Structural studies on some fluoromanganates(III)

Abstract

Abstract The influence of the Jahn-Teller-effect of the Mn3+ ion in octahedral fluorine coordination (5Eg ground state) on the crystal structure and in part on the magnetism has been studied in some new fluoromanganates(III) with 3-, 2-, 1- and O-dimensional connected [MnF6] octahedra. 3-dimensional connection. RbZnMnF6 shows an undistorted cubic HT-β- modification (a = 1014 pm) of the RbNiCrF6 type [1] and a low symmetric α- form. Layer structures. CsMnF4 with a TlAlF4 related layer structure is a 2-dimensional ferromagnet due to σ-superexchange interaction in antiferrodistortiv ordered planes of elongated [MnF6] octahedra [2]. KMnF4 however exhibits antiferromagnetism probably because of the dominating π-superexchange interaction in more puckered layers. LiMnF4·H2O (space group P212121, a = b = 536.7 c = 1232.7 pm) has a similar strongly distorted layer structure and is paramagnetic. Chain structures. The crystal structure determination of the hither- to unknown AMnF5·H2O (A  Ba,Sr) compounds yielded infinite chains of transconnected [MnF6] octahedra - the J.T.-elongated axes in the chain direction - separated from each other by the alkaline earth ions and the water molecules. The F-bridges are asymmetric and bent. Space group: P21, z = 2, Ba: a = 537.0 b = 817.2 c = 628.0 pm, β = 111.17°, Sr: a = 512.3 b = 792.9 c = 611.6 pm, β = 110.34°. Structures with isolated octahedra. Preliminary results of a single crystal investigation on ZnMnF5·7H2O indicate a disordered structure of the CoAlF5·7H2O type whose structure has been solved: Space group C2/m, Z = 4, CoAlF5·7H2O: a = 1091.7 b = 1386.3 c = 652.5 pm β = 100.3°, ZnMnF5·7H2O: a = 1114.6 b = 1428.9 c = 646.1 pm β = 100.4°. The CoAl compound shows disordered [AlF5H2O] octahedra connected with [Co(H2O)6] octahedra by a 3- dimensional network of H-bonds. The first cubic fluoroelpasolites of Mn3+ (space group Fm3m) has been found in K2.2Li0.8MnF6 (a = 811.0 pm) and in the metastable high pressure form α-Cs2NaMnF6 (a = 876.2 pm). Single crystals of the metastable HT-γ- form show the rhombohedral 12L-Cs2NaCrF6 type structure [3] (space group R 3 m, Z = 6, a = 626.5 c = 3054 pm), the related LT-β-form [4] has lower symmetry. In the γ-form there is - first times in the structural chemistry of fluoromanganates(III) - no crystallographic evidence for a static or dynamic Jahn-Teller-effect.