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

The effect of hydrostatic pressure on the transition temperature and the hy steresis widths of first order spin crossover phase transitions has been st udied. A decrease as well as an increase of the hysteresis width with incre asing pressure was reported in the literature. The increase of width with i ncreasing pressure contradicts the expectation derived from mean field theo ry. We remeasured the pressure dependence of the temperature hysteresis of the compound [Fe(phy)(2)](BF4)(2) (phy = 1, 10-phenanthroline-2-carbaldehyd ephenylhydrazone). 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 incr ease 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(dow n 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 modu lus of the compound, the anomalous increase of hysteresis width can be inte rpreted. (C) 1999 Elsevier Science Ltd. All rights reserved.