TRANSPORT-PROPERTIES AND MAGNETIC-BEHAVIOR IN THE POLYCRYSTALLINE LANTHANUM-DEFICIENT MANGANATE PEROVSKITES (APPROXIMATE-TO-LA1-XMNO3)

The rhombohedral lanthanum manganates approximate to La1-xMnO3 (x = 0. 0, 0.05, 0.1) were prepared by a soft chemistry route to test first as catalysts. These three weak ferromagnets exhibit a structural transit ion at T-t slightly below T-c. approximate to La0.9MnO3 undergoes a gr oup-subgroup R (3) over bar c <---->, R3c transition most likely of se cond order. approximate to LaMnO3 does not show any insulator-to-metal transition, but does show a semiconductor-semiconductor at T-t simila r to 130 K. Analysis of the transport data (conductivity and thermopow er) indicates hopping of polarons at both low and high temperature. An effective magnetic moment of 3.50 mu(B)/Mn is deduced from the sponta neous magnetization value at 5 K. The slight difference from the spin- only value (3.67 mu B) is assigned to a spin-canting effect which is r eflected in the divergence of the FC and ZFC susceptibility curves at low fields (H less than or equal to 10(-2) T) and low temperature. Com pared with recent results on thin films, the compositions x = 0.05, 0. 1 are expected to show magnetoresistive effects as well as a semicondu ctor-to-metal transition. If it is verified on bulk samples which all contain a similar Mn4+ content of 35%, the structural transition at T- t could be considered as magnetically driven. (C) 1997 Elsevier Scienc e Ltd.