Average versus local structure in K 2 NiF 4 -type LaSrAlO 4 : direct experimental evidence of local cationic ordering Cristina Tealdi, a Chiara Ferrara, a Lorenzo Malavasi, a Piercarlo Mustarelli, * a Clemens Ritter, b Alberto Spinella, c Dominique Massiot d and Pierre Florian d Received 31st January 2012, Accepted 11th March 2012 DOI: 10.1039/c2jm30603d 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 LaSrAlO 4 , a representative example of compounds with the K 2 NiF 4 -type structure. By the combined use of synchrotron and neutron diffraction, pair distribution function analysis, 27 Al 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. 1. Introduction AA’BO 4 compounds, a subgroup of the A 2 BO 4 -based oxides with K 2 NiF 4 -type structure, are the subject of great attention due to the variety of their potential technological applications which strongly depend on composition. In addition to the nature of the B site cation (Mn, Co, Ni, Cu) a wide range of substitutions on the A site (usually alkaline-earth elements for lanthanides) increases the possibility to modulate the physical properties of this class of materials. For example, cuprates and ruthenates are known as superconductors, manganites present interesting magnetoresistive properties, and nickelates and cobaltates are investigated as catalysts for CO oxidation, as well as cathode materials for solid oxide fuel cells due to their high mixed oxide- ion/electronic conductivity. 1 In this context, LaSrAlO 4 is in particular known as the most suitable substrate for thin film deposition of the single layer high-T C cuprate superconductors. 2 LaSrAlO 4 crystallizes in the tetragonal space group I4/mmm. The structure (Fig. 1) is composed of AlO 6 octahedra connected to form a two-dimensional network by corner-shared oxygens. These layers, similar to those found in the perovskite structure, are separated by (La,Sr)O slabs with the rock-salt structure. Within this structure, La and Sr are 9-coordinated to oxygen and each Al has 10 La/Sr nearest-neighbours. In the ideal K 2 NiF 4 - type structure only one crystallographic site (named A in Fig. 1) is available for La and Sr (0, 0, z) and the cations are supposed to be randomly distributed over this site throughout the structure. Consistently, only one distinct crystallographic site (0, 0, 0) is available for Al (named B in Fig. 1). Previous studies on Fig. 1 Representation of the tetragonal K 2 NiF 4 -type structure of A 2 BO 4 -based compounds showing the octahedral coordination around the B site and the layered nature of the structure. In the case of LaSrAlO 4 , B ¼ Al and A ¼ La/Sr. a INSTM - UdR Pavia, Department of Chemistry-Physical Chemistry Division, University of Pavia, Viale Taramelli 16, 27100 Pavia, Italy. E-mail: piercarlo.mustarelli@unipv.it; Fax: +39 0382 987575; Tel: +39 0382 987205 b Institute Laue-Langevin, Boite Postale 156, F-38042 Grenoble, France c Centro Grandi Apparecchiature - UniNetLab, Universit  a di Palermo, Via F. Marini 14, 90128 Palermo, Italy d CEMHTI-CNRS, 1D, Av. de la Recherche Scientifique, 45071 Orl eans Cedex 2, France 10488 | J. Mater. Chem., 2012, 22, 10488–10495 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 10488 www.rsc.org/materials PAPER Downloaded by Universita di Pavia on 27 July 2012 Published on 13 March 2012 on http://pubs.rsc.org | doi:10.1039/C2JM30603D View Online / Journal Homepage / Table of Contents for this issue