Effects of alloying and magnetic field on the stability of the energy gaps in the compounds CeNiSn and CeRhSb

CeRhSb and CeNiSn are valence fluctuating compounds which develop energy gaps in the electronic density of states at the Fermi level at temperatures below 10 K. Electrical transport and magnetic measurements on the alloys Ce_1-xLa_xRhSb, CeRh_1-xPd_xSb, Ce_1-xR_xNiSn (R = La, Y) and CeNi_1-xT_xSn (T = Pd, P t) have been used to compare and contrast the effects of alloying and lattice coherence in these two compounds. Magnetoresistance measurements show that the gaps decrease with increasing applied magnetic fields up to 12 T. Disruption of the Ce lattice coherence by alloying with La or Y rapidly eliminates the energy gaps. The substitution of 15% Pd for Rh in CeRhSb, and of 15% Pd and Pt for Ni in CeNiSn also suppresses the low temperature gap and drives the systems into a Kondo lattice regime. The resistivity of CeRh_0.85Pd_0.15Sb shows the presence of Kondo and crystalline field effects. The magnetic susceptibility results for CeRhSb, Ce_0.8La_0.2RhSb and Ce_0.8La_0.2NiSn have been analyzed on the basis of the Coqblin-Schrieffer model.