(NH4)2FeF5: Crystal Structures of Its α and β Forms.

α and β-(NH 4 ) 2 FeF 5 are obtained by dehydration of (NH 4 ) 2 FeF 5 •H 2 O. The formation of the α form is concomitant with the water loss at 98 o C. α-(NH 4 ) 2 FeF 5 transforms irreversibly into β at 109 o C. The structure of the α form ― a new A 2 MF 5 structural type ― is solved «ab initio»; the material exhibits an orthorhombic symmetry: Pbcn, Z=8, a=7.6223(5), b=13.4408(8) and c=10.4167(6) A. The network is built from infinite zigzag cis chains of FeF 6 sharing corners octahedra, running along [001]. The β form is isostructural with (NH 4 ) 2 MnF 5 ; the symmetry is orthorhombic: Pnma, Z=4, a=6.3379(6), b=7.6165(4) and c=11.0305(9) A. Its network presents infinite and kinked chains of FeF 6 trans linked octahedra, running along [010]