Crystal-field and Racah parameters of Mn4+ ion in red and deep red-emitting phosphors: Fluoride versus oxide phosphor

Abstract

A variety of rare earth-free phosphors enabling light emission in the red spectral region have drawn much interest for use in creating a warm white-LED device with a high color rendering index and a high luminous efficacy. The Mn4+-activated phosphor properties are discussed in detail from an aspect of spectroscopic point of view. First, we classify the various host materials used in red or deep red-emitting phosphors from a different kind of host materials and from a view point of the photoluminescence (PL) and PL excitation (PLE) properties. The ever-reported Mn4+-activated phosphors can be divided into five groups, namely, fluorides, oxides, oxyfluorides, and two fluorine-based compounds. They are then classified into totally eleven subgroups from an aspect of the spectroscopic properties, e.g., whether the zero-phonon emission peak can be clearly observed or not. Next, the PL and PLE properties of the Mn4+-activated “fluoride” and “oxide” phosphors as representatives of those phosphor groups are discussed in more detail. Finally, a method is proposed for determining reliable crystal-field (Dq) and Racah parameters (B and C) with drawing an attention to difficulty in the exact estimation of such parameters using a ligand field theory. The intra-d-shell electronic states, such as 2Eg, 4T2g, and 4T1g, of Mn4+ ions activated in the fluoride and oxide hosts are obtained and plotted against Dq in the Tanabe−Sugano diagram.