Trigonal Lattice Distortion and Ferro-Quadrupole Ordering in Phase IV of Ce_xLa_1−xB₆ (x=0.75 and 0.70)

Abstract

The low-temperature lattice effects of the Kondo compounds Ce_xLa_1−xB₆ (x=0.75 and 0.70) with a Γ₈ ground state have been investigated by dilatometric measurements. In an ordered phase IV below T_C=1.6 K (1.4 K) for x=0.75 (0.70), the lattice length along the [001] direction reveals an increase, while the length along the [111] direction shows an appreciable decrease. This lattice distortion is described in terms of the trigonal strain ⟨ε_yz⟩=⟨ε_zx⟩=⟨ε_xy⟩≠0 with Γ₅ symmetry in addition to a volume expansion ε_B=ε_xx+ε_yy+ε_zz with full symmetry Γ₁. The trigonal strain in phase IV indicates a ferro-quadrupole ordering of ⟨O_yz⟩=⟨O_zx⟩=⟨O_xy⟩≠0.