Structural and optical characterization of Er-doped CaMoO4 down-converting phosphors.

xEr2O3-(1-x)CaMoO4 (x = 1, 3, 5, 7 and 10 mol%) nanoparticles were synthesized by solid-state sintering at 800°C. X-ray diffraction studies confirmed the tetragonal crystal structure of CaMoO4, while the doped samples show the co-existence of cubic Er2O3 and tetragonal CaMoO4 and rule out the replacement of Ca2+ by Er3+ in the structure. The crystal unit-cell dimensions, phase concentration and atomic position coordinates were determined by Rietveld refinement. The short-range structure of CaMoO4 consists of tetrahedral MoO4 and snub disphenoid deltahedral CaO8 units, while the unit cell of Er2O3 consists of two types of ErO6 octahedral units. All MoO4 units contain Mo-O bonds of equal lengths, whereas two types of slightly different Ca-O bond lengths exist in CaO8. Raman spectra of the doped samples show only Mo-O vibrational modes and the Raman peaks of Er2O3 are masked by Mo-O bond vibrations. CaMoO4 shows bluish-green emission at 500 nm, while Er-doped samples show strong green emission under UV excitation. UV irradiation (380 nm) induces down-conversion green emission at 531 nm and 552 nm and good color purity in 1 mol% Er2O3-CaMoO4 sample which makes it a potential candidate for applications in optical devices.