Pressure Study of the Thermal Spin Transition and the High-Spin → Low-Spin Relaxation in theR3̄ andP1̄ Crystallographic Phases of [Zn_1-xFe_x(ptz)₆](BF₄)₂Single Crystals (x= 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)

Abstract

In the iron(II) spin-crossover compound [Fe(ptz) 6 ](BF 4 ) 2 , the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with T c V ) 128 K and T c v ) 135 K at ambient pressure [Franke, P. L.;Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1982, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R3 h f P1 h crystallographic phase transition with a large low-spin fraction stabilizing the P1 h phase at low temperatures. In the mixed crystal [Zn 1-x Fe x (ptz) 6 ](BF 4 ) 2 , x ) 0.1, with the iron complexes imbedded into the isomorphous zinc lattice, the crystallographic phase transition can be induced by an external pressure Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743. Thus the P1 h phase is additionally stabilized by external pressure. The interaction constant Γ, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Γ(P1 h) of 144(8) cm -1 and the volume difference ∆V°H L of 29(4) Å 3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P1 h phase. These values are compared to the corresponding values for the R3 h phase, viz. Γ(R3 h) of 170(9) cm -1 andThe high-spin f low-spin relaxation at temperatures above ∼80 K is thermally activated, while below ∼40 K temperature independent tunnelling takes place. An external pressure of 1 kbar accelerates the high-spin f low-spin relaxation exponentially by 1 order of magnitude in the tunnelling region in both crystallographic phases and regardless of x. In the concentrated material the high-spin f low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. Chem. Phys. Lett. 1992, 192, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the 1 A 1 f 1 T 1 absorption band.

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

“…2(b)]. The relaxation curves show the typical sigmoidal shape for a cooperative process [18], where the energy barrier progressively decreases while the relaxation proceeds. As in the thermally activated regime of real spin-crossover complexes, the relaxation is faster at higher temperature and for increasing pressures.…”

“…The probability for switching between the HS and LS states depends on the temperature and the local pressure on a given molecule in a given state, according to the classical activation formula presented in [18]:…”

Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid

“…We observe the lack of cooperativity in relaxation, coming from almost non-cooperative relaxation in the triclinic phase [18]. The formation of steady states is a combination of two different relaxation kinetics in two different crystallographic phases, too.…”

“…This further means that the treatment of two different kinetics with h(R3i )= 5 and h(P1i )= 1 [10,18]. Without knowing the proportion of the two phases the quantitative interpretation of the curves here is not possible.…”

“…After several hours waiting at one temperature, with a constant photoexcitation intensity indicated in the figure, the hysteresis is not broader than 1.5 K for the R3i phase and it is absent for the P1i phase. This difference is explained on the basis of different cooperativities in the two phases: h(R3i )=5 while h(P1i ) :1 [18].…”