Unusual Phase Behavior in the Piezoelectric Perovskite System, Li_<i>x</i>Na_1–<i>x</i>NbO₃

Abstract

The system Li_<i>x</i>Na_1–<i>x</i>NbO₃ has been studied by using a combination of X-ray and neutron powder diffraction and ²³Na solid-state NMR spectroscopy. For <i>x</i> = 0.05 we confirm a single polar orthorhombic phase. For 0.08 ≤ <i>x</i> ≤ 0.20 phase mixtures of this orthorhombic phase, together with a rhombohedral phase, isostructural with the low-temperature ferroelectric polymorph of NaNbO₃, are observed. The relative fractions of these two phases are shown to be critically dependent on synthetic conditions: the rhombohedral phase is favored by higher annealing temperatures and rapid cooling. We also observe that the orthorhombic phase transforms slowly to the rhombohedral phase on standing in air at ambient temperature. For 0.25 ≤ <i>x</i> ≤ 0.90 two rhombohedral phases coexist, one Na-rich and the other Li-rich. In this region the phase behavior is independent of reaction conditions