%0 Journal Article
%A Schenker, S.
%A Hauser, A.
%A Wang, W.
%A Chan, I.Y.
%T High-spin -> low-spin relaxation in [Zn1-xFex(6-mepy)3-y(py)ytren](PF6)2
%J Journal of Chemical Physics
%@ 0021-9606
%V 109
%N 22
%P 9870-9878
%D 1998
%U http://jcp.aip.org/jcpsa6/v109/i22/p9870_s1?isAuthorized=no
%R 10.1063/1.477681
%U http://archive-ouverte.unige.ch/unige:2763
%X The thermal spin transition in the diluted mixed crystalÂ [Zn1âxFex(6-mepy)3tren](PF6)2Â (xÂ = 0.00025,Â (6-mepy)3tren =Â tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine)Â is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values forÂ ÎHHL0Â andÂ ÎSHL0Â of 1551(50) cmâ1Â and 7.5(5) cmâ1/K and a molecular volume of reaction,Â ÎVHL0,Â of 22(2) Ã
3result. Reconsideration of the cooperative effects in the neatÂ [Fe(6-mepy)3tren](PF6)2from Adler et al. [Hyperfine Interact.Â 47, 343 (1989)] result in a lattice shift, Î, of 208(15) cmâ1Â and an interaction constant, Î, of 109(15) cmâ1. Temperature-dependent laser flash photolysis experiments in the spin-crossover systemÂ [Zn1âxFex(6-mepy)3tren](PF6)2Â and the LS systemÂ [Zn1âxFex(py)3tren](PF6)2Â in the pressure range between 1 bar and 1 kbar are presented. Above â100 K theÂ HSâLSÂ (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in[Zn1âxFex(py)3tren](PF6)2Â is more than three orders of magnitude larger than the one in[Zn1âxFex(6-mepy)3tren](PF6)2.Â External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.