TKWB-type lead-free oxyfluoride relaxors derived from Ba2NaNb5O15

Three solid solutions were prepared from Ba2NaNb5O15 by coupled cationic an d/or anionic substitutions: Ba2-xNa1+x(Nb5-xWx)(15), Ba2-xNa1+xNb5O15-xFx a nd Ba2NaNb5-xTixO15-xFx. They crystallize with a TKWB-type structure. In th e case of oxyfluoride compositions, large frequency range dielectric studie s performed on ceramics demonstrated a relaxor behavior. The greater the su bstitution rate, the higher the relaxor corresponding characteristics. Such a behavior is in agreement with the cationic disorder of Ba2+ and Na+ in t he same crystallographic site of BaNa2Nb5O14F. In addition. the following t ransition sequence was obtained: relaxor --> ferroelectric --> paraelectric . Such ceramics constitute a new family of lead-free relaxor compositions w ith TKWB structure and can be used in environmental-friendly applications.