Magnetic Phase Diagrams of the Dense Kondo Compounds CeB₆ and Ce_0.5La_0.5B₆

We have studied the magnetic phase diagrams of the dense Kondo compounds CeB₆ and Ce_0.5La_0.5B₆ by use of low temperature ultrasonic measurements. A transition associated with a quadrupolar reorientation has clearly been observed around 1 kOe in the antiferroquadrupolar phase ii of CeB₆. In the paramagnetic phase I of Ce_0.5La_0.5B₆ a crossover from the ordinaly Γ₈ ground state to the Kondo singlet state has been found. The field induced transition from the Kondo singlet state to the antiferromagnetic phase III or the phase II has been found at a critical field 17 kOe. The field angle dependence of the phase diagram of Ce_0.5La_0.5Be₆ is also presented.