Emergent phenomena in spin crossover systems

Abstract

In general, a spin crossover (SCO) system is any complex, material or framework containing two thermodynamically accessible spin-states: one high-spin (HS) and one low-spin (LS). The transition between spin-states is addressable by temperature, pressure, light irradiation, electric and magnetic fields, and chemical environment. The transition itself can be first-order, exhibiting hysteresis, continuous or a crossover. Typically, accompanied by the ferroelastic ordering of spin-states. It can also be part of an incomplete or multi-step transition accompanied by the antiferroelastic ordering of spin-states. In general, any alterations to the structural characteristics of SCO systems can have an effect on their bulk properties and behaviours. Consequently, constructing structure-property relations has traditionally been an extremely challenging task, and one of both great theoretical and experimental interest. Understanding the mechanisms behind these bulk properties and behaviours could lead to the rational design of SCO systems with enhanced applications and the synthesis of novel properties and behaviours. In this thesis we show that a simple, elastic model of SCO systems hosts almost all experimentally reported SCO properties and behaviours. We demonstrate clear structure-property relations that explain these results, derive the mechanisms of multi-step transitions and explain why and how intermolecular interactions play a role. We also propose that a new exotic state of matter could exist in elastically frustrated SCO materials and frameworks. In this phase “spin-state ice”, so-called in analogy to water- and spin- ice, the metal ions lack any kind of long-range order. Instead, local clusters of metal ions follow a local ‘ice rule’. For example on the kagome lattice, each triangle is constrained to have two metal ions in one spin-state and one in the other. The excitations are deconfined quasi-particles, with a fractionalised spin midway between that of the HS and LS states. We show that distinctive signatures of spin-state ice can be measured by neutron scattering, electron paramagnetic resonance, and thermodynamic experiments. Unlike other examples of ices that have been theorized to exist, the unique nature of SCO systems allows for multiple spin-state ice phases to exist on the same lattice with unique properties that can be tuned with external parameters, like temperature and pressure.