Thermal spin-crossover with a large hysteresis spanning room temperature in a mononuclear complex

Abstract

A large hysteresis centered around room temperature represents one of the mandatory goals of research on functional switchable materials. In the thoroughly studied field of spin-crossover, such behaviour appears very rarely and essentially concerns coordination networks. A new compound showing a large spin-crossover hysteresis spanning room temperature demonstrates in a definitive manner that this goal is achievable in molecular discrete compounds without damaging the single-crystal character.

Mosaicity of Spin-Crossover Crystals

“…It is nevertheless clear that SCO degrades the sample through a form of mechanical fatigability, as already observed in other SCO materials [16,17]. It can be noted that there are no significant effects of this mechanical degradation on the magnetic characterization of the SCO, since the magnetic curve is not affected by repeated cycles [13]. It is interesting from the methodological point of view that SCO can also be tracked by the temperature dependence of the mosaicity in this compound.…”

“…The loss of crystal quality is not reversible; there is no return of mosaicity to its initial value and there is even a tendency for the crystal to crack if warmed. The compound [Fe(PM-PeA)2(NCSe)2] has recently been reported; it is one of the very few compounds exhibiting SCO at room temperature together with a large hysteresis (T1/2down = 266 K and T1/2up = 307 K) [13]. As with sulphur, [Fe(PM-PeA)2(NCS)2] [11], this compound undergoes a highly cooperative phase transition associated with SCO and characterized by relatively distinct HS and LS crystal packings.…”

Pressure‐Induced Spin‐Crossover Features at Variable Temperature Revealed by In Situ Synchrotron Powder X‐ray Diffraction

“…Within the crystal‐packing of the title compound, the main directions of compression correspond indeed to directions where there is a relative free space between molecules, that is, where intermolecular interactions appear weak. For example, the well‐known H–H interaction that is a typical feature of this family of compounds, and that is directed along the b axis, is known to disappear in the LS state . Furthermore, in order to construct the ( P , T ) diagram and to investigate the temperature dependence of the pressure‐induced SCO in this compound, similar experiments and analyses as above have been performed at three higher temperatures: 313, 333 and 353 K. A fresh powder, fully HS, has been used for each temperature studied.…”

“…For example,t he well-known H-H interaction that is at ypical feature of this familyo fc ompounds, and that is directed along the b axis, is known to disappear in the LS state. [18] Furthermore, in order to constructt he (P, T)d iagram and to investigate the temperature dependence of the pressure-induced SCO in this compound, similar experiments and analyses as above have been performeda tt hree highert emperatures:3 13, 333 and 353 K. Af resh powder,f ully HS, has been used for each temperatures tudied. The SCO curves deduced from the HS ratio were determined at each temperature as shown in Figure 3.…”

“…Moreover, PXRD also potentially brings data at the microstructural scale, an aspect that is, to date, almost unexplored in SCO materials under pressure. In this article, the high‐pressure study is focused on one of the very rare compounds showing a thermal SCO truly centered on room‐temperature and reported recently: [Fe(PM‐PeA) 2 (NCSe) 2 ] [with PM‐PeA= N‐ (2′‐pyridylmethylene)‐4‐(phenylethynyl) aniline] presents a temperature induced SCO centered at 286 K with a hysteresis width of 41 K. SCO occurs in this compound, together with a structural transition corresponding to an increase of symmetry at low temperature, from a HS monoclinic P 2 1 / c phase to a LS orthorhombic Pccn phase. This rather unusual transition promises a priori a noteworthy behavior under pressure at variable temperature.…”