Coexistence of Two Different Distorted Octahedral [MnF6 ]3- Sites in K3 [MnF6 ]: Manifestation in Spectroscopy and Magnetism.

Abstract

As a consequence of the static Jahn‐Teller effect of the 5E ground state of MnIII in cubic structures with octahedral parent geometries, their octahedral coordination spheres become distorted. In the case of six fluorido ligands, [MnF6]3− anions with two longer and four shorter Mn−F bonds making elongated octahedra are usually observed. Herein, we report the synthesis of the compound K3[MnF6] through a high‐temperature approach and its crystallization by a high‐pressure/high‐temperature route. The main structural motifs are two quasi‐isolated, octahedron‐like [MnF6]3− anions of quite different nature compared to that met in ideal octahedral MnIII Jahn‐Teller systems. Owing to the internal electric field of Ci symmetry dominated by the next‐neighbour K+ ions acting on the MnIII sites, both sites, the pseudo‐rhombic (site 1) and the pseudo‐tetragonally elongated (site 2) [MnF6]3− anions are present in K3[MnF6]. The compound was characterized by single‐crystal and powder X‐ray diffraction, and magnetometry as well as by FTIR, Raman, and ligand field spectroscopy. A theoretical interpretation of the electronic structure and molecular geometry of the two Mn sites in the lattice is given by using a vibronic coupling model with parameters adjusted from multireference ab‐initio cluster calculations.