Spin transition and relaxation dynamics coupled to a crystallographic phase transition in a polymeric iron(II) spin-crossover system

Abstract

a b s t r a c tThe spin-crossover compound [Fe(bbtr) 3 ](ClO 4 ) 2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin?low-spin transition on cooling as well as the relaxation kinetics just below T c ; depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin?low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.

The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations

“…LS relaxation even slows down as the thermal transition temperature of 100 K is approached. This can only be explained by quite large short-range interactions leading to fluctuations and the formation of clusters or domains of equal spins [33,36,37]. Fig.…”

“…All this leads to the conclusion that the thermal spin transition triggers a crystallographic phase transition and that below T # c the low-temperature phase has a high degree of disorder. On the first cooling cycle this crystallographic phase transition occurs as a nucleation and growth process and results in the formation of a domain structure [33] depending upon the individual crystal. Upon heating to 120 K, the system returns to the HS state, but retains some memory of this domain structure.…”

“…This results in a high degree of cooperativity, which manifests itself in a thermal spin transition with a 15 K wide hysteresis centred around 105 K. The system presents two crystallographic phases: an ordered phase at above the spin transition temperature having space group P 3, and a disordered phase below the spin transition temperature for which to date it has not been possible to determine the crystal structure [32]. The crystallographic phase transition from ordered to disordered phase has been associated with a nucleation and growth process and domain formation in a previous communication [33]. In order to clarify the complex behaviour of this system, we provide in this paper a full analysis of its thermal spin transition, low temperature photoexcitation and finally of the relaxation curves inside the thermal hysteresis.…”

Model for Elastic Relaxation Phenomena in Finite 2D Hexagonal Molecular Lattices

“…In this case the deformations of the crystal are bigger and may lead to its collapse. Reference [20] provides an illustrative example of relaxation curves with a shape typical for a nucleation and growth mechanism and the formation of domains. Previously, domains have been experimentally observed during the thermal transition or photoexcitation [23]; a very recent study by optical microscopy at cryogenic temperatures shows that domains most often grow starting from a corner and present an evolution similar to our simulations [24].…”

“…In contrast to rectangular lattices [13,14], the hexagonal lattice is inherently stable even with only nearest neighbor interactions, thus keeping the number of parameters at a minimum. There are several examples of spin-crossover compounds with hexagonal sheet layers [20].…”