Dilatometric Measurements and Multipole Ordering in DyB2C2 and HoB2C2

Abstract

We have performed dilatometric measurements by the capacitance method on tetragonal rare earth compounds DyB 2 C 2 and HoB 2 C 2 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 DyB 2 C 2 . In the case of intermediate phase IV of HoB 2 C 2 , 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 e y z = e z x ≠0 indicating the spontaneous ferro-quadrupole moment = ≠0. A plausible model, based on an octupole ordering = = ≠0, in phase IV of HoB 2 C 2 , is argued.