Selective synthesis of monazite- and zircon-type LaVO4 nanocrystals

Pure monoclinic (m-) and tetragonal phased (t-) LaVO4 nanocrystals could be obtained by a hydrothermal method in a controllable way with additives. It is found that chelating ligands, such as ethylenediaminetetraacetic acid [EDTA or H4L, where L4- = (CH2COO)(2)N(CH2)(2)N(CH2COO)(2)(4-)], favor the formation of t-LaVO4 and can induce the polymorph transformation from stable m-LaVO4 to metastable t-LaVO4. Further studies demonstrated the important roles of chelating ligands in this transformation process. Careful investigation over the phase transition from t- to m-LaVO4 was also conducted with high-temperature X-ray diffraction (HTXRD) studies. The phase transition occurred at 850degreesC, which is about 250degreesC higher than for the bulk. The enhanced thermal stability of the nanosized metastable t-LaVO4 may come from the small size effect. Our capability of obtaining and stabilizing t-LaVO4 not only benefits the wider applications based on LaVO4 due to the improved luminescent and catalytic performance but also provides a new idea in the studies of polymorph control and selective synthesis of inorganic materials.