NEUTRON POWDER DIFFRACTION STUDY OF THE NUCLEAR AND MAGNETIC STRUCTURES OF La_1-xM_xMnO₃ (M=Ca and Sr). Q. Huang^1,2 A. Santoro¹, J.W. Lynn^1,2, R. Erwin¹, J. Borchers¹, J. Peng² & Z.Y. Li^2, Reactor Radiation Division, NIST, Gaithersburg, MD 20899¹, University of Maryland, College Park, MD 20742²

A study of the nuclear and magnetic structures of the Ca and Sr doped perovskite-type compounds La_1-xCa_xMnO₃ (x=0, 0.15, 0.175, and 0.33) and La_0.67Sr_0.33MnO₃ has been made using neutron powder diffraction. For x=0, the Rietveld refinements indicate that two phases coexist. One phase is orthorhombic (Pnma), and the fraction of the Pnma phase increases smoothly from 14wt% at 450 K to 85wt% at 100 K, and remains constant below 100 K. The other phase is rhombohedral (R3c) at room temperature and above, while below 300 K a monoclinic model of P112₁/a provides a significantly better fit. For x=0.15, 0.175, and 0.33 the structure is orthorhombic in the temperature regime investigated (10-300 K). Finally, for La_0.67Sr_0.33MnO₃ two structural models having R3c and P112₁/a symmetry were assumed in the refinements, and the results indicate that the monoclinic model yields significantly smaller R-factors and better fits.

A ferromagnetic ordering occurs for all the compounds studied. For the Ca-doped system the Curie temperature T_c increases from ~150 K for x=0 to ~250 K at x=1/3. For the x=0.33 material where the magnetoresistance anomalies are largest a strong structural anomaly is observed at Tc, while at smaller x the variation with T is smooth through Tc. For the magnetic structures, a model with orthorhombic symmetry Pn'ma' and the moments parallel to the c-axis was used for the phases with the Pnma nuclear structure. For the P112₁/a nuclear structure a magnetic P112₁/a' model and moments parallel to c-axis was found to be appropriate, while for the R-3c nuclear structure a triclinic P1 magnetic structure was used, where the moments can point along either the a- or b- axis.