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 A 2 B 2 X 6 X′ 0.5 -type oxyfluoride pyrochlore, Pb 2 Ti 2 O 5.4 F 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 Pb 2 Ti 2 O 5.4 F 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 Ti 4+, 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.