OVERALL AND LOCAL DISTORTION EFFECTS ON THE METAL-NONMETAL TRANSITIONS OF MIXED-VALENT PEROVSKITE-TYPE MANGANESE OXIDES

The metal-nonmetal (MN) transition data of mixed valent manganese oxid e perovskites A(1-x)A'xMnO3 (A: trivalent lanthanide metals, A': dival ent metals, 0.2<x<0.5) are analyzed in terms of the tolerance factor ( t), and the standard deviation (sigma) of the A-site cation sizes. The interplay of these two parameters that measure the overall and atomic scale distortions, respectively, determine whether a compound can sho w a MN transition or not and its transition temperature (T-MN) In Orde r to show a MN transition, a compound should have t>0.906 and sigma le ss than or similar to sigma=232t-202 (pm). Large t and small sigma are favorable for high T-MN . T-MN data of A(0.7)Ba(0.15)Sr(0.15)MnO(3) ( A=La, Pr, Nd, Sm), A(0.7)Ba(0.15)Ca(0.15)MnO(3) (A=La, Pr, Nd, Sm), (L a0.7Ca0.3)(x)(Gd0.7Ba0.3)(1-x)MnO3 (x = 0.1, 0.3, 0.5, 0.7, 0.9), and (Nd0.7Ca0.3)(x) (Gd0.7Ba0.3)(1-x)MnO3 (x=0.25, 0.3, 0.5, 0.55, 0.7, 0. 75) newly synthesized are reported. (C) 1997 American Institute of Phy sics.