High-resolution high energy X-ray diffraction studies of charge ordering in CMR manganites and nickelates

High-resolution, high-energy, X-ray diffraction results are presented for the study of weak charge ordering phenomenon. By utilizing X-rays in the 100 keV region the dramatic increase in the penetration depth allows for both bulk-sensitive and high- resolution measurements to be made. The strontium doped La₂NiO₄ system is a prototypical system in the understanding of strong electron-phonon coupling, and the resultant effects on material properties. At doping levels of 1/3 and 1/2 commensurate charge modulations are observed indicating real-space charge stripes. We have measured the correlation lengths of these charge stripes using both 1013 keV X-rays and 8.3 keV X-rays. In comparing our results we have observed that the charge stripes appear to be well correlated in the near-surface region with correlation lengths ξ ≈ 2400Å. However, our bulk sensitive measurements show that the charge stripes appear in a possible stripe glass phase with a correlation length of only ξ ≈ 300Å. Our measurements on the 3D charge order manganite system Nd_0.5Sr_0.5MnO₃ show that the charge ordering appears to be well correlated in the bulk of the sample in contrast to our nickelate results.