Two C-type antiferromagnets with different magnetoresistive properties:: Sm0.15Ca0.85MnO3 and Pr0.15Sr0.85MnO3

The investigation of Sm0.15Ca0.85MnO3 and Pr0.15Sr0.85MnO3 manganites shows that both oxides exhibit a metal to insulator transition around 110 and 260 K, respectively, but they differ by the existence of CMR properties for the former, in contrast to the latter. The neutron powder diffraction and electron microscopy studies show that the resistive transitions are linked to the structural and magnetic ones. Both oxides are C-type antiferromagnets at low temperature, with a similar d(3z2-r2) orbital polarization which is associated with ordering of the orbitals into chains, no charge ordering being detected. Although they exhibit a similar distortion of their MnO6 octahedra, these two C-type antiferromagnets differ mainly by the tilt arrangement of the octahedra, leading to the P2(l)/m and I4/mcm structures for Sm0.15Ca0.85MnO3 and Pr0.15Sr0.85MnO3 respectively. Moreover, few percents of a G-type antiferromagnetic phase has also been evidenced in Sm0.15Ca0.85MnO3. The existence of this secondary phase is discussed in connection with its CMR properties. (C) 1999 Elsevier Science B.V. All rights reserved.