This folder contains all raw data underlying the results presented in a manuscript, submitted to Angewandte Chemie, and entitled: Suppression of cation intermixing highly boosts the performance of core-shell lanthanide upconversion nanoparticles Authored by: Fuhua Huang2,3, Niusha Bagheri1, Li Wang *2,3, Hans Ågren*2,3, Jinglai Zhang*2,3, Jerker Widengren1, Haichun Liu*1 1 Department of Applied Physics, KTH Royal Institute of Technology, S-10691, Stockholm, Sweden 2 College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P. R. China 3 Henan Center for Outstanding Overseas Scientists, Henan University, Kaifeng 475004, P. R. China *Corresponding author: E-mail: haichun@kth.se Data files containing the raw data and the results of the analysis are grouped according to the order of the figures in the manuscript. ABSTRACT Lanthanide upconversion nanoparticles (UCNPs) have been extensively explored as biomarkers, energy transducers and information carriers in wide-ranging applications in areas from healthcare and energy to information technology. In boosting the brightness and enriching the functionalities of UCNPs, core-shell structural engineering has been well established as an important approach. Despite its importance, a strong limiting issue has been identified, namely cation intermixing in the interfacial region of the synthesized core-shell nanoparticles. Currently there still exists confusion regarding this destructive phenomenon and there is still a lack of facile means to reach a delicate control of it. By means of a new set of experiments, we provide in this work a clear picture for the physical mechanism of cation intermixing occurring in core-shell UCNPs, i.e. partial or substantial core nanoparticle dissolution followed by epitaxial growth of the outer layer and ripening of the entire particle. Based on this picture, we provide an easy but effective approach to tackle this issue that enables producing UCNPs with highly boosted optical properties. *Corresponding authors. E-mail addresses:haichun@kth.se (H. Liu), hans.agren@physics.uu.se (H. Ågren), chemwangl@henu.edu.cn (L. Wang), zhangjinglai@henu.edu.cn (J. Zhang)