The heavy-fermion compound URu₂Si₂ has provided an excellent arena to study phase transition and related symmetry breaking in correlated electron system. Despite of relentless efforts for several decades, elucidating the nature of hidden order (HO) phase at 17.5 K still remains enigmatic. Even though the experimental evidences for both broken rotational [1] and chiral symmetry [2] were found recently, little is known about crystal structure of the system. Here, we report a hard x-ray diffraction study of the single crystal URu₂Si₂ under hydrostatic pressure. For pressure above 3.4 kbar, we found a rotational symmetry breaking from tetragonal to orthorhombic crystal structure with onset temperatures near 100 K. Contrast to recent x-ray diffraction study [3], the pressure-induced orthorhomicity is not coincided with the HO transition. Instead, its temperature-dependence suggests possible relevance with the antiferromagnetic (AF) order. This observation has not yet been predicted by theories describing an adiabatic continuity of a complex order parameter. Therefore, our findings provide new perspectives to the long-standing conundrum.

[1] R. Okazaki et al., Science 331, 439 (2011).
[2] H. –H. Kung et al., Science 347, 1339 (2015).
[3] S. Tonegawa et al., Nat. Commun. 5, 4188 (2014).