Structural phase transition in the spin-crossover complex [Fe(ptz)6](BF4)2 studied by x-ray diffraction

The spin-crossover system $[\text{Fe}{(\text{ptz})}_{6}]{({\text{BF}}_{4})}_{2}$ 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.