Local structural distortions in manganites probed by comparative x-ray-emission and x-ray-absorption near-edge measurements

Comparison studies of the perovskite compounds system R_1-xCaₓMnO₃ (R=La,Bi) by both Mn K_β-emission spectroscopy and x-ray-absorption near-edge spectroscopy (XANES) are presented. The insensitivity of x-ray-emission measurements to structural distortions coupled with the sensitivity of x-ray-absorption near-edge measurements to changes in both structure and valence enable one to detect the presence of structural distortions, such as local Jahn-Teller (JT) distortions. Theoretical XANES computations for pure cubic perovskite and locally distorted endmembers are used to show the effect of distortions on XANES spectra as well as to comment on the nature of the pre-edge features in the spectra. We show by explicit computations that the near-edge spectra are determined by dipole transitions while pure $1s$ to $3d$ electric quadrupole transitions determine a very limited section of the pre-edge region. Simulations of the pre-edge features reveal a direct connection between local distortions and the $a1$ feature amplitude. The Bi-containing system is found to have significantly higher levels of distortions than the La system. XANES studies of the AMnO₃ (A=La, Pr, and Nd) system reveal a direct relationship between the main line width and the magnitude of the JT distortions.