We have studied the phase diagram of La 1Ϫx Ca x MnO 3 for 0.5рxр0.9 using neutron powder diffraction and magnetization measurements. At 300 K all samples are paramagnetic and single phase with crystallographic symmetry Pnma. As the temperature is reduced a structural transition is observed which is to a charge-ordered state only for certain x. On further cooling the material passes to an antiferromagnetic ground state with Néel temperature T N that depends on x. For 0.8рxр0.9 the structural transformation occurs at the same temperature as the magnetic transition. Overall, the neutron-diffraction patterns were explained by considering four phase boundaries for which La 1Ϫx Ca x MnO 3 forms a distinct phase: the charge-ordered phases at xϭ0.5 and x ϭ2/3, the monoclinic and C-type magnetic structure at xϭ0. 80-0.85, and the G-type magnetic structure at xϭ1. Between these phase boundaries the magnetic reflections suggest the existence of mixed compounds containing both phases of the adjacent phase boundaries.

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We report on magnetometry, resistivity and Mössbauer spectroscopy measurements on single-phase Fe3BO5 in a wide temperature range from 4.2 to 620 K. Magnetically split Mössbauer spectra were observed below TN = 114 K, although a proportion of the Fe2+ ions in the basal plane (crystallographic site Fe1) remain paramagnetic down to the ferrimagnetic transition at Tw = 74 K. This unusual magnetic behaviour known as `idle-spin behaviour' can be explained by the competitive magnetic interactions with its neighbouring ordered ions and should be related to the topology of the octahedral Fe1 sites in the crystal structure of Fe3BO5. The behaviour change recorded in the dc magnetization and Mössbauer data indicates that the magnetic structure of Fe3BO5 should change at 74 K, where the spin frustration gradually ceases to exist. Electron delocalization in the Fe3-Fe2-Fe3 triads between pairs of Fe2+ and Fe3+ ions was observed at all temperatures between 4.2 and 180 K and coexists with localized ferric and ferrous (Fe1 and Fe4 crystallographic sites) states. Above 180 K the delocalization in the Fe3-Fe2-Fe3 triads includes the rest of the Fe3+ ions and some evidence can be seen in the resistivity data as well.

55Mn NMR line shape measurements in La1-xCaxMnO3 for 0.20< or =x< or =0.50 provide experimental evidence about the existence of two distinct regions in the T-x magnetic phase diagram, where the homogeneous ferromagnetic (FM) metallic state is separated into FM metallic and FM insulating regions. These results are in agreement with recent theoretical predictions, which reveal a novel electronic phase separation in two FM states, providing orbital ordering and Jahn-Teller phonons are taken into consideration.

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