Preparation, structural characterization and ferroelectric properties of compensated Te-doped n=2 Aurivillius oxides ceramics

New solid solutions Bi2-xTexSrNb2-xBxOy (B = Zr, Hf; 0 less than or equal t o x less than or equal to 0.5) and Bi(2-y)Te(y)Sr(1-y)A(y)Nb(2)O(9) (A = K; 0 less than or equal to y less than or equal to 0.25) have been synthesize d and characterized by powder X-ray diffraction methods, thermal analysis a nd dielectric measurements. Evolution of the unit-cell parameters at room t emperature shows that doping the A or B perovskite sites dominates the size of (001) plane in the structure, whereas the c value is largely influenced by the cations which occupy the bismuth position in its layer. Thermogravi metric studies show that these materials are stable up to 800 degrees C for x, y = 0.25 and 750 degrees C for x = 0.5. Differential thermal analysis c urves of x = 0.25 compounds exhibit one exothermic peak at 400 and 500 degr ees C fur zirconium and hafnium phases, respectively. This effect is relate d to the expansion of their network, without changing the symmetry or space group. All compositions are ferroelectric at room temperature, and exhibit a non-zero d(33) value after poling. Measurements of the dielectric permit tivity as a function of the temperature reveal signs of at least two possib le phase transitions. The disappearance of the induced spontaneous polariza tion when heating samples above the lower temperature anomaly in the dielec tric permittivity clarifies the T-c value of the compounds. Assuming they a re displacive-type ferroelectrics, it can be shown that the variation of T- c with the composition is an effect of the change of the unit cell size. Th is result offers the possibility of tailoring a ferroelectric material with the desired thermal working range. (C) 2000 Elsevier Science Ltd. All righ ts reserved.