Observation of Two Length Scales for the Critical Fluctuations of RbCaF3

X-ray scattering measurements of the critical fluctuations in RbCa${\mathrm{F}}_3$ above $T_c$ have revealed two different length scales. The shorter one arises from the usual critical fluctuations and does not diverge at the transition, which is of first order. The longer one is associated with the presence of defects and appears to diverge at $T_c$. We suggest that this new feature can be interpreted in terms of large-scale fluctuations into the low-temperature phase, mediated by extended defects such as dislocations.