| Summary: | Systematic synthesis of REVO4 nano/microcrystals (RE = La-Lu lanthanides) was performed via hydrothermal reaction under 200 °C and pH = 12 for 30 h, and the effects of malate (Mal2−) chelate and lanthanide contraction on phase structure and crystal morphology were revealed. With RE = Y0.95Eu0.05 as representative, the influence of solution pH, reaction temperature/duration and VO43- concentration was also systematically investigated. It was shown that, under the optimized VO43-:RE3+:Mal2− molar ratio of 1.5:1.0:2.0, the products are tetragonal (t-) structured REVO4 for RE = Ce-Lu while a mixture of monoclinic (m-) and tetragonal (∼10.8 wt%) phases for RE = La. Mal2− was demonstrated to promote crystallization of phase-pure REVO4 through suppressing RE3+ hydrolysis to form hydroxyl nitrate impurity. The t-REVO4 crystals were identified to be single-crystalline quasi-squares significantly exposed with high energy {001} facets (∼80% coverage for t-LuVO4) owing to the capping effect of Mal2-, and the lateral size of the crystals gradually increased from ∼20 to 850 nm with decreasing RE3+ size from Ce3+ to Lu3+. Aside from structure details and IR response of the full series of REVO4, photoluminescence features were analyzed for RE = Y0.95Eu0.05, Eu, Dy, Er and Tm, and the 600 °C calcined (Y0.95Eu0.05)VO4 red phosphor was determined to have favorably-high external and internal quantum efficiencies of ∼56% and 80%, respectively, a fluorescence lifetime of ∼1.45 ± 0.01 ms and color coordinates of around (0.67, 0.33) under 313 nm excitation.
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