title: Na(1.5)Ag(1.5)MO3F3 (M = Mo, W): an ordered oxyfluoride derivative of the LiNbO3 structure.
abstract: Na(1.5)Ag(1.5)MoO(3)F(3) and Na(1.5)Ag(1.5)WO(3)F(3) have been synthesized by solid state reactions and structurally characterized using synchrotron X-ray and neutron powder diffraction. Unlike the vast majority of salts containing [MO(3)F(3)](3-) anions (M = Mo, W) the oxyfluoride groups in Na(1.5)Ag(1.5)MoO(3)F(3) and Na(1.5)Ag(1.5)WO(3)F(3) are orientationally ordered, so that the Na(+) ions are coordinated by fluorine and the Ag(+) ions by oxygen. The resulting structure type, which has not previously been reported, is related to the LiNbO(3) structure, but the combination of Na/Ag ordering and orientational ordering of the [MO(3)F(3)](3-) anions produces a supercell that doubles the c-axis and changes the space group symmetry from R3 to R3. The use of hard (Na(+)) and soft (Ag(+)) cations to direct the orientational ordering of polar oxyfluoride building units provides a new approach to the design of polar materials.
authors.affiliation: Department of Chemistry, The Ohio State University, Columbus, Ohio 43210-1185, USA.
authors.forename: Allyson M
authors.initials: AM
authors.lastname: Fry
authors.forename: Harry A
authors.initials: HA
authors.lastname: Seibel
authors.forename: Indunil N
authors.initials: IN
authors.lastname: Lokuhewa
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Woodward