Novel Red Phosphors KBaEu(XO) (X = Mo, W) Show High Color Purity and High Thermostability from a Disordered Chained Structure

Two novel red phosphors KBaEu(XO) (X = Mo, W) have been synthesized by high-temperature solid-state reactions and the crystal structures were determined for the first time. Single-crystal X-ray diffraction data reveal that their space groups are C2/c. The crystalline structure is constituted of K/BaO distorted square antiprisms and distorted EuO polyhedra which form chains lying along the c-axis and two kinds of distorted XO tetrahedra. This high disorder of K/Ba which might lower the crystal field symmetry around Eu results in the high purity of red emission around 615 nm originating from D → F transition under near-ultraviolet (NUV) excitation. With increasing temperature, the luminescence of KBaEu(XO) (X = Mo, W) phosphors decreases almost linearly with subtle alteration for the CIE coordinate. As the temperature reaches 550 K, the red emission intensity decreases to 37.3% and 50.7% of that at 300 K for KBaEu(MoO) and KBaEu(WO), respectively. The analysis of the decay curves of the D → F emission at variable temperatures indicates the weak cross relaxation and non-radiative energy transfer between Eu ions. These results demonstrate that the investigated phosphors are attractive for application in high power NUV excited white LEDs.