Pressure Study of the Thermal Spin Transition and the High-Spin → Low-Spin Relaxation in theR3 andP1 Crystallographic Phases of [Zn1-xFex(ptz)₆](BF₄)₂Single Crystals (x= 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)

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 ↓ = 128 K and T c ↑ = 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 R 3̄ → P 1̄ crystallographic phase transition with a large low-spin fraction stabilizing the P 1̄ 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 [Jeftić, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the P 1̄ 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( P 1̄) of 144(8) cm -1 and the volume difference 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 P 1̄ phase. These values are compared to the corresponding values for the R 3̄ phase, viz. Γ( R 3̄) of 170(9) cm -1 and of 26(3) Å 3 . Surprisingly Γ( R 3̄) is larger than Γ( P 1̄) despite the fact that is smaller than The high-spin → 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 → 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 → 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 → 1 T 1 absorption band.