Jahn-Teller active fluoroperovskites ACrF3 (A=Na+, K+): Magnetic and thermo-optical properties

Abstract

Chromium (II) fluoroperovskites A CrF 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 NaCrF 3 increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for KCrF 3 below its Néel temperature. In 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 CrF 4 − 6 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 A CrF 3 , opening up the possibility of varying the composition to create novel states of matter with tunable properties.