Study of Jahn-Teller phase transitions in nonstoichiometric RMnO(3+x) orthomanganites (R=La,Nd,Sm,Eu)

High temperature X-ray structural analysis, dilatometric, DTA and elastic properties investigations have been performed for the RMnO(3+x) orthomanganites (R = La --> Eu). These compounds show first-order transitions to their orbital disordered structures. The orbital ordered and disordered phases coexist in a wide temperature interval. The transition temperature decreases with increasing oxygen content and increases with the rare earth atomic number. In the RMnO(3+x) compounds (R = Nd --> Dy) the shape memory effect (SME) is observed. The SME is due to O-I <-> O transformation plasticity. The character of shape recovery is shown to depend on oxygen content and the orthorhombic distortion value of the unit cell.