Charge density waves (CDWs) in rare-earth nickel dicarbides (RNiC₂) provide a versatile platform to investigate the interplay between electronic instabilities, lattice effects, and magnetism in quasi-one-dimensional systems. In this work, we focus on the tuning of CDW order by external parameters, with particular emphasis on hydrostatic pressure.

Our central result is a high-pressure X-ray diffraction study on GdNiC₂ performed at ESRF (ID15B). At ambient pressure, an incommensurate CDW with modulation vector q = (1/2, 1/2 + η, 0) emerges below ~175 K, followed by a lock-in transition into a commensurate phase with q = (1/2, 1/2, 0) upon cooling. Under hydrostatic pressure above ~2.85 GPa, this CDW state is suppressed and replaced by a distinct phase characterized by q = (1/2, 1/2, 1/2). This observation reveals a pressure-induced reconstruction of the CDW ground state and a change in the underlying ordering mechanism.

These results demonstrate the pronounced sensitivity of CDW ordering vectors to pressure and provide new insight into competing electronic instabilities in RNiC₂.