Combining X‐Ray Whole Powder Pattern Modeling, Rietveld and Pair Distribution Function Analyses as a Novel Bulk Approach to Study Interfaces in Heteronanostructures: Oxidation Front in FeO/Fe3O4 Core/Shell Nanoparticles as a Case Study

R.U.I. acknowledges CAPES and CNPq (No. 206983/2014‐0) Brazilian agencies. A.G.R. and J.N. thanks the support of the Generalitat de Catalunya through the 2017‐SGR‐292 project and the Beatriu de Pinos Program (2011 BPB 00209) and the Spanish Ministerio de Economia y Competitividad (MINECO) through the MAT2016‐77391‐R project. I.P. was supported by the National Research Fund of Luxembourg (Grant No. FNR‐Inter2015/LRSF). A.L.‐O. acknowledges the MINECO through the Juan de la Cierva Program (IJCI‐2014‐21530). X.T. would like to acknowledge the financial support from the MINECO projects MAT2015‐67593‐P and BIA2014‐57658‐C2‐1‐R. The ALBA Synchrotron is acknowledged for the provision of beamtime and the MSPD beamline staff is for their help during the XRD measurements. Dr. Luis G. Martinez from IPEN/CNEN is acknowledged for useful discussions about XRD techniques. Dr. Paolo Scardi from Universita degli Studi di Trento is acknowledged for the helpful discussions about WPPM and for providing PM2K v.3 software. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya. ICN2 also acknowledges support from the Severo Ochoa Program (MINECO, Grant No. SEV‐2013‐0295).