  Spin-orbit coupling (SOC) is essential in understanding the properties of 5d
transition metal compounds, whose SOC value is large and almost comparable to
other key parameters. Over the past few years, there have been numerous studies
on the SOC-driven effects of the electronic bands, magnetism, and spin-orbit
entanglement for those materials with a large SOC. However, it is less studied
and remains an unsolved problem in how the SOC affects the lattice dynamics.
We, therefore, measured the phonon spectra of 5d pyrochlore Cd2Os2O7 over the
full Brillouin zone to address the question by using inelastic x-ray scattering
(IXS). Our main finding is a visible mode-dependence in the phonon spectra,
measured across the metal-insulator transition at 227 K. We examined the SOC
strength dependence of the lattice dynamics and its spin-phonon (SP) coupling,
with first-principle calculations. Our experimental data taken at 100 K are in
good agreement with the theoretical results obtained with the optimized U = 2.0
eV with SOC. By scaling the SOC strength and the U value in the DFT
calculations, we demonstrate that SOC is more relevant than U to explaining the
observed mode-dependent phonon energy shifts with temperature. Furthermore, the
temperature dependence of the phonon energy can be effectively described by
scaling SOC. Our work provides clear evidence of SOC producing a non-negligible
and essential effect on the lattice dynamics of Cd2Os2O7 and its SP coupling.
