Synthesis, structural characterization, and electrical properties of new oxygen-deficient tetragonal tungsten bronzes Ba2NdTi(2+x)Nb(3-x)O(15-x/2).

ABSTRACT

Oxygen-deficient tetragonal tungsten bronzes ceramics with general formula Ba(2)NdTi(2+x)Nb(3-x)O(15-x/2) (0 ≤ x ≤ 1) have been prepared by low temperature solvothermal synthesis with final firing of ceramics at 1100-1300 °C in air. Rietveld refinement of X-ray powder diffraction (XRD) and neutron powder diffraction (ND) data at room temperature of Ba(2)NdTi(3)Nb(2)O(14.5) shows that Ba and Nd are ordered on the 15-coordinate and 12-coordinate sites, respectively, Ti and Nb are disordered nonrandomly over the two octahedral sites, and oxygen vacancies locate preferentially in the coordination sphere of Nd and Ti/Nb(2) atoms. Variable frequency impedance measurements show that samples are poor electronic conductors with activation energies ∼0.8-1.7 eV, conductivities ∼1 × 10(-5) S cm(-1) at ∼725 °C and with some evidence of oxide ion conduction at high x values. Composition dependence of the dielectric properties shows a transition from classic ferroelectric behavior with Ba(2)NdTi(2)Nb(3)O(15) to a relaxor-like behavior with Ba(2)NdTi(3)Nb(2)O(14.5). At intermediate compositions, both a first-order phase transition and relaxor-like behavior are observed.