Influence of annealing conditions on the structure and luminescence properties of KGd1−xEux(MoO4)2 (0 ≤ x ≤ 1)†
This study describes the influence of annealing temperature on the structure and luminescence properties of KGd1−xEux(MoO4)2 (0 ≤ x ≤ 1). Compounds with the general formula (A′, A′′)n[(W, Mo)O4]m are investigated as luminescent materials for photonic applications such as phosphor-converted LEDs (light-emitting diodes). Herein, the KGd0.8Eu0.2(MoO4)2 light-rose crystal was grown by the Czochralski technique. Moreover, three polymorphs of KGd1−xEux(MoO4)2 were present in the 923–1223 K range of annealing temperatures under ambient pressure: a triclinic α-phase, a disproportionately modulated monoclinic β-phase and an orthorhombic γ-phase with a KY(MoO4)2-type structure. The different behaviors of KGd(MoO4)2 and KEu(MoO4)2 were revealed by DSC studies. The number and the character of phase transitions for KGd1−xEux(MoO4)2 depended on the elemental composition. The formation of a continuous range of solid solutions with the triclinic α-KEu(MoO4)2-type structure and ordering of K+ and Eu3+/Gd3+ cations were observed only for α-KGd1−xEux(MoO4)2 (0 ≤ x ≤ 1) prepared at 923 K. The structures of γ-KGd1−xEux(MoO4)2 (x = 0 and 0.2) were studied using electron diffraction and refined using the powder X-ray diffraction data. The luminescence properties of KGd1−xEux(MoO4)2 prepared at different annealing temperatures were studied and related to their different structures. The maxima of the 5D0 → 7F2 integral emission intensities were found under excitation at λex = 300 nm and λex = 395 nm for triclinic scheelite-type α-KGd0.6Eu0.4(MoO4)2 and monoclinic scheelite-type β-KGd0.4Eu0.6(MoO4)2 prepared at 1173 K, respectively. The latter shows the brightest red light emission among the KGd1−xEux(MoO4)2 phosphors. The maximum and integral emission intensity of β-KGd0.4Eu0.6(MoO4)2 in the 5D0 → 7F2 transition region is ∼20% higher than that of the commercially used red phosphor Gd2O2S:Eu3+. Thus, β-KGd0.4Eu0.6(MoO4)2 is very attractive for application as a near-UV convertible red-emitting phosphor for LEDs.