Details of structural and magnetic transitions in Pr0.5Ca0.5-xSrxMnO3 perovskites

Magnetic measurements supported in some cases by neutron diffraction and resistivity measurements were used to investigate the changes of crystal and magnetic structures between 2 and 400 K across the system Pr0.5Ca0.5 - xSrxMnO3, 0 less than or equal to x less than or equal to 0.5. While for x = 0, the low-temperature antiferromagnetic (AF) state is reached through mediating incommensurate charge ordered (CO) structures, a ferromagnetic (F) charge-disordered phase is intercalated for x greater than or equal to 0.3 between paramagnetic (P) and AF states. A two-phase region CO + F was found in between, for 0.1 less than or equal to x less than or equal to 0.2. The transition to the AF state is always accompanied by large changes of lattice parameters that are compatible with cooperative Jahn-Teller (JT) effect and a corresponding polarization of e(g) electron orbitals to definite planes. The elementary cell volume shrinks more than 0.5% during the transition and a local minimum close to T-N was found to exist in x = 0.15 and 0.3. It was also discovered that the actual low-temperature AF state often contains some residua of F phase as well as some weakly coupled or disordered spins exhibiting spin glass behaviour. The broad spectrum of results including the observed deviations from the Curie-Weiss law in the P region has been consistently interpreted on the basis of an interplay of double exchange and the J-T effect, in combination with steric effects influencing the tolerance factor t and the e(g) bandwidth, (C) 1999 Elsevier Science B.V. All rights reserved.