Sánchez-Movellán Inés, Carrasco-Busturia David, García-Lastra Juan M, García-Fernández Pablo, Aramburu José A, Moreno Miguel
Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria, Avenida de los Castros s/n, 39005, Santander, Spain.
Department of Energy Conversion and Storage, Technical University of Denmark, Anker Engelunds Vej. Building 301, 2800, Kgs. Lyngby, Denmark.
Chemistry. 2022 Aug 1;28(43):e202200948. doi: 10.1002/chem.202200948. Epub 2022 Jun 14.
The pressure-induced switch of the long axis of MnF units in the monoclinic Na MnF compound and Mn -doped Na FeF is explored with the help of first principles calculations. Although the switch phenomenon is usually related to the Jahn-Teller effect, we show that, due to symmetry reasons, it cannot take place in 3d (n=4, 9) systems displaying a static Jahn-Teller effect. By contrast, we prove that in Na MnF the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF unit at ambient pressure corresponds to the Mn -F direction, close to the crystal c axis, at 2.79 GPa the c axis is reduced by 0.29 Å while b is unmodified. This fact is shown to force a change of the HOMO wavefunction favoring that the long axis becomes the Mn -F direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d-d transitions observed for Na MnF and CrF at ambient pressure is also discussed together with changes induced by pressure in Na MnF . The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d or d ions.
借助第一性原理计算，对单斜晶系的NaMnF化合物和Mn掺杂的NaFeF中MnF单元长轴的压力诱导开关进行了探索。尽管开关现象通常与 Jahn-Teller 效应有关，但我们表明，由于对称性原因，它不会在显示静态 Jahn-Teller 效应的 3d（n = 4, 9）系统中发生。相比之下，我们证明在NaMnF中，开关源于低对称晶格对静水压力的各向异性响应。实际上，在常压下MnF单元的长轴对应于Mn-F方向，靠近晶体c轴，而在2.79 GPa时，c轴缩短了0.29 Å，而b轴未改变。这一事实表明，在随后的弛豫过程之后，这会迫使HOMO波函数发生变化，从而使长轴变为Mn-F方向，离晶体b轴不远。还讨论了在常压下NaMnF和CrF中观察到的不同d-d跃迁的起源以及NaMnF中压力引起的变化。目前的工作为理解压力对含d或d离子的低对称绝缘化合物的影响打开了一扇窗口。
