Tricritical behavior in uniaxially stressed RbCaF3

We have investigated the Oh1 to D4h18 structural phase transition in RbCaF3 under near-[100] stress, by monitoring the electron-paramagnetic resonance lines of Gd3+ (S=72) on a Ca2+ site. We find that the first-order transition at zero stress becomes second order at t=1.90.3 kg/mm2 and Tt=195.30.1 K. At this tricritical point, and up to =4.80.3 kg/mm2, the exponent of the temperature dependence of the rotational order parameter is =0.180.02. © 1979 The American Physical Society.