@Article{JPhysChemB_101_10262,
author = {J. Jeftic and A. Hauser},
title = {{Pressure Study of the Thermal Spin Transition and the High-Spin -> Low-Spin Relaxation in the R3 and P1 Crystallographic Phases of [Zn$_{1-x}$Fe$_{x}$(ptz)$_{6}$](BF$_{4}$)$_{2}$ Single Crystals (x = 0.1, 0.32, and 1; ptz = 1-n-propyltetrazole)}},
journal= {J. Phys. Chem. B},
ISSN = {1520-6106},
volume= {101},
number= {49},
pages = {10262-10270},
url = {http://pubs.acs.org/doi/abs/10.1021/jp972083k},
doi= {10.1021/jp972083k},
abstract = {{In the iron(II) spin-crossover compound [Fe(ptz)$_6$](BF$_4$)$_2$, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with {\em T}$_c^{â}$ = 128 K and {\em T}$_c^{â}$ = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. {\em Inorg. Chim. }{\em Acta} 1982, {\em 59}, 5]. The hysteresis is due to an interplay between the spin-transition and the {\em R}$^{3}$ â {\em P}â crystallographic phase transition with a large low-spin fraction stabilizing the {\em P}â phase at low temperatures. In the mixed crystal [Zn$_1_-${\em $_x$}Fe{\em $_x$}(ptz)$_6$](BF$_4$)$_2$, {\em 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. {\em J. Phys. Chem. Solids} 1996, {\em 57}, 1743]. Thus the {\em P}â 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 $\Gamma$({\em P}â ) of 144(8) cm$^{-1}$ and the volume difference $\Delta$V$^{0}$$_{HL}$ of 29(4) Ã
$^{3}$ are determined from a simultaneous fit to a series of transition curves for different pressures and iron content {\em x} in the {\em P}â phase. These values are compared to the corresponding values for the {\em R$^{3}$} phase, viz. $\Gamma$({\em R}$^{3}$) of 170(9) cm$^{-1}$ and $\Delta$V$^{0}$$_{HL}$(R3) of 26(3) Ã
$^{3}$. Surprisingly $\Gamma$({\em R}$^{3}$) is larger than $\Gamma$({\em P}â ) despite the fact that $\Delta$V$^{0}$$_{HL}$(R3) is smaller than $\Delta$V$^{0}$$_{HL}$(P1). 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 {\em x}. In the concentrated material the high-spin â low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. {\em Chem. Phys. Lett.} 1992, {\em 192}, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the $^{1}$A$_{1}$ â $^{1}$T$_{1}$ absorption band.}},
year = {1997}
}