Cation size variance effects in high-tolerance factor Ln0.7M0.3MnO3 perovskites

A series of fi five Ln(0.7)M(0.3)MnO(3) perovskites (Ln = trivalent lanthanide; M = divalent alkaline earth) has been prepared. These have the same mean A-cation site radius of 1.26 Angstrom (equivalent to a perovskite tolerance factor of 0.98) but varying amounts of A-cation size disparity quantified by the size variance sigma(2). As this increases, the temperatures of the metal-insulator and Curie transitions show a strong linear decrease but that of the structural transition between rhombohedral R (3) over bar c and orthorhombic Imma phases increases. Extrapolation from these and previous data indicates that the maximum metal-insulator temperature for an Ln(0.7)M(0.3)MnO(3) perovskite is similar to 550 K. Different methods for estimating the Curie temperature are compared. (C) 1999 Academic Press.