Internal friction investigation of phase transformation in nearly stoichiometric LaMnO3+δ

Rhombohedral LaMnO3+δ powders, prepared by two different soft chemistry routes (co-precipitation and hydrothermal synthesis), are sintered at 1400 °C for 2 h in air. Measurements of internal friction Q−1(T) and shear modulus G(T), at low frequencies from −180 to 700 °C under vacuum, evidence three structural transitions of nearly stoichiometric orthorhombic LaMnO3+δ. The first one, at 250 or 290 °C, depending on the processing followed, is associated to either a Jahn–Teller structural transition or a phase transformation from orthorhombic to pseudo-cubic. The second one at 610 or 630 °C is related to a phase transformation from pseudo-cubic or orthorhombic to rhombohedral. Below the Neel temperature, around −170 °C, a relaxation peak could be associated, for samples prepared according to both processing routes, to the motion of Weiss domains.