Temperature-Independent, Large Dielectric Constant Induced by Vacancy and Partial Anion Order in the Oxyfluoride Pyrochlore Pb₂Ti₂O_6-δF_2δ

In mixed-anion systems, partial anion order can be the key to realizing enhanced dielectric properties. A novel A₂B₂X₆X′_0.5-type oxyfluoride pyrochlore, Pb₂Ti₂O_5.4F_1.2, was prepared through the conventional solid-state reaction. Characterization of the oxyfluoride pyrochlore by synchrotron X-ray diffraction and electron diffraction revealed the existence of partial O^2-/F^- order at the X site, in which F^- is involved in fac-type Ti(O/F)₆ coordination, associated with the O^2-/vacancy order at the X′ site. Although Pb₂Ti₂O_5.4F_1.2 adopts a centrosymmetric structure, the refined occupancy factors suggested the existence of local polarization. Partial O^2-/F^- anion order stabilized the large displacement of Ti⁴⁺, leading to a high dielectric constant (as high as 800) with a low-temperature coefficient. These results suggest that mixing heteroanions in pyrochlores provides a basis for exploring novel dielectric materials.