Neutron-diffraction study of the unusual ordered phases of Ce _0.75la_0.25B₆

Ce_0.75La_0.25B₆ shows paramagnetic phase (phase I), 4f-electron quadrupolar ordered phase (II), magnetically ordered phase (III), and an unknown phase IV. In order to investigate order parameters in phases III and IV, neutron diffraction experiments were performed down to 74 mK. Magnetic diffraction pattern measured in phase III is reproduced by a model in which spin arrangement is modified from the previously suggested structure for CeB₆. No magnetic reflections were observed in phase IV. Thus, the order parameter in phase IV is expected to be 4f-electron multipoles rather than magnetic dipoles.