The Thermal Dehydration of Magnesium Aluminum Pentafluoride Dihydrate: Crystal Structures of MgAlF5(H2O)2 and MgAlF5

Abstract

Small plate-like single crystals of MgAlF5(H2O)2 have been obtained during hydrothermal treatment (270°C) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al2(SO4)3 · 18H2O and Mg(NO3)2 · 6H2O with diluted hydrofluoric acid. The crystal structure of MgAlF5(H2O)2 has been refined from single crystal data (Imma (# 74), Z = 4, a = 7.0637(7), b = 10.1308(10), c = 6.7745(7) A, 398 structure factors, 33 parameters, R(F2 > σ(F 2)) = 0.0245, wR(F2 all) = 0.0525). Main features of the inverse weberite type structure are infinite chains of trans-bridged [AlF6] octahedra which are connected via common fluorine atoms by isolated [MgF4(H2O)2] octahedra. MgAlF5(H2O)2 dehydrates at temperatures above 300°C to give MgAlF5. XRPD analysis of this phase has revealed isotypism with FeAlF5. The crystal structure of MgAlF5 (Immm (# 71), Z = 2, a = 7.268(1), b = 6.123(2), c = 3.543(1) A) is built of infinite chains of edge-sharing [MgF6] octahedra and chains of corner-sharing [AlF6] octahedra along [001]. Upon further heating to temperatures above 500°C, MgAlF5 decomposes to MgF2 and α − AlF3.