Neutron-powder-diffraction measurements, combined with magnetization and resistivity data, have been carried out in the doped perovskite La1-xCaxMnO3 (x50.47, 0.50, and 0.53) to elucidate the structural, magnetic, and electronic properties of the system around the composition corresponding to an equal number of Mn3+ and Mn4+. At room temperature all three samples are paramagnetic and single phase, with crystallographic symmetry Pnma. The samples then all become ferromagnetic (FM) at Tc~265 K. At ~230 K, however, a second distinct crystallographic phase (denoted A-II) begins to form. Initially the intrinsic widths of the peaks are quite large, but they narrow as the temperature decreases and the phase fraction increases, indicating microscopic coexistence. The A-II phase has the same space-group symmetry and similar lattice parameters as the initial (F-I) phase. Both phases begin to exhibit weak Jahn-Teller distortions of the MnO6 octahedra below ~230 K, but the distortions are different in the two phases and vary in character with temperature, and can be interpreted in terms of ordering of the d2z orbitals. The fraction of the sample that exhibits the A-II phase increases with decreasing temperature and also increases with increasing Ca doping, but the transition never goes to completion to the lowest temperatures measured (5 K), and the two phases therefore coexist in this temperature-composition regime. Phase A-II orders antiferromagnetically (AFM) below a Ne´el temperature TN ~160 K, with the CE-type magnetic structure. For the x=0.47 sample the proportion of F-I at 10 K is about 60%, which has afforded us the opportunity to fully characterize the crystallographic and magnetic behavior of both phases over the entire temperature range. The F-I phase maintains the ferromagnetic state for all temperatures below Tc , with the moment direction remaining along the c axis. Resistivity measurements show that this phase is a conductor, while the CE phase is insulating. Application of magnetic fields up to 9 T progressively inhibits the formation of the A-II phase, but this suppression is path dependent, being much stronger, for example, if the sample is field-cooled compared to zero-field cooling and then applying the field. The H-T phase diagram obtained from the diffraction measurements is in good agreement with the results of magnetization and resistivity. Overall the measurements underscore the delicate energetic balance between the magnetic, structural, and electronic properties of the system.


Physics, Astronomy, and Materials Science

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© 2000 The American Physical Society

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Physical Review B