  X-ray diffraction experiments under pressure in a diamond anvil cell have
been performed to gauge any response of the crystalline lattice of URu2Si2 to
the "hidden order" or antiferromagnetic transitions, the latter of which is
accessible only with applied pressure. The ambient-pressure crystal structure
of URu2Si2 persists to high pressure, and structural characterization reveals a
reasonably robust crystal lattice with respect to both of the aforementioned
temperature-induced electronic transitions. Coupling between the lattice and
the "hidden order" transition is reconfirmed to be subtle, and that subtle
sensitivity is extended to the antiferromagnetic transition. The
pressure-dependent evolution of the lattice parameters indicates a slightly
anisotropic compression of the unit cell, which results in a decrease in the
lattice c/a ratio with increasing pressure. From compression data, the bulk
modulus is estimated to be B_0~190 GPa. While the unit cell volume undergoes
compression, the Ru-Si layers separating the U planes evince an apparent
"flattening," resulting in a more 2D-like structure and an increased volume per
U ion.
