| Sumario: | NaAgMoOF and NaAgWOF 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 [MOF] anions (M = Mo, W) the oxyfluoride groups in NaAgMoOF and NaAgWOF 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 structure, but the combination of Na/Ag ordering and orientational ordering of the [MOF] 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.
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