Pressure effects on NaMnF4: Structural correlations and Jahn-Teller effect from crystal-field spectroscopy

This work investigates the optical absorption spectrum of the NaMnF4-layered perovskite and its variation with pressure. The spectrum basically consists of three broadbands located at 1.916, 2.263, and 2.817 eV, which correspond to the crystal-field (CF) transitions 5B1g⃗5Γi (Γi=A1g, B2g, and Eg) with the Jahn-Teller- (JT-) distorted MnF63- complex (Mn3+ d4 configuration). In addition, there are two spin-flip 5B1g⃗3B1g peaks at 2.397 and 2.890 eV, which are activated by the exchange mechanism. Their variation with pressure reveals that the JT energy does not change significantly with pressure: ∂EJT/∂P=0.8 meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral eg and t2g, termed Δe and Δt, respectively, clearly indicate that ∂Δe/∂P≪∂Δt/∂P, although Δe≈4Δt. The CF energies and their pressure shift are explained in terms of local structural changes within the MnF63- complex induced by pressure. The structural correlation analysis reveals that the reduction of the MnF63- JT distortion is smaller than the expected one on the basis of the crystal volume reduction, thus indicating tilt phenomena. This interpretation is supported by the decrease of in-layer Mn-F-Mn superexchange, such as is derived from the optical spectra. We demonstrate that the equatorial and axial distances decrease from 1.839 to 1.808 Å and from 2.167 to 2.107 Å, respectively, in the 0–10 GPa range