New Layered Oxide-Fluoride Perovskites: KNaNbOF5 and KNaMO2F4 (M = Mo6+, W6+)

Abstract
KNaNbOF5 and KNaMO2F4 (M = Mo6+, W6+), three new layered oxide-fluoride perovskites with the general formula ABB’X6, form from the combination of a second-order Jahn-Teller d0 transition metal and an alkali metal (Na+) on the B-site. Alternating layers of cation vacancies and K+ cations on the A-site complete the structure. The K+ cations are found in the A-site layer where the fluoride ions are located. The A-site is vacant in the adjacent A-site layer where the axial oxides are located. This unusual layered arrangement of unoccupied A-sites and under bonded oxygen has not been observed previously although many perovskite-related structures are known.