In this study, Eu3+-containing amorphous silica nanoparticles and Eu compounds were successfully synthesized via a mechanochemical solid-state reaction between silica nanoparticles and EuCl3 center dot 6H(2)O. This reaction was induced by a grinding process, and the states of Eu3+ in the silica/europium composites were investigated. The silica/europium composites exhibited orange-red color luminescence owing to the D-5(0)-F-7(0), D-5(0)-F-7(1), and D-5(0)-F-7(2) transitions, which indicated the presence of Eu3+ in the silica framework and the newly formed Eu compounds such as EuOCl and Eu(OH)(2)Cl. The mechanochemical reaction because of the grinding process effectively induced an interaction between the silica surface and europium chloride; subsequently, Eu(OH)(2)Cl was formed in the silica/europium composites. Additionally, the Eu(OH)(2)Cl in the silica/europium composite exhibited a higher thermal stability than that of simple Eu(OH)(2)Cl, indicating that the mechanochemical reaction mediated the formation of Eu(OH)(2)Cl and new chemical bonding between the newly formed Eu(OH)(2)Cl and the silica surface, providing improved thermal stability to Eu(OH)(2)Cl. Thus, we successfully prepared silica nanoparticles containing not only Eu(III) in the silica framework but also Eu compounds that exhibit unique chemical bonding during a mechanochemical reaction. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.