The iron(II) spin-crossover compound [Fe(ptz)(6)](PF6)(2) (ptz group P (1) over bar, with a 10.6439(4) Angstrom, b = 10.8685(4) Angstrom, c = 60.815(1)degrees, and Z = 1. In [Fe(ptZ)(6)](PF6)(2), the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, Delta V(HL)degrees, is 24(2) Angstrom(3)/ molecule. The interaction constant Gamma, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm(-1). In contrast to the case of the related compound [Fe(ptz)(6)]-(BF4)(2) (Decurtins et al. Inorg. Chem. 1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn1-xFex(ptz)(6)](BF4)(2), x = 0.1 (Jeftic et al. J. Phys. Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)(6)](PF6)(2), the interaction constant Gamma is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin --> low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin --> low-spin relaxation by 1 order of magnitude.