near the hidden order transition using X-ray crystallography*

URu₂Si₂ is known for its hidden order transition at T=17.5K whose physical origin is still not agreed. We present an x-ray crystallographic study of the structure of URu2Si2 over the temperature range 23 K < T < 5.5 K. Using a large, image plate detector, we recorded 32 reflections and performed a full structure refinement at each temperature. We found that, as the temperature is lowered, the spacegroup of the crystal does not change, but the Si atoms translate along the b axis by 0.78Å (0.19 b) in response to the hidden order transition. Our study suggests that the order parameter associated with this transition is weakly coupled to a Si phonon of Eu symmetry.