Multiple charge density wave transitions in the antiferromagnets RNiC2 (R = Gd, Tb)

X-ray scattering and electrical resistivity measurements were performed on GdNiC2 and TbNiC2. We found a set of satellite peaks characterized by q(1) = (0.5, eta, 0) below T-1, at which the resistivity shows a sharp inflection, suggesting the charge density wave (CDW) formation. The value of eta decreases with decreasing temperature below T-1, and then a transition to a commensurate phase with q(1C) = (0.5,0.5,0) takes place. The diffuse scattering observed above T-1 indicates the presence of soft phonon modes associated with CDW instabilities at q(1) and q(2) = (0.5,0.5,0.5). The long-range order given by q(2) is developed in addition to that given by q(1C) in TbNiC2, while the short-range correlation with q(2) persists even at 6 K in GdNiC2. The amplitude of the q(1C) lattice modulation is anomalously reduced below an antiferromagnetic transition temperature T-N in GdNiC2. In contrast, the q(2) order vanishes below T-N in TbNiC2. We demonstrate that RNiC2 (R = rare earth) compounds exhibit similarities with respect to their CDW phenomena, and discuss the effects of magnetic transitions on CDWs. We offer a possible displacement pattern of the modulated structure characterized by q(1C) and q(2) in terms of frustration.