Neutron powder diffraction studies of two spin transition Fe^II 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.