On the order parameter of RbH3(SeO3)2

Rubidium trihydrogen selenite RbH₃(SeO₃)₂ undergoes an improper ferro-electric phase transition due to the condensation of a doubly degenerate soft mode at the zone boundary. Superlattice reflections have been measured using neutron diffraction, and after taking domains into account, it is shown that the measured reflections scale onto the same curve as a function of temperature. The consequences of this scaling are explained in terms of the temperature dependence of the order parameter of the transition.