Palii Andrew, Ostrovsky Serghei, Reu Oleg, Tsukerblat Boris, Decurtins Silvio, Liu Shi-Xia, Klokishner Sophia
Institute of Applied Physics, Academy of Sciences of Moldova, Academy Str. 5, MD-2028 Kishinev, Moldova.
Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel.
J Chem Phys. 2015 Aug 28;143(8):084502. doi: 10.1063/1.4928642.
In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the Fe(ptz)62 crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.
在本文中，我们提出了一种新的微观理论方法来描述分子晶体中的自旋交叉现象。所考虑的自旋交叉晶体由自旋交叉金属离子及其周围最近的配体形成的分子片段组成，并表现出明确的局域（分子）振动。与该现象的先前模型不同，所提出的方法不仅考虑了自旋交叉离子与声子的相互作用，还考虑了电子壳层与分子模式的强耦合。这导致了局域模式与声子的有效耦合，这被证明是伴随着结构重组的协同自旋转变的原因。该转变由两个序参量表征，分别代表自旋交叉配合物高自旋态和低自旋态下电子对角矩阵乘积与局域模式坐标的平均值。最后，我们证明该方法为在Fe(ptz)62晶体中观察到的自旋转变提供了合理的解释。该理论很好地再现了在宽温度范围内观察到的从低自旋到高自旋的突然转变以及高自旋分数的温度依赖性，以及明显的磁滞回线。同时，在所提出模型采用的极限近似下，高自旋分数对温度的评估表明，从高自旋分数在高温下未达到其最大值的意义上来说，协同自旋 - 晶格转变是不完全的。
