High-pressure X-ray Diffraction Study of SrSi₂O₂N₂:Eu²⁺

SrSi₂O₂N₂ oxynitride crystallizes in the P1 space group [1]. This compound is an excellent host for phosphors, due to its superior thermal and chemical stability and large energy bandgap. Eu-doped SrSi₂O₂N₂ can be used in green LEDs, and as a green component in phosphor mixtures; it is suitable, in particular, for white LEDs. For application in LEDs, the most important feature of SrSi₂O₂N₂:Eu²⁺ is the intense broadband luminescence at about 530 nm. In this study, high-pressure X-ray powder diffraction (XRD) experiments are used in order to experimentally determine, for the first time, the equation of state (EOS) of SrSi₂O₂N₂:Eu²⁺. The studied sample, with Eu content of 2% was prepared by solid state reaction. The in situ XRD experiment was performed at the I711 beamline of MAXII synchrotron (Lund, Sweden) for a sample mounted in a diamond anvil cell, using hydrostatic compression conditions. The applied X-ray wavelength was 0.9917 Å. In the pressure range studied (up to 9.6 GPa) the triclinic structure is found to be conserved. Lattice parameters a, b and c decrease smoothly but slightly anisotropically as a function of applied pressure, whereas the angles α, β and γ vary marginally. The material is the most compressible in the b direction and the least compressible in the a direction. Angles α and γ are almost constant whereas the value of β angle slightly increases with rising pressure. The variation of unit cell volume with pressure served for determination of the Birch-Murnaghan EOS: the resulting bulk modulus value is 103(5) GPa. The present bulk modulus value is by 22% smaller than those reported for other oxonitridosilicates such as SrSiAl₂O₃N₂ and Ce₄[Si₄O₄N₆]O (131.9(1) GPa and 131(2) GPa, respectively) [2].