Jahn-Teller Active Fluoroperovskites Acrf(3) (A = Na+, K+): Magnetic And Thermo-Optical Properties

Chromium (II) fluoroperovskites ACrF(3) (A = Na+, 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 = Na at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in NaCrF3 increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for KCrF3 below its Neel temperature. In NaCrF3 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 CrF64- octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic A(x)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 ACrF(3), opening up the possibility of varying the composition to create novel states of matter with tunable properties.