A comparative study of suppression of the energy gap with La substitution in the Kondo insulators: CeNiSn and CeRhSb

We report structural, transport and magnetic measurements on the solid solutions of Ce1-xLaxNiSn and Ce1-xLaxRhSb (x = 0 to 1). The results have been used to compare and contrast The effect of La substitution on the ground state properties of the Kondo insulators CeNiSn and CeRhSb. The resistivity measurements reveal that the energy gap in CeNiSn and CeRhSb decreases with increasing La concentration; no gap type behaviour was observed for La concentration greater than or equal to 0.2. At low temperature, Ce1-xLaxNiSn alloys show a cross-over from a Kondo insulator (x = 0) to a single ion Kondo ground state (x greater than or equal to 0.2) with increasing La concentration, while Ce1-xLaxRhSb alloys show a cross-over from a Kondo insulator (x = 0) to a coherent Kondo lattice (x = 0.2) and eventually to a single ion Kondo ground state at x = 0.5. Magnetic susceptibility measurements of Ce1-xLaxNiSn alloys exhibit a dramatic change in behaviour with La concentration; Curie-Weiss behaviour for x = 0, valence fluctuation behaviour for x = 0.2 and again C-W behaviour for x greater than or equal to 0.4. On the other hand, susceptibility of Ce1-xLaxRhSb alloys continue to show a valence fluctuation behaviour up to x = 0.7, and C-W behaviour for higher La concentrations. CeNiSn and CeRhSb exhibit a large negative magnetoresistance at 20 T field and at 1.5 K.