Processing and dielectric properties of the mixed-layer bismuth titanate niobate Bi7Ti4NbO21 by the metal-organic precursor synthesis method

The preparation of homogeneous Bi7Ti4NbO21 single phase nanosized ceramic powders at temperatures as low as 400–500°C using a metal-citrate complexes method, based on the Pechini-type reaction route, is described. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, Raman spectroscopy and SEM, led to the formation of a well defined orthorhombic Bi7Ti4NbO21 compound with lattice parameters a=0.544, b=0.540 and c=2.905 ± 0.0005 nm. Reaction formation takes place through an intermediate binary phase with a stoichiometry close to Bi20TiO32 which forms between 300 and 375°C. The metal-organic precursor synthesis method, allows the control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of the nanosized bismuth titanate niobate, Bi7Ti4NbO21, ceramic powders, at temperatures much lower than usually needed in the conventional route. Sintering of the as-prepared ceramic powders led to near full density samples in the temperature range 1100–1130°C. The mixed-layer structure Bi7Ti4NbO21 ceramics showed two phase transitions at about 650 and 850 ± 5°C, and a piezoelectric modulus, d33, as high as 20×10−12 C/N.