Eu²⁺-Doped Sr₂B_{2–2<i>x</i>}Si_2+3<i>x</i>Al_2–<i>x</i>N_8+<i>x</i>: A Boron-Containing Orange-Emitting Nitridosilicate with Interesting Composition-Dependent Photoluminescence Properties

Abstract

Novel Sr_2-yEu_yB_2-2xSi_2+3xAl_2-xN_8+x phosphors were investigated as a function of the boron and aluminum over silicon ratio and as a function of the Eu²⁺ concentration. Samples were prepared via solid-state reaction synthesis by carefully controlling the synthesis conditions and composition. At high boron and aluminum content, that is, x = 0, a Eu²⁺ 5d-4f emission is observed of which the maximum shifts from 595 nm for low Eu concentrations (y = 0.005) toward 623 nm for high Eu concentrations (y = 0.5). The samples can be excited by UV or blue light up to ∼475 nm. Substitution of [B₂Al]⁹⁺ units by [Si₃N]⁹⁺ units, increasing x up to 0.15, greatly improves the luminescence efficiency up to 46% and shows a very large redshift of the excitation bands with ∼100 nm, while the emission band shifts with ∼10 nm. The shifts are attributed to the lowering of the 5d level as a result of the decreased Eu-N distance upon substitution. Temperature-dependent measurements show that the Eu²⁺ 5d-4f emission is largely thermally quenched at room temperature for x = 0 due to thermal ionization toward the conduction band, explaining the low luminescence efficiency. The lowering of the 5d level at larger values of x reduces the thermal ionization and consequently increases the thermal stability and quantum efficiency, resulting in strongly luminescent blue-to-orange conversion phosphors that are interesting for light-emitting diode applications.