%0 Journal Article
%A Jeftic, J.
%A Hauser, A.
%T Pressure Study of the Thermal Spin Transition and the High-Spin -> Low-Spin Relaxation in the R3 and P1 Crystallographic Phases of [Zn1-xFex(ptz)6](BF4)2 Single Crystals (x = 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)
%J Journal of Physical Chemistry B
%@ 1520-6106
%V 101
%N 49
%P 10262-10270
%D 1997
%U http://pubs.acs.org/doi/abs/10.1021/jp972083k
%R 10.1021/jp972083k
%U http://archive-ouverte.unige.ch/unige:2804
%X 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 Tcâ = 128 K and Tcâ = 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 R3 â Pâ crystallographic phase transition with a large low-spin fraction stabilizing the Pâ 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 [JeftiÄ, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the Pâ phase is additionally stabilized by external pressure. The interaction constant Î, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Î(Pâ ) of 144(8) cm-1 and the volume difference ÎV0HL of 29(4) Ã
3 are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the Pâ phase. These values are compared to the corresponding values for the R3 phase, viz. Î(R3) of 170(9) cm-1 and ÎV0HL(R3) of 26(3) Ã
3. Surprisingly Î(R3) is larger than Î(Pâ ) despite the fact thatÂ ÎV0HL(R3)Â is smaller thanÂ ÎV0HL(P1).Â The high-spin â 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 â 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 â 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 â 1T1 absorption band.