Chromium (II) fluoroperovskites $A{\mathrm{CrF}}_3(A={\mathrm{Na}}^{+},{\mathrm{K}}^{+})$ are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the $A$-site ion plays in this family of fluoroperovskites using both experimental methods (x-ray diffraction, optical absorption spectroscopy, and magnetic fields) and density functional theory simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions $E_2$ and $E_3$ only merge completely for $A=\text{Na}$ at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in ${\mathrm{NaCrF}}_3$ increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for ${\mathrm{KCrF}}_3$ below its Néel temperature. In ${\mathrm{NaCrF}}_3$ the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations possibly linked to crystal structure changes. This suggests that as the long range ordering is reduced, local Jahn-Teller effects in the individual ${\mathrm{CrF}}_6^{4-}$ octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic $A_x{\mathrm{MnF}}_{3+x}$ system. The size of the $A$-site cation is shown to be central in dictating the physical properties and phase transitions in $A{\mathrm{CrF}}_3$, opening up the possibility of varying the composition to create novel states of matter with tunable properties.

中文翻译：

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Jahn-Teller 活性氟钙钛矿 ACrF3(A=Na+,K+)：磁性和热光特性

铬 (II) 氟钙钛矿 $\mathrm{一种}{\mathrm{氟利昂}}_3(\mathrm{一种}={钠}^{+},{\mathrm{钾}}^{+})$是低温下强相关的 Jahn-Teller 活性材料。在本文中，我们研究了$\mathrm{一种}$位离子使用实验方法（X 射线衍射、光吸收光谱和磁场）和密度泛函理论模拟在这个氟钙钛矿家族中发挥作用。温度相关的光吸收实验表明自旋允许的跃迁${乙}_2$ 和 ${乙}_3$ 只为完全合并 $\mathrm{一种}=\text{钠}$ 在 2 K 处。 2 K 处的场相关光吸收测量表明自旋允许跃迁的振荡强度 ${\mathrm{氟化钠}}_3$随着应用领域的增加而增加。直接磁光相关抑制自旋翻转跃迁${\mathrm{氟化钾}}_3$低于其尼尔温度。在${\mathrm{氟化钠}}_3$自旋翻转跃迁在 9 K 以下突然消失，揭示了可能与晶体结构变化有关的磁光相关性。这表明随着远程排序的减少，个体中的局部 Jahn-Teller 效应${\mathrm{氟利昂}}_6^{4-}$八面体控制观察到的行为。我们的结果显示出明显偏离为等电子发现的模式${\mathrm{一种}}_X{锰}_{3+X}$系统。的大小$\mathrm{一种}$-位阳离子被证明是决定物理性质和相变的核心 $\mathrm{一种}{\mathrm{氟利昂}}_3$，开辟了改变成分以创造具有可调特性的新物质状态的可能性。