Abstract :
The Kondo insulator Y bB12 is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, ( n 4 f ) and ( n 5 d ) , as a function of doping of Y bB12 with the rare earths Tm and Lu. We found that n 5 d exhibits an anomalous change at the critical concentration x c of the dopant, in agreement with experiment ( x c = 0.5 for Y b1−xLuxB12 and x c = 0.3 for Y b1−xTmxB12). We suggest that the critical behaviour seems to be strictly connected with the change of n 5 d and in consequence the change of the Yb valency.
