  Disentangling the primary order parameter from secondary order parameters in
phase transitions is critical to the interpretation of the transition
mechanisms in strongly correlated systems and quantum materials. Here we
present a study of structural phase transition pathways in superionic Cu2S
nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron
diffraction techniques sensitive in both momentum-space and the time-domain, we
distinguish the dynamics of crystal symmetry breaking and lattice expansion in
this system. We are able to follow the transient states along the transition
pathway and so observe the dynamics of both the primary and secondary order
parameters. Based on these observations we argue that the mechanism of the
structural phase transition in Cu2S is dominated by the electron-phonon
coupling. This mechanism advances the understanding from previous results where
the focus was solely on dynamic observations of the lattice expansion.
