Spin crossover molecular ceramics by Cool-SPS: consequences on switching features beyond the sole microstructural effect

Abstract

The sintering of spin crossover material using Spark Plasma Sintering at low temperature (Cool-SPS) lead to a new way of shaping such compounds into functional molecular ceramics. These ceramics reach a high relative density of 95%, what may address several issues for using spin crossover materials into barocaloric devices. Starting from the reference complex [Fe(Htrz)2(trz)]BF4, we first investigated the magnetic, structural, microstructural properties as well as the fatigability behavior of the starting powder using multiple magnetic measurements, X-ray diffraction and calorimetry to compare them to the elaborated ceramics. The best conditions of pressure and temperature during the SPS process to obtain reproductible molecular ceramics with high relative density where found to be between 250 and 300 °C, and 300 and 400 MPa. The same complete set of characterizations made on a molecular ceramic of 95% of relative density reveal that crystal structure as well as the abrupt hysteretic SCO of [Fe(Htrz)2(trz)]BF4 are perfectly conserved after sintering. However, ceramic presents a faster stabilization of their microstructural and magnetic properties upon cycling and a higher cooperativity at the macroscopic level was observed compared to the starting powder.