Inverse-Tunable Red Luminescence and Electronic Properties of Nitridoberylloaluminates Sr_2-x Ba_x [BeAl₃ N₅ ]:Eu²⁺...

The nitridoberylloaluminate Ba₂ [BeAl₃ N₅ ]:Eu²⁺ and solid solutions Sr_2-x Ba_x [BeAl₃ N₅ ]:Eu²⁺ (x=0.5, 1.0, 1.5) were synthesized in a hot isostatic press (HIP) under 50 MPa N₂ atmosphere at 1200 °C. Ba₂ [BeAl₃ N₅ ]:Eu²⁺ crystallizes in triclinic space group $$ (no. 2) (Z=2, a=6.1869(10), b=7.1736(13), c=8.0391(14) Å, α=102.754(8), β=112.032(6), γ=104.765(7)°), which was determined from single-crystal X-ray diffraction data. The lattice parameters of the solid solution series have been obtained from Rietveld refinements and show a nearly linear dependence on the atomic ratio Sr : Ba. The electronic properties and the band gaps of M₂ [BeAl₃ N₅ ] (M=Sr, Ba) have been investigated by a combination of soft X-ray spectroscopy and density functional theory (DFT) calculations. Upon irradiation with blue light (440-450 nm), the nitridoberylloaluminates exhibit intense orange to red luminescence, which can be tuned between 610 and 656 nm (fwhm=1922-2025 cm^-1 (72-87 nm)). In contrast to the usual trend, the substitution of the smaller Sr²⁺ by larger Ba²⁺ leads to an inverse-tunable luminescence to higher wavelengths. Low-temperature luminescence measurements have been performed to exclude anomalous emission.