INVESTIGATION OF THE FERROELECTRIC PHASE WITH THE TETRAGONAL TUNGSTEN BRONZE STRUCTURAL TYPE IN THE TERNARY SYSTEM PbNb2O6 -NaNb03 -LaFeO3

A crystal chemical study of the ternary system PbNb 2 O 6 - NaNbO 3 - LaFeO 3 , has allowed to delimit a wide homogeneous region within the triangle PbNb 2 O 6 - Pb 2 NaNb 5 O 15 - Pb 1.875 La 1.25 Nb 3.75 Fe 1.25 O 15 crystallising with the tetragonal tungsten bronze structural type. The compositional dependence of T C and the lattice parameters for the analysed solids solutions has evidenced. For example, two main variations could be pointed out versus composition: i) a diminution of the orthorhombic distortion δ = [(b-a)/(b+a)], with a and b representing regular lattice parameters; ii) the ferroelectric relaxor behaviour enhanced as the lattice contains more different ions, hi agreement with the expected correlative cationic disorder.