Kondo-semiconductor to Kondo-impurity transition in the heat capacity of Yb1-xLuxB12

We have measured the heat capacity and magnetic susceptibility of Yb1-xLuxB 12 single crystals in the temperature range from 4 to 300 K. For x less tha n or equal to 1/2, formation of an energy gap is manifested by a strong dec rease in the magnetic susceptibility below 100 K. The heat capacity exhibit s a Schottky anomaly with an activation energy of about 170 K for x less th an or equal to 1/2, whereas for x > 1/2 single-impurity Kondo behavior with T-K similar to 300 K. For 0 less than or equal to x less than or equal to 3/4, the magnetic entropy saturates to a value of R In 4/molYb around 300 K , indicating that the crystal-field ground state of Yb3+ is a quarter in th e whole series of alloys. (C) 1999 Elsevier Science B.V. All rights reserve d.