Preparation, characterization and dielectric study of new ferroelectric relaxors crystallizing with the tetragonal tungsten bronze structural type

The replacement of sodium and niobium by lanthanum and iron in Pb2NaNb5O15, have allowed to prepare the new solid state solution Pb2Na1−x La x Nb5−x Fe x 015 (0 ≤ x ≤ 1), crystallising with the tungsten bronze structure. The latter synthesized, in air, by solid state reaction, has been studied by means of X-ray and dielectric measurements. The composition dependencies of ferroelectric—paraelectric transition temperature and the lattice parameters are reported. The dielectric constants ɛ′r and ɛ″r have been also investigated as a function of temperature in the range 20°–700°C, and as a function of frequency in the range 1 k–13 MHz. In the composition range 0 ≤ x ≤ 0.35, the materials exhibit a typical ferroelectric–paraelectric transition while, in the composition range 0.35 < x ≤ 1, a relaxor effect has been evidenced.