Magnetic Phase Diagrams of Kondo Compounds Ce0.75La0.25B6and Ce0.6La0.4B6

We measured the elastic and thermal properties to examine the competing effect between the intersite interactions and the Kondo coupling in Ce 0.75 La 0.25 B 6 and Ce 0.6 La 0.4 B 6 with the Γ 8 ground state. The huge softening of the transverse C 44 mode of both compounds at low temperatures indicates a remarkable fluctuation of the quadrupolar moments O y z , O z x and O x y with Γ 5 symmetry. In the magnetic phase diagram of Ce 0.75 La 0.25 B 6 , we found a new ordered phase IV in addition to the antiferroquadrupolar phase II and the antiferromagnetic phase III. The compound Ce 0.6 La 0.4 B 6 , however, shows the Fermi liquid state without the long-range order at low fields and transits into the ordered phase II or III in the magnetic field above 16 kOe.