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

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.