Local structural distortions in manganites probed by comparative x-ray-emission and x-ray-absorption near-edge measurements - art. no. 024430

Comparison studies of the perovskite compounds system R1-xCaxMnO3 (R=La,Bi) by both Mn K-beta-emission spectroscopy and x-ray-absorption near-edge spe ctroscopy (XANES) are presented. The insensitivity of x-ray-emission measur ements to structural distortions coupled with the sensitivity of x-ray-abso rption near-edge measurements to changes in both structure and valence enab le one to detect the presence of structural distortions, such as local Jahn -Teller (JT) distortions. Theoretical XANES computations fur pure cubic per ovskite and locally distorted endmembers are used to show the effect of dis tortions on XANES spectra as well as to comment on the nature of the pre-ed ge features in the spectra. We show by explicit computations: that the near -edge spectra are determined by dipole transitions while pure 1s to 3d elec tric quadrupole transitions determine a very limited section of the pre-edg e region. Simulations of the pre-edge features reveal a direct connection b etween local distortions and the a1 feature amplitude. The Bi-containing sy stem is found to have significantly higher levels of distortions than the L a system. XANES studies of the A MnO3 (A = La, Pr, and Nd) system reveal a direct relationship between the main line width and the magnitude of the JT distortions.