Stress-induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex.

: Domain wall motion is detected for the first time during the transition to a ferroelastic and spin-state ordered phase of a spin crossover complex. Single crystal X-ray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5-diBr-sal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc → Pc and is thermodynamically continuous, while the second, Pc → P1 at 85 K, is discontinuous and related to spin crossover and spin-state ordering. Stress-induced domain wall mobility was detected as softening of the phonon modes at the Pc → P1 transition.