Dependence of the photoluminescence properties of Eu2+ doped M–Si–N (M = alkali, alkaline earth or rare earth metal) nitridosilicates on their structure and composition

Abstract

Optical data of the Eu²⁺ doped nitridosilicates (M_xSi_yN_z) have been collected from the literature and have been analysed with regard to their dependence on structure and composition. Nitridosilicates with a higher degree of condensation, i.e. a higher Si/N ratio, generally have a higher Eu²⁺ 4f–5d absorption energy, a higher 5d–4f emission energy and a larger Stokes shift. The higher absorption and emission energies are due to the increase of the N by Si coordination number with increasing Si/N ratio. This results in more electrons on N that participate in the bonding with Si, and thus less electrons are available for Eu–N bonding, reducing the covalency of the Eu–N bonds. The lower covalency gives a weaker nephelauxetic effect, reducing the centroid shift of the 5d level. The lowest 4f–5d absorption energy further increases due to the reduction of the crystal field splitting of the 5d levels, as the Eu–N bonds become longer with increasing Si/N ratio. The Stokes shift increases with increasing degree of condensation despite an increase of lattice rigidity, ascribed to a decrease of local rigidity around the Eu²⁺ ion caused by the larger Eu–N bond lengths. Some nitridosilicates show deviations from the general trends attributed to peculiarities in their crystal structure and the way Eu²⁺ is substituted in the lattice. The relationships established in the present work will be helpful for the design and exploration of new Eu²⁺ doped nitride-based luminescent materials for practical applications.