Ferroelastic domain wall motion and collective domain switching in RbSCN

Low frequency (0.05 - 40 Hz) dynamic elastic measurements and resonant ultrasound spectroscopy measurements (100-600 kHz) of RbSCN have been performed in the temperature region of the order-disorder improper ferroelastic phase transition at T$_c \approx$ 435~K. Quite similar to KSCN, the low frequency data show - in addition to the intrinsic phase transition anomalies - superelastic softening in a- and b-directions, resulting from movements of ferroelastic domain walls under dynamic stress. However, in contrast to KSCN, a sudden discontinuous increase of Young's modulus appears in RbSCN at { T$^{\ast} < T_c$}, which is accompanied by a frequency dependent damping peak. This behaviour is reminiscent of a first order phase transition.\\ Heating RbSCN slightly above T$^{\ast}$, followed by subseqent cooling, removes all {signs of domain wall dynamics}. The results demonstrate, that the anomalies in RbSCN around $T^{\ast}$ result from collective domain switching events that are induced when the {temperature dependent critical pinning stress, $\sigma_c(T)$ falls below the applied external stress $\sigma$, implying that $T^{\ast}(\sigma=\sigma_c)$. This interpretation is supported by calculations of the temperature dependences of twin boundary widths $w$ and energies $F_w$, as well as the Peierls potential $V_0$ using a compressible pseudospin model, which leads to a critical pinning stress, $\sigma_c(T)$ that is in excellent agreement with experimental values of $T^{\ast}(\sigma_c)$. }