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 low doping levels, the energetically favorable formation of isovalently substituted Ti⁴⁺ clusters in the monoclinic phase of ZrO₂ (m-ZrO₂) using photoluminescence spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and first-principles density functional theory (DFT) calculations. The oxygen vacancy–Ti dopant interactions critically reduce the doping energy cost, and Ti doping is favored for oxygen deficiency formation. The Ti⁴⁺ clusters feature a high degree of excited-state electron delocalization following charge-transfer (CT) excitation (O²⁻ → Ti⁴⁺) and are responsible for the reduced quenching concentration and doping-dependent excitation band redshift. The occurrence of Ti⁴⁺ clustering was analyzed using 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 a broader range of material systems.