The largest magnetoresistance ratios were found for com-We report the observation, in Pr 0.6 Sr 0.4 MnO 3 , of a low-temper-pounds around the 33% substitution level (7-10). ature phase transition between Pnma and the monoclinic space Pr 1ฯชx Sr x MnO 3 (0.2 ี
x ี
0.5) is also ferromagnetic with group I2/a. The I2/a allotype has never previously been obmetallic behavior below T C . However, for Ca or Sr concenserved in the (R 1ุx A x )MnO 3 manganese perovskite system. Furtrations of around 50%, the conducting/ferromagnetic state thermore, this space group symmetry is not present in the is replaced at lower temperatures by a charge-ordered/ recently published phase diagram obtained at 30% Mn ุ4 , indiantiferromagnetic state, where resistivity exhibits activated cating that electronic doping has a direct effect on the phase behavior (11). The application of a magnetic field can melt stability. The structural and magnetic parameters for the new the charge ordered lattice and lead to the restoration of a phase, as obtained from Rietveld refinements of neutron powder conducting/ferromagnetic regime. In the case of Pr 1ฯชx Ca x diffraction data, are also reported. ยฉ 1996 Academic Press MnO 3 (0.3 ี
x ี
0.45), the magnetic and charge ordering transitions occur at different temperatures (2, 12, 13). Below the charge ordering temperature, the application of a 276