Abstract
AbstractThe compositional limits of a previously reported (J. Am. Ceram. Soc., 61, 5‐8. (1978)) but relatively unstudied sodium‐bismuth titanate pyrochlore solid solution are revised and their electrical properties presented. The pyrochlore solid solution we report forms via a different mechanism to that originally reported and occurs in a different location within the Na2O‐Bi2O3‐TiO2 ternary system. In both cases, relatively large amounts of vacancies are required on the A‐sites and on the oxygen sites, similar to that reported for undoped ‘Bi2Ti2O7’ pyrochlore. In contrast to ‘Bi2Ti2O7’, this ternary pyrochlore solid solution can be prepared and ceramics sintered using conventional solid‐state methods; however, the processing requires several challenges to be overcome to obtain dense ceramics. This cubic pyrochlore series has low electrical conductivity (and does not exhibit any evidence of oxide‐ion conduction) and exhibits relaxor ferroelectric behavior with a broad permittivity maximum of ~100 near room temperature. Variable temperature neutron diffraction data do not provide any conclusive evidence for a phase transition in the pyrochlore solid solution between ~4 and 873 K.
Highlights
The Na2O-Bi2O3-TiO2 ternary system contains several phases with electrical functionalities
The review discusses how ferroelectric Bi4Ti3O12 is a common impurity in the synthesis of ‘Bi2Ti2O7’ that can be difficult to detect using laboratory XRD, since characteristic diffraction peaks overlap with intense peaks associated with the pyrochlore phase
They note, that since Bi4Ti3O12 and Bi2Ti4O11 have an intense yellow color compared to the pyrochlore, the color can be a better way to detect these common impurities in small quantities rather than via XRD data
Summary
The Na2O-Bi2O3-TiO2 ternary system contains several phases with electrical functionalities. Undoped ‘Bi2Ti2O7’ is a pyrochlore that has been extensively studied due to its interesting optical and electrical properties; it cannot be prepared through conventional solid-state synthesis and sintering methods due to a narrow window of stability and incongruent melting of the material.[9,10] The Bi2Ti2O7 pyrochlore has been studied through neutron diffraction by Radosavljevic et al[11] in 1998 at room temperature, by Hector and Wiggin[12] in 2004 at room temperature and 2 K, and by Kim et al[13] in 2009 at room temperature They all conclude that Bi2Ti2O7 is cubic (space group Fd3̅ m) with a lattice constant between 10.36 and 10.38 Å. They report a broad maximum in permittivity of ~115 at ~50°C between 500 and 2000 kHz in their microwave-assisted sintered pellets
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