Study of the properties of intermetallic semiconductor n-TiNiSn doped by Ag impurity

The structural, kinetic and energy properties of the intermetallic semiconductor n -TiNiSn, doped by Ag atoms, were investigated in the ranges: T = 80–400 K, N Ag = (0.45–6.3)×10 -2 cm -3 . Samples of n -TiNiSn, doped by Ag atoms were synthesized by arc melting the stoichiometric amounts of the constituent elements under high purity Ti-gettered argon atmosphere on a water-cooled copper hearth. The arc-melted ingots were annealed at 1073 K for 720 hours and subsequently quenched in cold water. For the characterization of the samples X-ray powder diffraction was performed. Measurements of the temperature dependences of the resistivity ρ ( Т , х ) and the thermopower coefficient α ( Т , х ) were carried out by a two-probe method in the temperature range of 80÷400 K on samples in the form of parallelepiped with a size of ~1.0×1.0×5 mm 3 . Measurements of the thermopower coefficient were carried out by a potentiometric method relative to copper. Structural studies of samples of the semiconductor n -TiNiSn, doped by Ag atoms at concentrations N Ag = (0.45–6.3)×10 -20 cm -3 ( х = 0.005–0.07) showed that impurity Ag atoms at different concentrations can occupy different crystallographic positions or located in the interstitial space of the crystal lattice. In this case, structural defects will be generated in the semiconductor, and in the band gap ε g , the corresponding energy states of different nature will be generated. At concentration range of Ag impurity atoms N Ag = (0.45–1.8)×10 -20 cm -3 ( х = 0.005–0.02) a decrease of the unit cell parameter a ( х ) was observed. At higher concentrations of Ag atoms in the region N Ag = (1.8–3.6)×10 -20 cm -3 ( х = 0.02–0.04), the values of the unit cell parameter a ( х ) increase rapidly, and at N Ag >3.6×10 -20 cm -3 ( х >0.04), the increase in the parameter a becomes smoother. The study of the temperature dependences of the resistivity ln( ρ (1/ T , х )) and the thermopower coefficient α (1/ Т , х ) of n -TiNiSn doped by Ag atoms revealed the presence of high-temperature activation regions at all concentrations of Ag impurity atoms, indicating the location of the Fermi level ε F in the band gap ε g . Since the sign of the thermo-power coefficient α (1/ Т , х ) is negative at all temperatures, the main carriers of the electric current of n -TiNiSn samples doped by Ag atoms are electrons, and the Fermi level ε F is located in the band gap ε g near the conduction band ε С . The experimental results show that donor and acceptor states are generated simultaneously in a semiconductor at different rates. Understanding the mechanisms of electrical conductivity of the intermetallic semiconductor n -TiNiSn, doped by Ag atoms, will allow us to predictably optimize the kinetic characteristics of the thermoelectric material and increase the efficiency of converting thermal energy into electrical energy by changing the type and concentration of the impurity, as well as the scattering mechanisms. Keywords: electrical conductivity, thermopower coefficient, Fermi level, structural defect.