An electron diffraction and XRPD study of superlattice ordering in the elpasolite-related oxyfluoride K₃MoO₃F₃

A detailed electron diffraction and XRPD study has been made of the room-temperature α polymorph of K₃MoO₃F₃. It is shown that the true symmetry of this polymorph is neither tetragonal, trigonal, nor triclinic as previously reported but rather monoclinic I1a1, a = 2a_p - c_p, b = 4b_p, c = a_p + 2c_p when expressed in terms of the underlying elpasolite (ordered perovskite) parent structure type. A highly structured, three-dimensional, continuous diffuse intensity distribution (presumably arising from local O/F ordering and associated structural relaxation) is shown to coexist with the sharp satellite reflections characteristic of the monoclinic supercell.