Uniaxial compressive stress and temperature dependent mechanical behavior of (1-x)BiFeO₃-xBaTiO₃ lead-free piezoelectric ceramics

The mechanical behavior of polycrystalline lead-free $(1-x)$BiFeO$_{3}$-xBaTiO$_{3}$ (BF-BT) piezoelectric ceramics was investigated under uniaxial compressive stress from room temperature up to 400 degrees C with macroscopic stress strain measurements and in situ stress-dependent neutron diffraction. Stress-strain curves revealed a changing mechanical response with BaTiO$_{3}$ content and temperature. With decreasing BaTiO$_{3}$ content there was an increase in the coercive stress, which reduced the remanent strain and hysteresis. Full pattern structural refinement of the neutron data reveals both rhombohedral distortion and magnetic moment decreases with increasing BaTiO$_{3}$ content. In situ stress-dependent neutron diffraction experiments showed that accommodation of external stress occurs through the changes in tilt magnitude and anisotropy of oxygen octahedra at room temperature. The origin of stress-induced strain at room temperature is a lattice deformation without any apparent change in average crystallographic symmetry or domain switching. Temperature-dependent in situ stress-induced measurement of BF-3OBT showed maximum strain close to the rhombohedral - pseudocubic transition temperature, which has been proposed to be due to the lattice deformation as well as to the differing degree of tilting of the (Fe/Ti)O-$_{6}$ octahedra.