Resonant X-Ray-Scattering Study Of Octahedral Tilt Ordering In Lamno3 And Pr1-Xcaxmno3

We report an x-ray-scattering study of octahedral tilt ordering in the manganite series Pr1-xCaMnO3 with x = 0.4 and 0.25 and in LaMnO3. The sensitivity to tilt ordering is achieved by tuning the incident x-ray energy to the L-I, L-II, and L-III absorption edges of Pr and La, respectively. The resulting energy-dependent profiles are characterized by a dipole-resonant peak and higher-energy fine structure. The polarization dependence is predominantly sigma to pi and the azimuthal dependence follows a sin-squared behavior. These results are similar to those obtained in recent x-ray-scattering studies of orbital ordering carried out in these same materials at the Mn K edge. They lead to a description of the cross section in terms of Templeton scattering in which the tilt ordering breaks the symmetry at the rare-earth site. The most interesting result of the present work is our observation that octahedral tilt ordering persists above the orbital ordering transition temperatures in all three samples. Indeed, we identify separate structural transitions which may be associated with the onset of orbital and tilt ordering, respectively, and characterize the loss of tilt ordering versus temperature in LaMnO3.