Structural changes, clustering, and photoinduced phase segregation in Pr0.7Ca0.3MnO3

The structural properties of Pr0.7Ca0.3MnO3 were studied by x-ray synchrotron and neutron-powder diffraction as a function of temperature (15 < T < 300 K), and as a function of x-ray fluence at 15 and 20 K. The temperature evolution of the lattice parameters and of the superlattice reflections is consistent with the development of charge and orbital ordering below T-CO similar to 180 K, followed by antiferromagnetic ordering below T-N similar to 140 K, similar to what was previously observed for La0.5Ca0.5MnO3. Below T-C similar to 120 K, the magnetic structure develops a ferromagnetic component along the a axis on the Mn ions. At low temperatures, a small ferromagnetic moment of 0.45(2)mu(B), oriented in the same direction appears on the Pr ions as well. The observation in Pr0.7Ca0.3MnO3 of significant lattice strain developing below T-CO, as well as the development of a ferromagnetic component to the magnetic structure at T-C similar to 120 K, can be interpreted in terms of the presence of ferromagnetic clusters with an associated lattice distortion from the average structure. At low temperatures, exposure to the x-ray beam produces a phase-segregation phenomenon, whereby the ferromagnetic droplets coalesce into larger aggregates. Further exposure results in a gradual melting of the charge-ordered phase and the formation of a second phase, recently shown to be a ferromagnetic metallic phase by Kiryukhin et al. [Nature (London) 386, 813 (1997)]. The ferromagnetic phase has a significantly smaller a lattice parameter and unit-cell volume (Delta V/V similar to 0.4%) than that of the charge-ordered phase.