Spin transition with a very large thermal hysteresis in a molecular crystal:: an EPR study of Fe(PM-PEA)2(NCS)2

Magnetic susceptibility and X- and Q-band EPR studies are reported for the Fe2+ spin transition compound (Fe(PM-PEA)(2)(NCS)(2)) in powder form. An exceptionally large thermal hysteresis loop, about 60 K, has been found by both techniques. The EPR studies have been carried out on compound doped with Mn2+ (0.1 parts per thousand of Mn/Fe). An original EPR spectra simulation program has been elaborated in order to yield data on the structural modifications accompanying the spin transition. The fine structure parameters obtained by simulations in the high-spin state of Fe2+, D = 460, E = 25, and in the low-spin state of this ion, D = 235, E = 20 (in 10(-4) cm(-1)) confirm that the spin transition is accompanied by a crystalline phase transformation. The Mn2+ EPR spectra in the spin transition region can be convincingly fitted by linear combinations of the spectra observed in the high-spin and low-spin states of Fe2+, indicating that the spin transition occurs in domains of like-spin ions. (C) 1999 Elsevier Science B.V, All rights reserved.