The influence of hydrostatic pressure on hysteresis phase transition in spin crossover compounds

The effect of hydrostatic pressure on the transition temperature and the hysteresis widths of first order spin crossover phase transitions has been studied. A decrease as well as an increase of the hysteresis width with increasing pressure was reported in the literature. The increase of width with increasing pressure contradicts the expectation derived from mean field theory. We remeasured the pressure dependence of the temperature hysteresis of the compound [Fe(phy)(2)](BF4)(2) (phy = 1, 10-phenanthroline-2-carbaldehydephenylhydrazone). The spin transition temperatures at ambient temperature are T-1/2(up arrow) = 289 +/- 1 K, T-1/2(down arrow) = 283 +/- 1 K. An increase of the hysteresis width of d root Delta T(1/2)dp = 0.29 root K kbar(-1) up to p = 4.1 kbar has been measured confirming the reported anomaly. The change of the average temperatures T-1/2 = 1/2(T-1/2(up arrow) + T-1/2(down arrow)) at which the high spin fraction gamma is 1/2 is determined to be 3.0 +/- 0.2 K kbar(-1). Allowing for a pressure dependence of the bulk modulus of the compound, the anomalous increase of hysteresis width can be interpreted. (C) 1999 Elsevier Science Ltd. All rights reserved.