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 A2BMO3F3 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) A, c = 34.86739(8) A for α-K3MoO3F3 and a = 20.0748(4) A, c = 36.1694(1) A 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 MoO3F33– 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...

[1]  P. Woodward,et al.  Na(1.5)Ag(1.5)MO3F3 (M = Mo, W): an ordered oxyfluoride derivative of the LiNbO3 structure. , 2012, Journal of the American Chemical Society.

[2]  P. Woodward,et al.  The high-temperature polymorphs of K3AlF6. , 2011, Inorganic chemistry.

[3]  M. G. Rozova,et al.  Crystal structure and phase transitions in Sr3WO6. , 2010, Inorganic chemistry.

[4]  M. G. Rozova,et al.  The crystal structure of alpha-K3AlF6: elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework. , 2009, Inorganic chemistry.

[5]  C. Stern,et al.  Cation-anion interactions and polar structures in the solid state. , 2007, Journal of the American Chemical Society.

[6]  A. Tressaud,et al.  The low-temperature form of Rb2KCrF6 and Rb2KGaF6: The first example of an elpasolite-derived structure with pentagonal bipyramid in the B-sublattice , 2006 .

[7]  David E. Tanner,et al.  ISODISPLACE: a web-based tool for exploring structural distortions , 2006 .

[8]  G. Tendeloo,et al.  Phase transitions in K3AlF6 , 2006 .

[9]  P. Maggard,et al.  Alignment of acentric MoO3F33− anions in a polar material: (Ag3MoO3F3)(Ag3MoO4)Cl , 2003 .

[10]  Yun Liu,et al.  A combined diffraction (XRD, electron and neutron) and electrical study of Na3MoO3F3 , 2003 .

[11]  P. Woodward,et al.  Ordered double perovskites -- a group-theoretical analysis. , 2003, Acta crystallographica. Section B, Structural science.

[12]  R. Withers,et al.  Oxygen/fluorine ordering, structured diffuse scattering and the local crystal chemistry of K3MoO3F3 , 2003 .

[13]  Brian H. Toby,et al.  EXPGUI, a graphical user interface for GSAS , 2001 .

[14]  Z. Ye,et al.  Optical and dielectric studies on ferroelectric oxyfluoride K3MoO3F3 single crystals , 1991 .

[15]  A. Tressaud,et al.  Structural Phase Transitions in Rb2KMIIIF6 Elpasolites. I. Crystal Chemistry and Calorimetric Studies , 1986 .

[16]  P. Hagenmuller,et al.  Study of phase transitions in A3MO3F3 compounds (A = K, Rb, Cs; M = Mo, W) , 1978 .

[17]  H. P. Rooksby,et al.  Pseudo-cubic alkaline-earth tungstates and molybdates of the R3MX6 type , 1951 .

[18]  L. Pauling.  THE CRYSTAL STRUCTURES OF AMMONIUM FLUOFERRATE, FLUO-ALUMINATE AND OXYFLUOMOLYBDATE , 1924 .

[19]  R. Withers,et al.  An Electron Diffraction and XRPD Study of Superlattice Ordering in the Elpasolite-Related Oxyfluoride K3MoO3F3 , 2002 .

[20]  P. Hagenmuller,et al.  A new family of ferroelectric materials with composition A2BMO3F3 (A, B = K, Rb, Cs, for rA+ ⩾ rB+ and M = Mo, W) , 1980 .

[21]  W. Rüdorff,et al.  Über Alkali‐oxofluorometallate der Übergangsmetalle. A 3IMeOxF6−x‐Verbindungen mit x = 1, 2, 3 , 1969 .