Micro- and nanostructured layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2.

Pure micro- and nanocrystalline powders of the layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2 have been successfully prepared and thoroughly characterised. Microstructured samples (BaZn3-MPs) have been produced by hydrothermal reaction using synthetic martyite Zn3V2O7(OH)2·2H2O as the starting reagent. Nanoparticles (NPs) with an average size of ≈ 60 nm (BaZn3-NPs-7h) or ≈ 50 nm (BaZn3-NPs-25min) have been obtained by using a coprecipitation method at ambient pressure, and by varying the stirring time. Rietveld refinements of X-ray diffraction data indicate that micro- and nanostructured BaZn3(VO4)2(OH)2 both crystallize in a R3̄m structure very similar to that of the known layered-kagome compound BaCo3(VO4)2(OH)2. Transmission electron microscopy observation of BaZn3-NPs-7h and BaZn3-NPs-25min reveals crystallized NPs with homogenous distributions of Ba, Zn, and V elements. FT-IR and Raman spectra show subtle differences between micro- and nanostructured samples which cannot be linked to any differences in the average crystal structures. The high resolution 51V MAS NMR spectrum of BaZn3-MPs shows a single isotropic line attributed to VO43- groups with C3v point group. The spectra of the nanostructured samples reveal the presence of a weak additional signal which decreases in intensity with increasing the NPs size, and which has been tentatively assigned to the presence at the surface of the NPs of a small amount of V5+ ions in a different chemical environment. Nanostructuring also impacts the optical properties of BaZn3(VO4)2(OH)2. The UV-vis absorption spectra of NPs exhibit an additional weak transition in the visible domain which is not observed for the microstructured sample.

Microstructured layered-kagome BaCo3(VO4)2(OH)2 with variable crystallite size: alternative synthetic route and comparison with nanostructured samples.

Microplatelets of the layered-kagome compound BaCo3(VO4)2(OH)2, which is the Co2+ analogue of mineral vesignieite BaCu3(VO4)2(OH)2, have been prepared with very high yield by hydrothermal reaction using synthetic karpenkoite Co3V2O7(OH)2·2H2O as starting reagent. The Rietveld refinement of X-ray diffraction data indicates that Co3V2O7(OH)2·2H2O is isostructural with martyite Zn3V2O7(OH)2·2H2O. Two single-phased samples of microstructured BaCo3(VO4)2(OH)2 have been characterized using powder X-ray diffraction, FT-IR and Raman spectroscopies, thermal analyses, scanning electron microscopy, energy-dispersive X-ray spectroscopy and magnetisation measurements. Their crystallite sizes perpendicular to the c-axis are in the range of 92(3) to 146(6) nm and depend on the synthesis conditions. Results have been compared to those previously obtained for quasi-spherical nanoparticles having a crystallite size of the order of 20 nm, to explore the effect of the crystallite size on the properties of BaCo3(VO4)2(OH)2. This study highlights that the magnetic properties depend on the crystallite sizes only at low temperatures.