11 - Photoluminescence in pyrochlore structures

Abstract

This chapter provides an overview of the binary and quaternary pyrochlore type oxides as host materials for photoluminescence phenomena that play a critical role in the advancement of many technologies, including agriculture, biomedicine, displays, solid-state lighting, solar cells, thermometry, and others. Photoluminescence has been reported in various hosts such as oxides, halides, nitrides, sulfides, and silicates, with several structural families: perovskites, scheelites, pyrochlores, weberites, and fergusonites, among others. Among these structures, pyrochlore-type luminescent materials have recently attracted attention for their unique properties, including high chemical and thermal stability, ability to accommodate a wide range of elements, lattice stiffness, and tolerance of both cation and anion disorder. Current solid-state lighting technology demands orange-red emitting materials to improve the color rendering index (CRI) and reduce the correlated color temperature (CCT) to develop warm white lighting systems for general illumination applications. Among the activators, Eu3+ is an ultimate choice for orange and red luminescence in any host, due to its characteristic intraconfigurational transitions. However, these transitions strongly depend on the chemical environment, coordination, and symmetry of the Eu3+ ion in the host. In this view, the Eu3+-doped pyrochlore system has received much attention for the development of orange-red emission phosphors, since the excitation transitions ~394nm (7F0–5L6) and~464nm (7F05D2) match very well with the emission bands of near-UV and blue LED chips. Here we review the Eu3+-doped binary-based pyrochlore-type oxides RE2M2O7 (RE=La to Lu, Y; M=Ti, Zr, Hf, Ce), in which the orange emission is dominant due to the allowed magnetic dipole transition (MD) (5D07F1) of the D3d inversion symmetry in the pyrochlore structure. Further, the quaternary pyrochlore system offers great scope for the structural maneuverability, symmetry distortion, and chemical environment to enhance the Eu3+ orange-red luminescence by various cation substitutions on both A- and B-sites. This chapter thus examines the results of the influence of structural transitions, crystal chemistry of A and B cations, and displaced pyrochlores on the Eu3+ luminescence in quaternary pyrochlore systems.