This work investigates the optical absorption spectrum of the ${\mathrm{NaMnF}}_4$-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 ${}^5{\overrightarrow{B_{1g}}}^5{\Gamma }_i$ $({\Gamma }_i{=A}_{1g},$ $B_{2g},$ and $E_g)$ with the Jahn-Teller- (JT-) distorted ${\mathrm{MnF}}_6^{3\mathrm{-}}$ complex $({\mathrm{Mn}}^{3+}\hspace{0.5em}{d}^4$ configuration). In addition, there are two spin-flip ${}^5{\overrightarrow{B_{1g}}}^3{B}_{1g}$ 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: $\partial E_{\mathrm{JT}}/\partial P=0.8\mathrm{}$ meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral $e_g$ and $t_{2g},$ termed ${\Delta }_e$ and ${\Delta }_t,$ respectively, clearly indicate that $\partial {\Delta }_e/\partial P\ll \partial {\Delta }_t/\partial P,$ although ${\Delta }_e\approx 4{\Delta }_t.$ The CF energies and their pressure shift are explained in terms of local structural changes within the ${\mathrm{MnF}}_6^{3\mathrm{-}}$ complex induced by pressure. The structural correlation analysis reveals that the reduction of the ${\mathrm{MnF}}_6^{3\mathrm{-}}$ 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.

• Received 18 October 2002

DOI:https://doi.org/10.1103/PhysRevB.67.205101