Hydrothermal Synthesis of Fibrous Vanadate Crystal in the System V2O5-H2O

Fibrous vanadate crystals of the system V205-H20 were synthesized hydrothermally at temperature ranging from 200 to 600°C under water vapor pressures of 50 to 400 atm for a duration within one month. A reagent grade V20, and its crystalline mass obtained by cooling after melting at 800°C for five hours were used as the starting materials. The products were examined by X-ray diffractometry, microscopic observation, infrared spectrometry, differential thermal analysis etc. A solubility curve of V205 in water at a constant pressure of 200 atm for two days showed that about 1.9g of V205 dissolved in water at 320°C and above 400°C its solubility decreased with increasing temperature. The length of the fibrous vanadate crystals increased with the treatment time below 320°C, but decreased above 400°C, at a constant pressure of 200 atm. Vanadate fiber length had a tendency to increase with the increase of temperatures and pressures in synthesis. The optimum conditions were found to be near 500°C and 200 atm for one hour treatment. The pH of the solutions in the reaction at 400°C and 200 atm for two days, affected the length of fiber. Long fibers were synthesized at the pH of 2.8, 4.2 and 6.0 after the reaction. X-ray powder diffraction patterns showed that V205 H2O (navajoite) was stable under present experimental conditions and that above about 500°C V20520 and an unknown phase with platy crystal habit were produced with navajoite. DTA curves showed that a large peak for the endothermic reaction near 610°C was attributed to the evaporation of bond water. An absorption band near 3500 cm-' on IR spectrum was assigned to the vo. or vH20 stretching vibrations and a band near 1000 cm-' to the vv =0 stretching vibration. A silky fiber (navajoite) having a maximum length of 3.6 mm and 1 pm in diameter was synthesized. The fiber was elongated along [010], and found to have a light brown color and flexibility.