Spin crossover molecular ceramics by Cool-SPS: consequences on switching features beyond microstructural effects

Abstract

The sintering of spin crossover materials using spark plasma sintering at low temperature (Cool-SPS) provides a new way of shaping such compounds into functional molecular ceramics. These ceramics reach a high relative density of 95%, which may address several issues for using spin crossover materials in barocaloric devices. Starting from the reference complex [Fe(Htrz)₂(trz)](BF₄), we first investigated the magnetic, structural, and microstructural properties as well as the fatigability behavior of the starting powder using multiple magnetic measurements, X-ray diffraction and calorimetry to compare them with the elaborated ceramics. The best conditions of pressure and temperature during the SPS process to obtain reproducible molecular ceramics with high relative density were found to be between 250 and 300 °C, and 300 and 400 MPa. The same complete set of characterization performed on a molecular ceramic of 95% of relative density reveals that the crystal structures, as well as the abrupt hysteretic SCO of [Fe(Htrz)₂(trz)]BF₄, are perfectly conserved after sintering. However, ceramics present 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.