Rhombohedral LaMnO 3+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 LaMnO 3+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.
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