RESISTIVITY STUDIES IN THE KONDO INSULATING SYSTEM, FESI1-XGEX

Electrical resistivity measurements have been carried out in the FeSi1 -xGex system in the 1.4-300K temperature range for 0.0 less than or eq ual to x less than or equal to 0.25, the composition range in which th e samples are monophasic. In FeSi, the resistance shows a two-step inc rease with decrease in temperature; the high-temperature region (200-1 00K) showing an activated semiconducting behaviour corresponding to a gap value, Delta, of 707 K. With Ge substitution at the Si site, the r esistivity continues to show a semiconducting behaviour and the gap va lues obtained from the experimental data systematically decrease to 11 5K for x = 0.25. The observed decrease in Delta is discussed in conjun ction with band-structure calculations. In FeSi and in the Ge substitu ted sample with x = 0.05, the resistivity in the 5-50K range could be fitted either to an activated behaviour with lower Delta or to the var iable-range hopping mechanism. In the samples with x > 0.05, the data in the 5-50 K region fits better to the variable-range hopping mechani sm. Below 5 K, the resistance tends to saturate to a constant Value in FeSi and the low-temperature saturation of resistance persists upon G e substitution, possibly implying a metallic sound state as T --> 0.