Tetragonal tungsten bronze compounds represent the second largest family of ferroelectric materials. Here, the Nb(V) distorts out-of-center in response to an electric field, which is why tetragonal tungsten bronzes are known insulating dielectrics. If any appreciable conductivity can be introduced into these normally insulating compounds, one may be able to achieve exotic properties such as superconductivity(1) and transparent conductivity (2). Work by the Hagenmuller group 50 years ago showed that the introduction of fluorine decreases the covalency of the Nb-O/F bond, which removes the out-of-center distortion and restores local symmetry(3). Furthermore, the substitution of oxygen with fluorine should introduce carriers (from the extra electron) and the greater ionicity should suppress cation vacancy formation. Here, we present the preparation and properties of the reduced solid state oxyfluorides, and the samples are phase pure as confirmed with powder X-ray diffraction. Fluorine doping avoids the formation of secondary phases as compared to other potential n-type dopants (Ce, Ba, and W).

Tetragonal tungsten bronzes revisited

2019 Spring National Meeting

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