The effect of Li-substitution on the M-phases of AgNbO3

Ceramics in the solid solution, LixAg1−xNbO3 (LAN) with 0 ≤ x ≤ 0.20, have been synthesized by the mixed oxide method. X-ray diffraction (XRD) revealed that only compositions with x ≤ 0.05 were single phase which were indexed according to Pbcm symmetry with a unit cell, √2ap × √2ap × 4ap (where ap is the fundamental perovskite lattice parameter). For compositions with x ≥ 0.1, second phase peaks attributed to LiNbO3 were observed and the matrix phase transformed to a metrically rhombohedral structure according to XRD. A phase transition at 0.05 < x ≤ 0.1 was confirmed by Raman spectroscopy and electron diffraction but the latter technique revealed superstructure reflections for x ≥ 0.1 at ±1/6{00l} and 1/2{ooo} ± 1/6{00l} (where o = odd). These support the premise that Li doping invokes a new complex modulated tilt system with a √2ap × √2ap × 6ap unit cell rather than √2ap × √2ap × 4ap (Pbcm). Reflections of the type ±1/6{00l} are, however, streaked along the [001] direction indicating that the structure may be incommensurate or only partially ordered. The appearance of the √2ap × √2ap × 6ap structure is associated with strong asymmetry in the peak in permittivity attributed to the M3–M2 and a decrease in intensity of the peak in permittivity associated with the M2–M1 transition. Slim polarization versus field curves characteristic of weak ferroelectricity were observed for compositions with x ≤ 0.05 but for higher values of x the loops were unsaturated albeit with larger remanent polarization.