Structural phase transition in the spin-crossover complex[Fe(ptz)6](BF…

International audienceThe spin-crossover system [Fe(ptz)₆](BF₄)₂ has been studied for more than 25 years and is used as a model system to the understanding of the eponymous phenomenon in solid-state materials. However, the structural properties of the low-spin phase formed at low temperature after a slow cooling have never been elucidated due to a splitting of the Bragg peaks. We report here a reinvestigation of this low-spin phase by single-crystal x-ray diffraction. This study demonstrates the perfect matching between the structural and magnetic transitions temperatures and hysteresis width through careful unit-cell temperature dependence. The Bragg splitting is also unambiguously associated to the spin transition. Above all, this work reveals a reversible doubling of the unit-cell parameters a and b corresponding to the spin transition. A preliminary solution for the crystal structure of the low-spin phase notably shows the potential major role of the deformation of the n-propyl groups in the physical behavior of this material