Spin-transition materials can exhibit thermal hysteresis due to cooperative elastic interactions in between active molecular sites. It results from the coupling of the nonsymmetry-breaking high spin fraction order parameter to the lattice volume strain. However, a symmetry-breaking order parameter responsible for crystallographic phase transition, like a ferroelastic distortion, can also couple to the volume strain. Here, we use the Landau theory to study the elastic coupling between a spin-crossover instability and a discontinuous ferroelastic distortion and the different phase transition lines. Below the triple point, the first-order line involves simultaneous ferroelastic distortion and spin transition. Above, the purely first-order ferroelastic distortion and the spin crossover occur sequentially. Our model, related to the coupling and decoupling of the ferroelastic phase transition and spin crossover, explains exotic behaviors reported experimentally in the literature for diverse spin-crossover systems, plastic crystals, or Prussian blue analogues. The unsymmetric hysteresis loops and the stepwise evolution of the spin conversion or volume strain under pressure result from different sequences of phase transitions. The model shows that the ferroelastic phase transition is the driving force of the cooperative spin transition hysteresis in this case.

中文翻译：

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自旋交叉和铁弹性畸变的耦合和解耦：非对称磁滞回线，相图和相序

由于活性分子位点之间的协同弹性相互作用，自旋转变材料会表现出热滞后作用。它是由于非对称破坏高自旋分数阶参数与晶格体积应变的耦合而产生的。但是，负责结晶相变的对称破坏阶数参数（如铁弹性畸变）也可以耦合到体积应变。在这里，我们使用Landau理论研究自旋交叉不稳定性与不连续铁弹性畸变以及不同相变线之间的弹性耦合。在三点以下，一阶线同时发生铁弹性变形和自旋跃迁。在上面，纯一阶铁弹性变形和自旋交叉依次发生。我们的模型 与铁弹性相变和自旋交叉的耦合和解耦有关，解释了文献中通过实验报道的奇异行为，涉及各种自旋交叉系统，塑料晶体或普鲁士蓝类似物。不对称的磁滞回线和自旋转换或在压力下的体积应变的逐步演化是由不同的相变序列引起的。该模型表明，在这种情况下，铁弹性相变是协同自旋跃迁迟滞的驱动力。不对称的磁滞回线和自旋转换或在压力下的体积应变的逐步演化是由不同的相变序列引起的。该模型表明，在这种情况下，铁弹性相变是协同自旋跃迁迟滞的驱动力。不对称的磁滞回线和自旋转换或在压力下的体积应变的逐步演化是由不同的相变序列引起的。该模型表明，在这种情况下，铁弹性相变是协同自旋跃迁迟滞的驱动力。