Jahn-Teller active fluoroperovskites A CrF 3 ( A = Na + , K + ) : Magnetic and thermo-optical properties

Chromium (II) fluoroperovskites $A{\mathrm{CrF}}_{3}\phantom{\rule{4pt}{0ex}}(A={\mathrm{Na}}^{+},{\mathrm{K}}^{+})$ 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 N\'eel 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 ${\mathrm{CrF}}_{6}^{4\ensuremath{-}}$ 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.