Site-Selective Eu3+ Luminescence in the Monoclinic Phase of YSiO2N

Abstract

Eu3+-doped YSiO2N phosphor was synthesized, and its crystal structure was analyzed by single-crystal X-ray and neutron powder diffraction techniques. The new crystal structure of YSiO2N with the monoclinic lattice (space group C2/c) composed of nonequivalent [YO6N2] dodecahedra was identified, and, in this structure, five different Y3+ sites take Ci or Cn symmetry. Based on the experimentally determined crystal structure, we characterized the luminescence properties of the Eu3+ ions with site-selective and time-resolved spectroscopy. The Eu3+ ions at the Y: Cn sites show intense 5D0 → 7F2 electric dipole luminescence peaking at 626 nm with a short lifetime (0.61 ms) due to the lack of inversion center. In the centrosymmetric Ci sites, the lifetime of the 5D0 level was quite long (5.51 ms), and the strongest three peaks are assigned to the 5D0 → 7F1 magnetic dipole transition instead of 5D0 → 7F2. The anomalous temperature dependence of the long lifetime for Eu3+: 5D0 at this Ci site at low temperatures suggests that the slow radiative rate can be influenced by the thermal distortion of centrosymmetry. The thermal quenching mechanism of the Eu3+: 5D0 luminescence at both sites is considered to be due to the charge transfer states related to the N3– ions in the [Eu3+O6N2] dodecahedra rather than the multiphonon relaxation mechanism. This study shows the potentials of Eu3+ luminescence in oxynitride coordination and provides new insights into the material design guidelines for near-UV excitable red phosphor activated with Eu3+ ions.