The orthorhombic twinned microstructure of stoichiometric LaMnO3 was explained within the framework of ferroelastic theory. It was found to be necessary to propose the existence of a very high-temperature, and previously unobserved, cubic phase. The orthorhombic and rhombohedral microstructures of La1-xSrxMnO3 solid solutions, where x was between 0 and 0.35, could then be easily interpreted, and the characteristics of their twinning systems were predicted. This experimental determination of the twinning elements of LaMnO3 was consistent with predictions; assuming that the features of the orthorhombic ferroelastic form of LaMnO3 resulted from a cubic-orthorhombic transition. This assumption was validated in spite of the existence of a transient rhombohedral form which was stable at intermediate temperatures. This could be understood if it was considered that the 2 ferroic phases were slight modifications of an ideal common cubic prototype phase. The fact that the lattice parameter of this hypothetical cubic solid solution varied continuously with the Sr content was also consistent with this assumption. In particular, an excellent continuity of the lattices across the walls was an important criterion for minimizing the interface energy of the twinned structure. It was noted that the shear angle that was associated with this twinning mechanism, calculated according to the lattice parameters of La1-xSrxMnO3 solid solutions at room temperature, tended towards zero when the replacement of La by Sr was close to 17%. When x was greater than 0.17, it was unclear whether the same twinning mechanism was still valid for the different symmetry of the rhombohedral ferroelastic phase.

Twinned Microstructure of La1-xSrxMnO3 Solid Solutions. M.Déchamps, A.M.De Leon Guevara, L.Pinsard, A.Revcolevschi: Philosophical Magazine A, 2000, 80[1], 119-27