Structures of α‑K3MoO3F3 and α‑Rb3MoO3F3: Ferroelectricity from Anion Ordering and Noncooperative Octahedral Tilting

The room temperature crystal structures of α-K3MoO3F3 and α-Rb3MoO3F3 have been solved via combined Rietveld refinements of synchrotron and neutron powder diffraction data. These two compounds are part of a broader family of A 2 BMO3F3 compounds that have been studied for their dielectric properties, but until now the complex crystal structures of the ferroelectric phases of these compounds were not known. At room temperature and below, these two isostructural compounds are tetragonal with I41 space group symmetry and unit cell parameters of a = 19.38613(3) Å, c = 34.86739(8) Å for α-K3MoO3F3 and a = 20.0748(4) Å, c = 36.1694(1) Å for α-Rb3MoO3F3. Their structures are related to the cubic double perovskite structure but are considerably more complicated due to noncooperative octahedral tilting and long-range orientational ordering of the polar MoO3F3 3– units. The pattern of octahedral tilting is equivalent to that seen in the α-K3AlF6 structure, which has I41/a symmetry, but orientational ordering of MoO3F3 3– units lowers the symmetry to I41. The polar space group symmetry is consistent with earlier reports of ferroelectricity in these compounds. Hence orientational ordering of the MoO3F3 3– units is directly responsible for the ferroelectric behavior.