Lattice distortion and spontaneous Γ_5g ferro-quadrupole moment in phase IV of Ce_xLa_1-xB₆

We have performed low-temperature thermal expansion measurements to investigate lattice effect of the phase IV in Ce_xLa_{1- x}B₆, for x=0.75 and 0.70. In phase IV of both compounds, lattice length along the [1 1 1] direction shrinks notably, on the other hand length along the [0 0 1] direction expands. These results provide evidence that the spontaneous Γ_5g ferro-quadrupole moment 〈Oyz〉=〈Ozx〉=〈Oxy〉≠0 is relevant in phase IV because the trigonal lattice distortion is described by the symmetry strain 〈εyz〉=〈εzx〉=〈εxy〉≠0 simultaneously.