Shift of the first-order transition in RbCaF3under hydrostatic pressure

We have investigated the ${O_h}^1$ to $D_{4h}^{18}$ structural phase transition in RbCa${\mathrm{F}}_3$ under hydrostatic pressure by monitoring the electron-paramagnetic resonance lines of ${\mathrm{Gd}}^{3+}$ ($S=\frac{7}{2}$) on ${\mathrm{Ca}}^{2+}$ sites. We find the first-order transition temperature shifts linearly by (3.6±0.1) deg/kbar but neither the amount of the first-order discontinuity nor its character is altered up to the highest pressure $p_h=1.2\mathrm{}$ kbar we attained. The present experiment, which does not break the symmetry, confirms a uniaxial-stress experiment of Buzaré et al., who reached a tricritical point at ${\sigma }_t=0.19\mathrm{}$ kbar, and attributed it to the symmetry-breaking stress effect on the highly anisotropic cubic fluctuations which induce the first-order transition.