Crystal chemistry of layered manganites Ln_1.4Sr_1.6Mn₂O₇ (Ln = La, Pr, Nd, Sm, Eu, and Gd)

The powder X-ray diffraction patterns of a series of layered manganite compounds, Ln_1.4Sr_1.6Mn₂O₇ (Ln = La, Pr, Nd, Sin, Eu, Gd), were refined with the Rietveld method. The Ln occupancy in the rock salt site increases on moving from La through Gd. The variations of the lattice parameters and the bond distances correlate and can be accounted for with the distribution of Ln and Sr in the perovskite and rock salt sites. The structural analysis implies that the physical properties of these compounds have to vary monotonically from La through Sm and then more or less the same from Sm through Gd, which is not the case. Therefore, in order to understand the various physical properties of these compounds depending on Ln another factors such as the magnetic properties of Ln need to be considered.