Ferroelastic phase transition and the role of volume strain in the structural trapping of a metastable quenched low-spin high-symmetry phase in [Ru0.35Fe0.65(ptz)6](BF4)2

Abstract

Spin-crossover (SCO) materials exhibit thermal conversion of their electronic state from low (LS) to high (HS) spin state, which also involves both intra- and inter-molecular structural reorganisations within the crystal. The occurrence of coupled SCO and symmetry-breaking (SB), also responsible for structural reorganisations, is a current topic of interest because it can generate exotic behaviours such as unsymmetric hysteresis loops, stepwise evolution of the spin conversion, or sequences of phase transitions. Here we study the coupling and decoupling of SCO and SB during the thermal conversion of the prototypical [Fe(ptz)₆](BF₄)₂ compound diluted in the isostructural ruthenium host lattice [Ru(ptz)₆](BF₄)₂. We show that the [Ru_0.35Fe_0.65(ptz)₆](BF₄)₂ system undergoes a SB between the high-spin high symmetry (HShs) phase (R space group) and the low-spin low symmetry (LSls) phase (P space group). We present a structural analysis of the ferroelastic R → P phase transition and highlight the structural SB signatures. We also study the metastable low-spin high symmetry (LShs) phase (R space group) reached upon fast cooling. In the framework of the Landau theory of phase transition adapted by Collet, and considering the elastic coupling between SCO and SB, we discuss the structural distortion between the HShs, LShs, and LSls phases and explain the metastable nature of the LShs phase.