Study of thermoelectric material Hf Ni1-x CO x Sn

The electron energy state, magnetic and transport characteristics of thermometric materials HfNi1-xCoxSn were investigated in the T = 80 ¸ 400 K temperature range and at charge carriers concentration from Co » 9,5×1019 NA см-3 ( x = 0,005 )÷ 5,7 ×1021 см-3 ( x = 0,30 ) and H £ 10 kGs. The material HfNi1-xCoxSn is sensitive to the temperature change and could be used as the basis for the sensitive thermoelectric devices. We investigated the crystal structure, electron density of states (DOS) and the kinetic and energy characteristics of n-HfNiSn heavily doped with the Co impurity. Samples were synthesized at the laboratory of the Institute of Physical Chemistry, Vienna University. The HfNi1-xCoxSn crystal-lattice periods were determined by X_ray analysis with the use of the Fullprof software. We employed a data array obtained by the powder method using a Guinier-Huber image plate system. The chemical and phase compositions of the samples were determined using a Ziess Supra 55VP scanning electron microscope and an EMPA energy dispersive X-ray analyzer. The electronic structure was calculated by the Korringa–Kohn–Rostoker (KKR) technique in the coherent potential approximation (CPA) and local density approximation (LDA), as well as the full-potential linearized plane wave (FP-LAPW) method within density functional theory (DFT). In the calculations, we used experimental values of the lattice constant on a k grid 10 × 10 × 10 in size and the Moruzzi–Janak–Williams exchange correlation potential parametrization. The width of the contoured energy window was 16 eV. The number of energy values for DOS calculations was 1000. To predict the behavior of the Fermi level, band gap, and electrokinetic characteristics of n-HfNiSn doped with Co atoms, the electron density distribution (DoS) was calculated. The calculated results pretending to be adequate to experimental studies should account for complete information on the semiconductor’s crystalline structure. To obtain more accurate results, we calculated the DoS for almost all possible cases of the mutual substitution of atoms at sites of the HfNiSn unit cell. Shows the result most consistent with experimental data. It was found that the disordered structure (Hf1-xNix)NiSn, x = 0.01, of the HfNiSn compound is most probable. We note that the same result was obtained from structural studies of HfNiSn. The partial (to 1 at%) substitution of Hf atoms with Ni atoms generates donor-type structural defects in the crystal, and the Fermi level is in the band gap which becomes narrower. It was also found that the minimum in the dependence of variations in the DoS at the Fermi level (DoSF(x)) for the disordered structure (Hf1-xNix)NiSn of the HfNiSn compound corresponds to the (Hf0.99Ni0.01)NiSn composition. In this semiconductor model, the Fermi level is in the band gap which is εg ≈ 282 meV. The same question arises when analyzing the behavior of the dependences (x) and (x) in HfNi1-xCoxSn. For example, the (x) variation in the concentration range 0.05 ≤ x ≤ 0.20 shows that the modulation amplitude of the continuous energy bands of HfNi1-xCoxSn HDCSs increases. Indeed, the activation energies (x) increase from (x = 0.05) = 38.3 meV to (x) (x = 0.20) = 59.2 meV. As we already noted, such behavior is possible only when compensating electrons appear in the p-type semiconductor due to the ionization of donors whose appearance was not initially assumed. In HfNi1-xCoxSn samples, x > 0.20, the decrease in (x) indicates a decrease in the modulation amplitude of the continuous energy bands, which is possible only when the degree of compensation of HfNi1-xCoxSn decreases due to a decrease or termination of the generation of donor-type structural defects. Thus, the initial assumption that n-HfNiSn doping with Co atoms by substituting Ni atoms is accompanied by the generation of only acceptor-type structural defects in the crystal does not allow consistent explanation of the behavior of the energy characteristics of HfNi1-xCoxSn HDCSs. The variations in the activation energy of hopping conduction (x) and the modulation amplitude of the continuous energy bands (x) unambiguously prove the existence of a donor source in HfNi1-xCoxSn. Further, we will identify the possible mechanism for the appearance of donors. The series of studies on the crystalline structure, energy spectrum, and electrokinetic parameters of the n_HfNiSn intermetallic semiconductor heavily doped with the Co impurity allowed determination of the variation in the degree of compensation of the semiconductor due to the generation of both structural defects of acceptor nature during the substitution of Ni atoms with Co atoms and defects of donor nature during the partial substitution of Ni sites with Sn atoms. The n_HfNiSn crystalline structure is disordered, and the Hf site can be occupied by Ni to ~1 at%, which generates structural defects of donor nature in the semiconductor and explains the mechanism of its “a priori doping with donors”. The mechanism of the degree of compensation of the semiconductor as the result of the crystal structure transformation during doping, leading to the generation of structural defects of the acceptor and donor nature was established. The results of the electronic structure calculation are in agreement with experimental data and the HfNi1-xCoxSn semiconductor is a promising thermoelectric material. The results are discussed in the framework of the heavily doped and compensated semiconductor model by Shklovsky–Efros.