On two new K2FeF5 forms

Abstract

Two additional forms of K2FeF5 are disclosed and characterized by powder diffraction. One (form-IV) adopts a new A2MF5 structural type, space group C2/m, a = 13.0919(2) Å, b = 5.7311(1) Å, c = 7.3114(1) Å, β = 118.241(1), and Z = 4, it is obtained from the thermal evolution of form-III, isostructural with Rb2FeF5, space group Pnma, a = 7.3964(2)  Å, b = 5.6779(1) Å, c = 11.5445(3) Å, and Z = 4. Both forms are compared to two previously reported polymorphs. The III → IV transition is topotactic, conserving the same zigzag cis-connected octahedral (FeF5)n2n− chains after an ~90° rotation and 1/2b translation of half of them.