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 Ce_1-xLa_xNiSn and Ce_1-xLa_xRhSb (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 ≥ 0.2. At low temperature, Ce_1-xLa_xNiSn alloys show a cross-over from a Kondo insulator (x = 0) to a single ion Kondo ground state (x ≥ 0.2) with increasing La concentration, while Ce_1-xLa_xRhSb 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 Ce_1-xLa_xNiSn 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 ≥ 0.4. On the other hand, susceptibility of Ce_1-xLa_xRhSb 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.