Magnetic Properties of the Planar AntiferromagnetRb2FeF4

${\mathrm{Rb}}_2$Fe${\mathrm{F}}_4$, a ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$-structure planar antiferromagnet with large anisotropy and spins perpendicular to the $c$ axis, has a broad susceptibility maximum at 90°K. Mössbauer effect shows no long-range order in the interval 60-90°K. Between 50 and 60°K there is a gradual transition to long-range order accompanied by a crystallographic distortion. Three-dimensional order exists only at low temperature in the distorted crystal.