Abstract
The peculiarities of crystal and electronic structures, thermodynamic and energy state characteristics of the ZrNi1-xRhxSn semiconductive solid solution were investigated. It has been shown that in the ZrNiSn compound simultaneously exist two types of structural defects of the donor nature which generate two donor bands with different ionization energy in the band gap: a) the donor band ɛD1, formed as a result of a partial, up to ~ 1%, occupation of 4a position of Zr atoms by Ni atoms (mechanism of “a priori doping”) and deep donor band ɛD2, formed as a result of partial occupation of the tetrahedral voids by Ni atoms (Vac). The substitution in 4c position of the Ni atoms by Rh ones in ZrNi1-xRhxSn generates structural defects of acceptor nature and creates an impurity acceptor band ɛA in the band gap, which, in addition to the existence of ɛD1 та ɛD2 donor bands, makes semiconductor highly doped and strongly compensated. The obtained results allow to understand the mechanisms of electrical conductivity of thermoelectric materials based on n-ZrNiSn and the ways of conscious optimization of their characteristics for obtaining the maximum efficiency of conversion of thermal energy into electric.
Highlights
Investigation of electrokinetic and magnetic characteristics of thermoelectric material based on the ZrNi1-xRhxSn semiconductive solid solution revealed an interesting feature in the behavior of Fermi level εF depending on the concentration of Rh atoms and temperature [1]
As known, according to the mechanism of “a priori doping” [2], the structure of ZrNiSn (MgAgAs-type, space group F 43m [3]) is disordered as a result of partial, up to ~ 1 % (z = 0.01), occupation of 4а position of Zr atoms (4d25s2) by Ni (3d84s2) atoms, which generates the structural defects of donor nature and corresponding donor band ɛD1 (Ni has more d-electrons), and formula of compound can be written as (Zr1-zNiz)NiSn
In Ref. [1] it was established that in ZrNi1-xRhxSn at Т = 80 K and the lowest concentration of Rh (х = 0.005) the sign of thermopower coefficient α(х) changes from negative to positive. Such change of α(х) sign was predicted because substitution of Ni atoms by Rh (4d85s1) in 4c position generates the structural defects of acceptor nature (Rh has less selectrons)
Summary
Investigation of electrokinetic and magnetic characteristics of thermoelectric material based on the ZrNi1-xRhxSn semiconductive solid solution revealed an interesting feature in the behavior of Fermi level εF depending on the concentration of Rh atoms and temperature [1]. In the ZrNi1-xRhxSn samples at х = 0.005, х = 0.01 and х = 0.03 with increase of temperature higher than Тinv.≥ 254 К, Тinv.≥ 295 К and Тinv.≥ 362 К, respectively, the α(х) sign unexpectedly became negative, indicating that electrons again, as in n-ZrNiSn, are major current carriers At these concentrations of ZrNi1-xRhxSn with increase of temperature Fermi level εF drifts from the valence band (low temperatures) to the conduction band (high temperatures), crossing the middle of the band gap ɛg (complete compensation) at Тinv.≈ 254 K, Тinv.≈ 295 K and Тinv.≈ 362 K, respectively.
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