Low-Doping Clustering of Ti4+ in Monoclinic ZrO2 : Photoluminescence and Formation Mechanism

Abstract

Clustering of dopant cations commonly occurs when they are incorporated into host matrices, particularly at high doping levels. In this work, we demonstrated, even at the low doping level, the energetically favorable formation of the isovalently substituted Ti 4+ clusters in monoclinic phase of ZrO 2 (m-ZrO 2 ) by photoluminescence spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and the first-principle density functional theory (DFT) calculation. The oxygen vacancy-Ti dopant interactions critically reduce the doping energy cost and Ti doping is in favor of oxygen deficiency formation. The Ti 4+ clusters feature a great amount of excited electron delocalization following charge-transfer (CT) excitation (O 2-→Ti 4+ ), and are responsible for the reduced quenching concentration and doping-dependent excitation band redshift. The occurrence of Ti 4+ clustering was analyzed by a model that involved the electronegativity of dopant/host cations and the inherent neutral oxygen vacancies. The formation mechanism established is anticipated to be extendable to broader material systems.