K5Eu(MoO4)4 red phosphor for solid state lighting applications, prepared by different techniques

Abstract

The influence of preparation techniques on the structure and luminescent properties of K₅Eu(MoO₄)₄ (KEMO) was investigated. KEMO phosphors were synthesized by three different techniques: solid state and sol–gel (sg) methods as well as the Czochralski (CZ) crystal growth technique. Laboratory powder X-ray diffraction (PXRD) studies revealed that all KEMO samples had a structure analogous to that of other high temperature α-K₅R(MoO₄)₄ palmierite-type phases (space group (SG) Rm). Contrary to laboratory PXRD data, electron diffraction revealed that the KEMO crystal grown by the CZ technique had a (3 + 1)D incommensurately modulated structure (super space group (SSG) C2/m(0β0)00) with the modulation vector q = 0.689b*. A detailed analysis of electron diffraction patterns has shown formation of three twin domains rotated along the c axis of the R-subcell at 60° with respect to each other. Synchrotron XRD patterns showed additional ultra-wide reflexes in addition to reflections of the R-subcell of the palmierite. However, the insufficient number of reflections, their low intensity and large width in the synchrotron X-ray diffraction patterns made it impossible to refine the structure as incommensurately modulated C2/m(0β0)00. An average structure was refined in the C2/m space group with random distribution of K1 and Eu1 in [M1A₂O₈]-layers of the palmierite-type structure. The dependence of luminescent properties on utilized synthesis techniques was studied. The emission spectra of all samples exhibit intense red emission originating from the ⁵D₀ → ⁷F₂ Eu³⁺ transition. The integrated intensity of the emission from the Eu^{3+ 5}D₀ term was found to be the highest in the crystal grown by the CZ technique. The quantum yield measured for KEMO crystals demonstrates a very high value of 66.5%. This fact confirms that KEMO crystals are exceptionally attractive for applications as a near-UV converting red phosphor for LEDs.