Orientation disorder in ammonium elpasolites: Crystal structures of (NH4)3AlF6, (NH4)3TiOF5 and (NH4)3FeF6

Abstract Crystal structures of the known (NH 4 ) 3 AlF 6 (I) and (NH 4 ) 3 FeF 6 (III) and new (NH 4 ) 3 TiOF 5 (II) elpasolites were refined by localizing anions (F − , O 2− ) in four acceptable positions of the cubic system Fm 3 m ( Z =4) with a =8.9401(3), 9.1104(3), 9.110(1) A, respectively. According to the refinement data and a rather large entropy change due to fluorine (oxygen) octahedra disordering in the above compounds and in (NH 4 ) 3 WO 3 F 3 (IV) elpasolite, it was found that fluorine (oxygen) atoms are randomly distributed in two ways, in general 192 l position or in mixed 24 e + 96 j one. Statistics in fluorine (oxygen) distribution is, probably, the result of domain structure of the crystals.