Thermal and Magnetic Field Switching in a Two‐Step Hysteretic Mn^III Spin Crossover Compound Coupled to Symmetry Breakings

Abstract

A MnIII spin crossover complex with atypical two‐step hysteretic thermal switching at 74 K and 84 K shows rich structural–magnetic interplay and magnetic‐field‐induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X‐ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where 1/4 are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry‐breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low‐temperature phase with two inequivalent low‐spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Elastic Properties of the Iron(II)–Triazole Spin Crossover Complexes [Fe(Htrz)2trz]BF4 and [Fe(NH2trz)3]SO4

“…Instead, they arise due to the spin transition itself. Similar to other types of phase transitions, and in-line with previously reported resonant ultrasonic spectroscopy measurements on a few molecular SCO compounds − and static compression of iron-bearing minerals, we therefore suggest that the elastic softening at the spin transition observed in our experiments should be induced by the coupling of the spin state of the molecules to lattice strains. Indeed, when an SCO material is subjected to an external stress, as in a DMA experiment, it will deform to some extent, which depends on its elastic constants.…”

“…Indeed, such low-frequency (∼1 Hz) mechanical responses at first-order phase transitions are usually associated with interfacial phenomena, which could be internal frictions at HS/LS phase boundaries in our case. Nevertheless, it is worth mentioning that pronounced loss peaks at the SCO were reported previously at much higher frequencies in resonant ultrasound spectroscopy experiments − (∼1 MHz) and MEMS-based measurements (∼50 kHz) even for materials displaying continuous transitions, for which no phase separation is expected. Further investigations over a broad range of frequencies and temperatures will be thus indispensable to properly “map” the anelastic response of SCO materials …”

Near‐Room‐Temperature Magnetoelectric Coupling via Spin Crossover in an Iron(II) Complex

“…In [Fe(1bpp) 2 ](BF 4 ) 2 , the coupling is enhanced by a strong and thermally switchable distortion of the lattice caused by the Jahn–Teller effect. Compared to the already reported magnetoelectric compounds where the coupling was accompanied by symmetry breaking, [11–15] the distortion in [Fe(1bpp) 2 ](BF 4 ) 2 occurs between phases of the same polar space group P 2 1 (an isostructural transition) indicating that large atomic displacements changing the value of electric polarization in neighboring phases are sufficient for the observation of the coupling.…”

Spin-Crossover and Slow Magnetic Relaxation Behavior in Hexachlororhenate(IV) Salts of Mn(III) Complexes [Mn(5-Hal-sal2323)]2[ReCl6] (Hal = Cl, Br)

“…To date, several important results have been obtained in this class of Mn(III) spin-crossover complexes. Variation of substituents and counterions led to the synthesis of the complexes, which showed sharp spin transitions with significant hysteresis [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ], including a conducting spin-crossover complex with an electroactive counterion TCNQ with a hysteresis width of 50 K, [Mn(5-Cl-sal 2 323)]TCNQ 1.5 ·2CH 3 CN [ 7 ]. The Mn(III) complexes with a 3,5-dihalo-substituted sal 2 323 ligand and tetraphenylborare as a counterion were recently synthesized and showed unusual structural and magnetic properties [ 8 , 9 , 10 , 11 , 12 ].…”