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

Abstract

The room temperature crystal structures\nof α-K₃MoO₃F₃ and α-Rb₃MoO₃F₃ have been solved via combined\nRietveld refinements\nof synchrotron and neutron powder diffraction data. These two compounds\nare part of a broader family of <i>A</i>₂<i>BM</i>O₃F₃ compounds that have been studied\nfor their dielectric properties, but until now the complex crystal\nstructures of the ferroelectric phases of these compounds were not\nknown. At room temperature and below, these two isostructural compounds\nare tetragonal with <i>I</i>4₁ space group symmetry\nand unit cell parameters of <i>a</i> = 19.38613(3) Å, <i>c</i> = 34.86739(8) Å for α-K₃MoO₃F₃ and <i>a</i> = 20.0748(4) Å, <i>c</i> = 36.1694(1) Å for α-Rb₃MoO₃F₃. Their structures are related to the cubic double\nperovskite structure but are considerably more complicated due to\nnoncooperative octahedral tilting and long-range orientational ordering\nof the polar MoO₃F₃^3– units.\nThe pattern of octahedral tilting is equivalent to that seen in the\nα-K₃AlF₆ structure, which has <i>I</i>4₁/<i>a</i> symmetry, but orientational\nordering of MoO₃F₃^3– units\nlowers the symmetry to <i>I</i>4₁. The polar\nspace group symmetry is consistent with earlier reports of ferroelectricity\nin these compounds. Hence orientational ordering of the MoO₃F₃^3– units is directly responsible for\nthe ferroelectric behavior.