MBe20N14:Eu2+ (M = Sr, Ba): Highly Condensed Nitridoberyllates with Exceptional Highly Energetic Eu2+ Luminescence

The isotypic nitridoberyllates MBe20N14 with M = Sr, Eu, Ba have been synthesized at high temperatures under a N2 atmosphere. The crystal structures were solved and refined from single-crystal X-ray diffraction data (I4/m (no. 87), Z = 2, SrBe20N14:Eu2+, a = 6.525(2), c = 12.133(4) Å; EuBe20N14, a = 6.5241(8), c = 12.136(2) Å; BaBe20N14:Eu2+, a = 6.540(6), c = 12.214(11) Å). MBe20N14 contains a rigid network of multiply edge-sharing BeN4 tetrahedra with a degree of condensation (i.e., atomic ratio Be/N) κ = 1.43. Eu2+-doped samples of MBe20N14 (M = Sr, Ba) show two emission bands in the blue-to-green spectral region: regular narrow-band blue Eu2+ emission (λem1 ≈ 426–428 nm, full width at half-maximum (fwhm) ≈ 22–25 nm/1180–1310 cm–1) and trapped exciton emission in the blue-green spectral region (λem2 ≈ 530–540 nm, fwhm > 100 nm). MBe20N14 show high chemical and thermal stability. For SrBe20N14, a wide band gap (Eg = 4.6 eV) and a high Debye temperature (θD = 1214 K) were calculated. Large band gaps, chemical and thermal stability, and rigid networks make nitridoberyllates a promising class of host compounds for narrow-band emitting light-emitting diode phosphors.