Anisotropic Thermal and Chemical Expansion in Sr-Substituted LaMnO_3+δ: Implications for Chemical Strain Relaxation

The anisotropic thermal and chemical expansion of rhombohedral ( R 3̅ c ) La 1– x Sr x MnO 3+δ ( x = 0.2, 0.3) was investigated by in situ high temperature X-ray diffraction of submicrometer size powders in pure oxygen and nitrogen (inert) atmospheres. The thermal expansion of the long axis c was found to be close to twice as high as the thermal expansion of the short axis a . The large thermal expansion of the c -axis is caused by rectification of the antiferrodistortive tilting and decompression of the MnO 6/2 octahedra. The unit cell parameters were shown to be strongly dependent on the partial pressure of oxygen, which was attributed to chemical expansion/contraction due to reduction/oxidation of Mn. Anisotropic chemical expansion/contraction was more pronounced for the unit cell parameter a than for c, and the chemical expansion/contraction was inferred to reflect the size of the MnO 6/2 octahedra. The onset of chemical expansion in nitrogen and contraction in oxygen atmosphere during heating was discussed in terms of a gradual transformation from a nonequilibrium to an equilibrium point defect population in La 1– x Sr x MnO 3+δ . Possible implications of slow relaxation of chemically induced stresses at the nanoscale and in epitaxial thin films are addressed. Finally, a second order phase transition from the ferroelastic ( R 3̅ c ) to paraelastic state ( Pm 3̅ m ) is reported for La 0.7 Sr 0.3 MnO 3+δ at 850 ± 25 °C. The temperature of the phase transition decreases with increasing Sr content in La 1– x Sr x MnO 3+δ .