The thermal dehydration of magnesium aluminum pentafluoride dihydrate:: Crystal structures of MgAlF5(H2O)2 and MgAlF5

Small plate-like single crystals of MgAlF5(H2O)(2) have been obtained during hydrothermal treatment (270 degreesC) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al-2(SO4)(3) . 18H(2)O and Mg(NO3)(2) . 6H(2)O 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) Angstrom, 398 structure factors, 33 parameters, R(F-2 > 2 sigma,(F-2)) = 0.0245, wR(F-2 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 degreesC 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) Angstrom) 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 degreesC, MgAlF5 decomposes to MgF2 and alpha -AlF3.