  We examine the relationship between astrometric stability and astrophysical
properties in flat-spectrum radio-loud quasars making up the celestial
reference frame. We use position determinations from geodetic VLBI
measurements, and develop a new metric for source position stability. We then
compare this quantity to two measures of source compactness: structure index,
which probes structure on milliarcsecond scales; and the presence of
interstellar scintillation associated with the quasar, which probes scales of
tens of microarcseconds.
  We find that persistent scintillators have greater position stability than
episodic scintillators, which are in turn more stable than non-scintillators.
Scintillators are also more likely to be compact on milliarcsecond scales, as
measured by the structure index. Persistent scintillators are therefore
excellent candidates for inclusion in any future realisation of the celestial
reference frame. A list of these sources is presented in the Appendix.
  We find that slow (characteristic timescale >3 days) scintillators have more
stable positions than rapid scintillators, suggesting they are more compact.
High-cadence, long term monitoring is therefore required to identify other
members of this population of compact, high brightness temperature quasars.
