Selective Synthesis of Monazite- and Zircon-type LaVO4 Nanocrystals

Abstract

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 L^4- = (CH₂COO)₂N(CH₂)₂N(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 850 °C, which is about 250 °C 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.