Moessbauer effect and x-ray diffraction at the high-spin (5T2) .dblharw. low-spin(1A1) transition in bis(thiocyanato)bis(4,7-dimethyl-1,10-phenanthroline)iron(II)-.alpha.-picoline: thermal hysteresis, associated crystallographic phase change, time dependence of 5T2 .fwdarw. 1A1 transition, particle size effects and related phenomena