Electron paramagnetic resonance of defects in β-iron disilicide ceramics

Abstract

The point defects in β-iron disilicide ceramics, which may influence the thermoelectric properties of the ceramics, have been investigated by electron paramagnetic resonance (EPR). The 77-K EPR spectra are dependent upon the Si/Fe ratio of the ceramics specimens. Several S=1/2 centers with g factors of about 2.0 are observed in both silicon-deficient p-type and iron-deficient n-type specimens prepared by sintering and subsequent annealing of FeSix(1.9<x<2.8) grains. These centers disappear in a specimen prepared by directly annealing a lump of FeSi2 alloy without sintering. By considering the difference of the sample-preparation procedures, we conclude that the S=1/2 centers exist in the oxidized intergrain region of the ceramics. An S=1/2 signal with an anisotropic g factor (g1=2.061, g2=2.047, g3=2.024) arises only in iron-deficient n-type specimens. This signal is attributable to unionized donor electrons. A multiplet signal with zero-field splitting arises in nearly stoichiometric p-type specimens. The multiplet signal is explained in terms of an S=3 center. Several possible models based on exchange interaction between two spins were examined to elucidate the origin of the S=3 center. The most plausible model is the pair of a high spin Fe3+ (S=5/2) and an S=1/2 center (a silicon vacancy or an acceptor hole).