Average versus local structure in K2NiF4-type LaSrAlO4: direct experimental evidence of local cationic ordering

Abstract

The long-range ordering of a crystalline material can be accurately determined by analyzing the Bragg intensities and positions. In contrast, direct observation of short-range ordering in crystalline materials, which is increasingly considered of fundamental importance to unravel the structure–property relationships that underpin their technological applications, is a challenging task. In this study we have investigated the structure of LaSrAlO₄, a representative example of compounds with the K₂NiF₄-type structure. By the combined use of synchrotron and neutron diffraction, pair distribution function analysis, ²⁷Al MQMAS NMR and atomistic simulations we have highlighted differences between the average and local structure of this material, directly determining the existence of at least two distinct Al sites. The results presented in this study confirm that La and Sr are, on average, randomly distributed throughout the structure, but demonstrate that the situation is considerably more complex at the short range.