Band Gap Engineering of New Lutetium-based Orthovanadates

A series of mixed orthovanadates with nominal composition Lu0.5R0.5VO4 (R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) were synthesized by a solid-state reaction method in the three-stage annealing process. The single-phase powders with smaller Eu, Tb, Dy, Ho, Er, Tm, Yb were obtained after the second calcined stage at 1200 C for 5h, while for large Ce, Pr, Nd, Sm needed a third calcining. Rietveld refinement analysis of the crystal structure confirms a zircon-type structure (space group I41/amd). Furthermore, a linear dependence of the obtained lattice parameters on the ionic radius of the substituted lanthanide R was observed in the mixed Lu0.5R0.5VO4 series. The lattice parameters optimized by ab initio methods are slightly smaller than parameters obtained by Rietveld refinement, which can be explained by inaccuracies in the description of f-electrons using density functional theory (DFT). The transmission and diffuse reflectance spectra of mixed compounds Lu0.5R0.5VO4 are similar to such spectra obtained for corresponding orthovanadates RVO4. Both obtained spectra indicated significant differences in transmission/reflection intensities between samples annealed at 1200 °C and 1400 °C, but the band gap width remained unchanged, suggesting an improvement their crystallinity. Both experimental and theoretical results reveal a barely noticeable reduction in the band gap observed with a reduction of the ionic radius of the lanthanides. A thorough analysis of the electronic structure of the materials and diffuse reflectance spectra suggests that the position of the f-levels of lanthanides is the determining factor in the formation of valence and conduction bands.