Elastic properties and magnetic phase diagrams of dense Kondo compound Ce_0.75La_0.25B₆

We have investigated the elastic properties of the cubic dense Kondo compound Ce_0.75La_0.25B₆ by means of ultrasonic measurements. We have obtained magnetic fields vs temperatures (H-T) phase diagrams under magnetic fields along the crystallographic [001], [110] and [111] axes. An ordered phase IV showing the elastic softening of c₄₄ locates in low temperature region between 1.6 and 1.1K below 0.7 T in all field directions. The phase IV shows an isotropic nature with regard to the field directions, while the antiferro-magnetic phase III shows an anisotropic character. A remarkable softening of c₄₄ and a spontaneous trigonal distortion ε_yz + ε_zx + ε_xy recently reported by Akatsu et al. [J. Phys. Soc. Jpn. 72 (2003) 205] in the phase IV favor a ferro-quadrupole (FQ) moment of O_yz + O_zx + O_xy induced by an octupole ordering.