Atomic and magnetic structure of perovskite manganites: A-cation size and oxygen isotope substitution effects and homogeneity of magnetic state

A series of (La1-yPry)(0.7)Ca0.3MnO3 samples with y = 0.5, 0.6, 0.7 and 0.7 5 has been systematically studied by neutron powder diffraction. The analys is shows that their basic structural parameters including the temperature o f metal-insulator-phase transition are the linear functions of an average A -cation radius. For composition with y = 0.75 the structural analysis was p erformed on two samples, one containing the natural mixture of oxygen isoto pes (99.7% O-16, metallic and ferromagnetic at T less than or equal to T-FM approximate to 110 K). and the other one enriched by O-18 in 75% (insulati ng in the whole temperature range and with collinear antiferromagnetic orde r at T less than or equal to T-AFM approximate to 150 K). These two samples have been found to be identical in crystal and magnetic structure down to transition of the sample with O-16 into the metallic ferromagnetic phase. T his means that their quite different transport and magnetic properties at T less than or equal to 110 K are driven solely by the different oxygen atom s dynamics. Evidences of low-temperature two-phase state: AFM-dielectric FM-metallic have been obtained for the samples with y greater than or equal to 0.6. (C) 2000 Elsevier Science B.V. All rights reserved.