Time and frequency dependent mechanical properties of LaCoO3-based perovskites: Neutron diffraction and domain mobility

Abstract

The study of domain wall movement and texture formation in ferroelastic LaCoO 3 perovskite under constant applied compressive stress has been performed using in situ neutron diffraction. It was established that under constant applied compressive stress the domain walls show mobility that may lead both to the shrinkage (creep strain) and to the expansion (negative creep strain) of LaCoO 3 perovskite. The domain wall movement and texture formation can be explained by the availability, mobility, and interaction of twins, stacking faults, antiphase boundaries, dislocations, and point defects, such as oxygen vacancies and their complexes as well as impurity atoms. The equation of motion was used to describe the possible mechanisms of domain wall movement under applied stress, and it was determined that the available solutions of this equation allow both for the shrinkage (creep strain) and for the expansion (negative creep strain) of LaCoO 3 perovskite to occur.

Time dependent deformation of LaCoO3 based perovskites at different temperatures: ferroelastic and non-ferroelastic creep behaviour

“…However, as the temperature of testing increased to 800°C and 900°C, one might expect, similar to La 0.8 Ca 0.2 CoO 3 behaviour, that the coercive stress might significantly decrease and even become less than 10 MPa. If this assumption is accurate, then the stiffening of LaCoO 3 at 800°C and 900°C, which is reported both in this work and in other works [11,12], can be explained by the fact that the measured loading portion of stress–strain diagrams presented in Figure 5E and G belongs to the curves above the inflection points, where domain switching has already occurred, texture has been formed, and stiffening of LaCoO 3 upon loading has already occurred [14]. Therefore, the inflection points corresponding to a coercive stress are not visible in LaCoO 3 perovskite upon loading for all four temperatures under the testing conditions used.…”

“…Using Eq. ( 7), (14), and ( 15) the ferroelastic creep strain contribution to the total bending creep strain can be obtained for short creep time for creep upon unloading as.…”

“…It was reported that these materials, such as LaCoO 3 , LaFeO 3 , LaMnO 3 or LaCrO 3 based perovskites, are ferroelastic and hysteretic upon mechanical loading due to the mobility of their defects, such as domain walls, stacking faults, multiple point defects among others [5][6][7][8][9][10][11][12]. Such defect mobility upon application of stress results in time-dependent continuous deformation that easily occurs in these perovskites even at room temperature [13][14][15][16]. Such room temperature ferroelastic creep is distinctly different from non-ferroelastic creep that mainly occurs in materials at high temperatures due to grain sliding, dislocation movements or even diffusion of atoms upon applied stress [17][18][19].…”

Room Temperature Ferroelastic Creep Behavior of Porous (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ

“…The ferroelastic time-dependent deformation has been reported in ferroelectric lead zirconate titanate [13,14] and ferroelastic lanthanum cobaltite-based perovskites [3,11,[15][16][17]. Unlike high-temperature creep which usually reaches a steady-state strain rate under a constant stress, ferroelastic time-dependent deformation exhibits a continuous decreasing creep rate [3,11,16,17] or an unusual expansion of the sample under a constant compressive stress called negative creep which has been reported for LaCoO 3 [12] for the first time. The ferroelastic creep mechanisms are different from those occurring in materials at high temperatures [11,14].…”

“…Room temperature time-dependent deformation is one of the characteristic features of ferroelastic perovskites, which occurs because of the mobility of domain (twin) walls in the microstructure and their interactions with lattice defects such as oxygen vacancies and grain boundaries to name a few [12]. The ferroelastic time-dependent deformation has been reported in ferroelectric lead zirconate titanate [13,14] and ferroelastic lanthanum cobaltite-based perovskites [3,11,[15][16][17].…”