Tricritical Behavior in Uniaxially Stressed RbCaF3

Abstract

We have investigated the ${O_h}^1$ to ${D_{4h}}^{18}$ structural phase transition in RbCa${\mathrm{F}}_3$ under near-[100] stress $\sigma$, by monitoring the electron-paramagnetic resonance lines of ${\mathrm{Gd}}^{3+}$ ($S=\frac{7}{2}$) on a ${\mathrm{Ca}}^{2+}$ site. We find that the first-order transition at zero stress becomes second order at ${\sigma }_t=1.9\mathrm{}\pm 0.3$ kg/${\mathrm{mm}}^2$ and $T_t=195.3\mathrm{}\pm 0.1$ K. At this tricritical point, and up to $\sigma =4.8\mathrm{}\pm 0.3$ kg/${\mathrm{mm}}^2$, the exponent of the temperature dependence of the rotational order parameter is $\beta =0.18\mathrm{}\pm 0.02$.