  The classification of structural phase transitions as displacive or
order-disorder in character is usually based on spectroscopic data above the
transition. We use single crystal x-ray diffraction to investigate structural
correlations in the quasi-skutterudites, (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$,
which have a quantum phase transition at $x\sim0.9$. Three-dimensional pair
distribution functions show that the amplitudes of local atomic displacements
are temperature-independent below the transition and persist to well above the
transition, a signature of order-disorder behavior. The implications for the
associated electronic transitions are discussed.
