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

Crystal structures of the known (NH4)3AlF6(I) and (NH4)3FeF6(III) and new (NH4)3TiOF5(II) elpasolites were refined by localizing anions (F−, O2−) in four acceptable positions of the cubic system Fm3m (Z=4) with a=8.9401(3), 9.1104(3), 9.110(1) Å, respectively. According to the refinement data and a rather large entropy change due to fluorine (oxygen) octahedra disordering in the above compounds and in (NH4)3WO3F3(IV) elpasolite, it was found that fluorine (oxygen) atoms are randomly distributed in two ways, in general 192l position or in mixed 24e + 96j one. Statistics in fluorine (oxygen) distribution is, probably, the result of domain structure of the crystals.