Spin-transition materials, including the families of spin-crossover and charge-transfer systems, and more generally molecular-based materials exhibiting electronic and/or structural bistability, may undergo various types of phase transitions. The change of electronic state is stabilized by molecular reorganizations and both phenomena, which are usually non-symmetry breaking, can be described through the evolution of an order parameter q. Due to symmetry, q linearly couples to volume change. It is known that such elastic interactions are responsible for cooperative phenomena in non-symmetry-breaking spin-transitions. However, spin-transition materials may also exhibit symmetry-breaking phenomena related to various types of orders such as structural order as well as spin-state concentration waves. The universal framework of the Landau theory of phase transition is relevant for describing such ordering processes through the evolution of a symmetry-breaking order parameter η. The simultaneous or sequential occurrence of spin-transition and symmetry-breaking phenomena are reported for numerous spin-transition materials, and the coupling between these two types of instabilities is responsible for the emergence of various types of functions. In this work, we use the Landau approach to describe both symmetry-breaking phenomena and non-symmetry-breaking spin transition. We discuss how their coupling can generate sequences of phase transitions, from simple spin-crossover to spin-transition, continuous or discontinuous symmetry breaking, including ferroelasticity or stepwise spin transitions.

中文翻译：

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非对称和非对称自旋跃迁材料的一般Landau理论

自旋转变材料，包括自旋交叉和电荷转移系统的家族，以及更普遍地表现出电子和/或结构双稳态的基于分子的材料，可以经历各种类型的相变。电子状态的变化通过分子重组得以稳定，并且通常通过非对称断裂的两种现象都可以通过阶跃参数q的演化来描述。由于对称，q线性耦合到体积变化。众所周知，这种弹性相互作用是非对称破坏自旋跃迁中合作现象的原因。但是，自旋转变材料也可能表现出与各种顺序有关的对称破坏现象，这些顺序包括结构顺序以及自旋态浓度波。Landau相变理论的通用框架与通过对称破坏阶数参数η的演化描述此类有序过程有关。。据报道，许多自旋转变材料同时或顺序发生自旋转变和对称破坏现象，而这两种类型的不稳定性之间的耦合是导致各种类型功能发生的原因。在这项工作中，我们使用Landau方法描述对称破坏现象和非对称破坏自旋跃迁。我们讨论了它们的耦合如何生成从简单的自旋交变到自旋跃迁，连续或不连续的对称断裂（包括铁弹性或逐步自旋跃迁）的相变序列。