Preparation, Electronic Structure, and Photoluminescence Properties of Eu2+- and Ce3+/Li+-Activated Alkaline Earth Silicon Nitride MSiN2 (M = Sr, Ba)

The electronic structure of alkaline-earth silicon nitride MSiN 2 (M = Sr, Ba) was calculated using the CASTEP code. BaSiN 2 is calculated to be an intermediate band gap semiconductor with a direct energy gap of about 2.9 eV, while SrSiN 2 is an intermediate band gap semiconductor with an indirect energy gap of about 3.0 eV. As expected, the calculated optical band gaps of MSiN 2 (M = Ba, Sr) are lower compared to the experimentally determined values (about 4.1 eV for BaSiN 2 and 4.2 eV for SrSiN 2 ). In addition, the luminescence properties of Eu 2+ and Ce 3+ in MSiN 2 (M = Sr, Ba) have been studied. Ba 1− x Eu x SiN 2 (0 < x ≤ 0.1) shows a broad emission band in the wavelength range of 500–750 nm with maxima from about 600 to 630 nm with increaseing Eu 2+ concentration, while Sr 1− x Eu x SiN 2 (0 < x ≤ 0.1) shows a broad emission band in the wavelength range of 550–850 nm with maxima from 670 to 685 nm with increasing Eu 2+ concentration. The high absorption and strong excitation bands of M 1− x Eu x SiN 2 (0 < x ≤ 0.1; M = Sr, Ba) in the wavelength range of 300–530 m are very favorable properties for application as light-emitting-diode conversion phosphors. Ce 3+ - and Li + - codoped MSiN 2 (M = Sr, Ba) exhibits a broad emission band in the wavelength range of 400–700 nm with a peak center at about 485 nm for BaSiN 2 and about 535 nm for SrSiN 2 . A comparison is made between the luminescence properties of Eu 2+ and Ce 3+ in the Sr versus Ba compounds. The long-wavelength excitation and emission of Eu 2+ and Ce 3+ ions in the host of MSiN 2 (M = Sr, Ba) are attributed to the effect of a high covalency and a large crystal field splitting on the 5d bands of Eu 2+ and Ce 3+ in the nitrogen coordination environment.