In the iron(II) spin-crossover compound [Fe(ptz)6](BF4)2, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with Tc↓ = 128 K and Tc↑ = 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 → P crystallographic phase transition with a large low-spin fraction stabilizing the P phase at low temperatures. In the mixed crystal [Zn1-xFex(ptz)6](BF4)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 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) of 144(8) cm-1 and the volume difference of 29(4) Å3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P phase. These values are compared to the corresponding values for the R phase, viz. Γ(R) of 170(9) cm-1 and of 26(3) Å3. Surprisingly Γ(R) is larger than Γ(P) 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 1A1 → 1T1 absorption band.