Stress-induced phase transition and its reversal by low-temperature annealing in Li-doped sodium niobate

An extensive phase transition of (Na1−xLix)NbO3 (x=0.04, 0.1) from orthorhombic (Pmc21) to rhombohedral (R3c) was induced by mechanical stress. A reverse phase transition was induced by thermal annealing at temperatures as low as 300 °C. A sintered pellet was subjected to mechanical stress by grinding to powder form in a mortar. The orthorhombic phase in the ceramic pellet transformed to R3c phase, by as much as 83-89% by grinding. The net cell volume of the R3c increased by 0.4-0.5 Å3 per four formula unit (z=4). The reverse transition from the R3c to Pmc21 was 84-97 mass% after annealing at 300 °C. The activation free energy for the reverse transition was calculated to be 2-10 meV using an Arrhenius-type equation. The crystal class of the Pmc21, i.e. mm2, conformed to the measured P-E hysteresis (Ps≅20 μC/cm2) of the pellet.