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 ultrasound 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 → P 1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc ‐ P 1 transition

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn 3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn III (3,5-diCl-sal 2 (323))]BPh 4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P 1 → P 1 (1/2) . The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn 3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc , the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P 1 and P 1 → P 1 (1/2) transitions, with opposite signs. All these phases are polar, while P 1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P 1→ P 1 (1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

We investigate giant magnetoelectric coupling at a Mn 3+ spin crossover in [Mn III L]BPh4 (L = (3,5-diBr-sal)2323) with field-induced permanent switch of the structural, electric and magnetic properties. An applied magnetic field induces a 1 st order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη 2 coupling between the order parameter γ related to the spin-state bistability, and the symmetry-breaking order parameter η, responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover, and evaluate the possibility that it results from conducting phase boundaries. We perform ab-initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Domain wall motion is detected for the first time during the transition to af erroelastic and spin state ordered phase of as pin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound 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 .T he first at 250 K, involves the space group change Cc!Pc and is thermodynamically continuous,w hile the second, Pc!P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of asteep increase in acoustic loss immediately belowt he the Pc-P1 transition

Enantiopure Δ and Λ forms of the spin crossover Mn 3+ chelate complex [Mn{5-OCF 3 -sal 2 (323)}] + were prepared by co-crystallization with the (R,R) and (S,S) forms of the chiral spiroborate anion bis[1,1′-binaphthyl-2,2′-diolato]boron. The absolute structures of the enantiomers were established by X-ray crystallography at 100 K and at 293 K. The enantiomeric [a]