Topotaktische Entwässerung von Cs2MnF5 · H2O zu Cs2MnF5 und die Kristallstrukturen beider Verbindungen / Topotactical Dehydration of Cs2MnF5 · H2O to Cs2MnF5 and the Crystal Structures of Both Compounds

Abstract

Redetermination of the crystal structure of Cs2MnF5 · H2O (space group Cmm 2, Z = 2, a = 974.0(2), b = 867.4(1), c = 426.0(1) pm, R = 0.026 for 478 reflections) revealed ordered instead of disordered positions of water molecules indicating ferroelectric properties. The dehydration of a twinned crystal gave an untwinned single crystal of Cs2MnF5 by a topotactical reaction. The crystal structure of Cs2MnF5 (space group P4/mmm, Z = 1, a = 642.0(1), c = 422.9(1) pm, R = 0.034 for 175 reflections) shows trans-connected octahedral chains [MnF5]∞ 2- with 180° Mn-F-Mn bridge angles like in the monohydrate. The bridging bonds in Cs2MnF5 · H2O and in Cs2MnF5 resp. are elongated by the Jahn-Teller effect (mean values Mn-Fbridge 213/211, Mn-Fterm 184/186 pm). Cs2MnF5 represents the “aristotype” of the so- called Tl2AlF5 type structure family