hide
Free keywords:
-
Abstract:
The substitutional series of Heusler compounds NiTi1-xMxSn (where M=Sc,
V and 0<x <= 0.2) were synthesized and investigated with respect to
their electronic structure and transport properties. The results show
the possibility to create n-type and p-type thermoelectrics within one
Heusler compound. The electronic structure and transport properties were
calculated by all-electron ab initio methods and compared to the
measurements. Hard x-ray photoelectron spectroscopy was carried out and
the results are compared to the calculated electronic structure. Pure
NiTiSn exhibits massive "in gap" states containing about 0.1 electrons
per cell. The comparison of calculations, x-ray diffraction, and
photoemission reveals that Ti atoms swapped into the vacant site are
responsible for these states. The carrier concentration and temperature
dependence of electrical conductivity, Seebeck coefficient, and thermal
conductivity were investigated in the range from 10 to 300 K. The
experimentally determined electronic structure and transport
measurements agree well with the calculations. The sign of the Seebeck
coefficient changes from negative for V to positive for Sc substitution.
The high n-type and low p-type power factors are explained by
differences in the chemical-disorder scattering-induced electric
resistivity. Major differences appear because p-type doping (Sc) creates
holes in the triply degenerate valence band at T whereas n-type doping
(V) fills electrons in the single conduction band above the indirect gap
at X what is typical for all semiconducting transition-metal-based
Heusler compounds with C1(b) structure.