EPR OF THE Gd**3** plus minus O**2** minus PAIR AS A PROBE TO ESTABLISH THE PHASE DIAGRAM OF (111)-STRESSED RbCaF//3.

RbCaF//3 undergoes a slightly first-order structural phase transition (SPT) (T//c congruent 195 K) from the cubic O//h1 to the tetragonal D//4//h18 space group connected with CaF//6 octahedra rotation. Previously, this SPT was investigated by monitoring the EPR lines of the Gd3** plus minus O2 minus center substituted to a Ca2 plus minus F** minus bond. Here, we report EPR investigations of (111)-stressed RbCaF//33 plus minus O2 minus providing accurate measurements of order-parameter components and clear determination of the phase symmetries which corroborate firmly the theoretically predicted phase diagram.