(INVITED) Eu3+ activated molybdates – Structure property relations

Abstract

Eu3+ activated phosphors have been a subject of scientific investigation for more than 100 years. Various host materials have been employed, among which molybdate hosts play an important role. They offer an array of advantageous properties such as high critical activator concentrations, facile solid state synthesis and excitation via the Mo6+/O2− charge transfer (CT) transition. This review paper discusses the basics of Eu3+ luminescence and presents the most important properties and synthesis conditions of various Eu3+ activated molybdates. Furthermore, relations between the crystallographic structure of the host and the optical properties of Eu3+ are shown. The point symmetry of the Eu3+ doping site can be determined from specific features of the Eu3+ emission spectrum and the number of crystallographic sites can be estimated. The concept of optical basicity is discussed in regard to the spectral position of the Mo6+/O2− charge transfer band. The influence of the CT band on the thermal quenching properties and consequently the photoluminescence efficiency of Eu3+ is investigated. Disadvantages of molybdates as phosphor host materials are discussed as well, such as the formation of Mo5+ defect states.