Dimensional modification of oxyfluoride lattice: Preparation and structure of A′ANb2O6F (A′=Na, K, A=Ca, Sr) Original Research Article

A series of oxyfluorides, A′ANb2O6F (A′=Na, K, A=Ca, Sr) have been prepared employing solid-state reaction. X-ray diffraction patterns show that NaCaNb2O6F, NaSrNb2O6F and KCaNb2O6F exhibit the cubic pyrochlore structure, while KSrNb2O6F crystallizes in the Dion–Jacobson type layered perovskite. For the A′ANb2O6F compounds, the size of A′ and A ion plays a crucial role in the modification of crystal structure. When A′ and A ions are large, the layered perovskite structure is more favorable than the pyrochlore structure. The phase boundary of pyrochlore and layered perovskte is around 2.1 of RA′A/RNb ratio for A′ANb2O6F, where RA′A is the average radius of A′ and A cations and RNb is the radius of Nb5+ ion. The plot of cell volume vs. RA′A/RNb ratio indicates that the layered perovskite has an expanded cell as compared with the pyrochlores, which may be closely related to the excellent ion-exchange ability of the layered perovskite.