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 T-c(down arrow) = 128 K and T-c(up arrow) = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1932, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R (3) over bar --> P (1) over bar crystallographic phase transition with a large low-spin fraction stabilizing the P (1) over bar 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 [Jeffic, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57; 1743]. Thus the P (1) over bar phase is additionally stabilized by external pressure. The interaction constant Gamma, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Gamma(P (1) over bar) of 144(8) cm(-1) and the volume difference Delta V-HL(o) of 29(4) Angstrom(3) are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P (1) over bar phase, These values are compared to the corresponding values for the R (3) over bar phase, viz. Gamma(R (3) over bar) of 170(9) cm(-1) and Delta V-HL(o)(P (3) over bar) Of 26(3) Angstrom(3). Surprisingly Gamma(R (3) over bar) is larger than T(Pi) despite the fact that Delta V-HL(o)(R (3) over bar) is smaller than Delta V-HL(o)(P (1) over bar) The high-spin --> low-spin relaxation at temperatures above similar to 80 K is thermally activated, while below similar to 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) --> T-1(1) absorption band.