Neutron powder diffraction studies of two spin transition FeII complexes under pressure

The pressure dependences of the lattice parameters of two spin transition compounds have been derived from neutron powder diffraction measurements at ambient temperature. The study of Fe(PM-AzA)(2)(NCS)(2) [ PM is N-2'-pyridylmethylene and AzA is 4-(phenylazo) aniline] has been used to validate this new type of investigation of spin crossover compounds, and the study of Fe(PM-BiA)(2)(NCS)(2) (BiA is 4-aminobiphenyl) has allowed the atypical spin crossover behaviour of this compound under pressure to be explained. In addition, this complex exhibits a pressure-induced structural transition with an associated symmetry change, inducing the transformation of Fe(PM-BiA)(2)(NCS)(2) into a different polymorph that avoids a first-order spin transition in favour of continuous transition.