The interplay between the spin transition and the crystallographic phase transition in the Fe(II) spin-crossover system [Zn1-xFex(ptz)6](BF4)(2) (x=0.1, 1; ptz=1-propyltetrazole)

In the [Fe(ptz)(6)](BF4)(2) (ptz = 1-propyltetrazole) spin-crossover system, the thermal spin transition is accompanied by a first order crystallographic phase transition (T-c(down arrow) = 128 K and T-c(up arrow) 135 K) from <R(3)over bar> above T-c(down arrow) to <P(1)over bar> at low temperatures (Wiehl L., Acta Cryst. B49, 289 (1993)). The high-symmetry phase can be super-cooled, in which case the spin transition is still complete and quite steep (T-1/2 = 125 +/- 2 K) but now without a hysteresis. The corresponding interaction constant Gamma is 170 cm(-1). In the diluted system [Zn1-xFex(ptz)(6)](BF4)(2), x = 0.1, the spin transition is gradual with T-1/2 = 95 +/- 2 K. From the shift of T-1/2 towards high temperatures with external pressure a value for Delta V-HL(0) of 26 Angstrom(3) molecule(-1) is obtained. Pressures above 250 bar induce a crystallographic phase transition even in the diluted system, as a result of which the spin transition is discontinuous. The interplay between the thermal spin transition and the crystallographic phase transition in the neat and the diluted system is discussed consistently.