Crystal data for a new family of phases, Li₃Mg₂XO₆:X=Nb, Ta, Sb

Three members of a new family of phases, Li₃Mg₂XO₆: X = Nb, Ta, Sb, have been prepared by solid-state reaction of MgO and Li₃XO₄. Unit-cell parameters of Li₃Mg₂TaO₆ determined by electron diffraction and refined by X-ray powder diffraction are a = 8.883(2), b = 5.802(2), c = 17.437(6) Å, with space group Fddd; z = 8. Powder diffraction data of Li₃Mg₂NbO₆ and Li₃Mg₂SbO₆ were indexed by analogy with those of Li₃Mg₂TaO₆ and unit-cell dimensions obtained by least-squares refinement: Li₃Mg₂NbO₆ a = 8.552(2), b = 5.897(1), c = 17.721(5) Å; Li₃Mg₂SbO₆ a = 8.614 (1), b = 5.908(1), c = 17.759(5) Å.