We have performed dilatometric measurements by the capacitance method on tetragonal rare earth compounds DyB₂C₂ and HoB₂C₂ that show antiferro-quadrupole (AFQ) ordering. The tetragonal structure is invariant across the paramagnetic phase to the AFQ phase at T_Q=24.7 K in DyB₂C₂. In the case of intermediate phase IV of HoB₂C₂, between T_C1=5.9 K and T_C2=5.0 K, the lattice length along the [101] direction reveals an abrupt increase, while the lengths along the [100] and [110] directions shrink slightly. This lattice distortion is described in terms of triclinic strain ε_yz = ε_zx≠0 indicating the spontaneous ferro-quadrupole moment <O_yz>= <O_zx>≠0. A plausible model, based on an octupole ordering <T_x>=<T_y>=<T_z>≠0, in phase IV of HoB₂C₂, is argued.

J. Phys. Soc. Jpn. 74 (2005) 1666.

(*This research was performed at Niigata University in 2004.)