Ferroelasticity of LaMO3 (M=Co, Fe, Al, Ga)
Authors: Araki Wakako , Takeda Kazutaka , Arai Yoshio 
Published: February 2020

Doped lanthanum perovskite oxides are promising candidates for solid oxide fuel cell materials. Recently, anomalous mechanical behaviours due to ferroelastic characteristics of these materials such as La-Sr-Co-Fe-O (LSCF)and La-Sr-Co-O (LSC) have been reported; however, there has been no systematic study on ferroelasticity of lanthanum perovskite oxides doped with different transition metal ions. In the present study, the ferroelasticity of doped lanthanum perovskite oxides LaMO3 (M = Co, Fe, Al, Ga) was investigated in detail.

LaCoO3 (LCO), LaFeO3 (LFO), LaAlO3 (LAO), and LaGaO3 (LGO) samples are prepared by solid-state reaction method. The crystal structures of the prepared samples were determined by X-ray diffraction analysis, whilst the grain structures as well as the ferroelastic domain structures were observed by scanning electron microscopy. The uniaxial compression test was carried out to examine the mechanical behaviour (stress – strain curve) of the samples. All the experiments were conducted at room temperature.

The colour of the prepared LCO was black, whilst LAO and LGO were white. LFO was either rufous (sintered at 1473 K, called LFO_R) or black (1673 K, LFO_B). This difference in colour can be attributed to the formation of oxygen vacancy accompanied by a slight amount of Fe segregation during the sintering. The porosity of all the samples determined by the Archimedes method was lower than 5%. The XRD analysis revealed that LCO and LAO have rhombohedral structure, where the rhombohedral angle of LCO is larger than LAO. On the other hand, LGO and also both LFOs are in orthorhombic structure at room temperature. The grain size of specimens observed by SEM is varied from 1 to 20 micron depending on the sample. The ferroelastic domains are clearly observed as striped patterns for LCO and LGO, whereas there is no clear ferroelastic domain for LAO and LFOs. The stress – strain curve of LCO obtained in the compressive test exhibits the largest hysteresis and residual strain. LAO also has a typical stress – strain relationship for ferroelastic materials but very low critical stress. LGO has the largest initial elastic modulus and exhibits a slight nonlinear stress – strain curve with a hysteresis due to the ferroelastic domain. LFO_R and LFO_B have a completely different mechanical behaviour: LFO_R exhibits an almost linear stress – strain relationship, whilst LFO_B shows a nonlinear curve with strong back-switching behaviour.

A comparison among these specimens indicates that the rhombohedral phase with a larger rhombohedral angle tends to exhibit a larger hysteresis and residual strain in the stress – strain curve, and also that the rhombohedral phase generally exhibits more distinctive ferroelastic mechanical behaviour than the orthorhombic phase. Furthermore, the significant difference between LFO_R and LFO_B suggests that the oxygen non-stoichiometry of the lanthanum perovskite oxides could affect the ferroelasticity. Further studies on the ferroelasticity of the lanthanum perovskite oxides at elevated temperatures are ongoing.