Chemical Properties
black powder(s); monoclinic, a=1.058 nm, b=0.658nm, c=1.388nm; sensitive to moisture; obtained by reaction of excess I2 with Nb metal in a sealed tube [STR93] [KIR81]
Preparation
A vertical tube of Vycor or Pyrex glass (I.D. approximately 23 mm., wall thickness 2.5 mm.) is charged with 4-12 g. of Nb metal (either solid or powder). A dense glass wool plug is placed over the charge and approximately 20 cm. from the closed tube end, followed by a 20% excess of pure, resublimed I
2 powder. The tube and its contents are thoroughly degassed in a high vacuum and melt-sealed under vacuum. Then the reactor is placed in a slightly inclined position (with the Nb metal at the higher and the I
3 at the lower end) and heated by means of two separate tubular electric furnaces (these meet at the center of the tube). The niobium is heated to 300℃ and the I
2 first to 180℃ and then to 250℃. The reaction is nearly quantitative after 10-15 hours and Nbl
5 crystals collect in the transition zone between the two temperature regions. The yields are lower with Nb powder than with solid Nb. The reactor is broken at the center, the Nbl
5 is removed under anhydrous conditions, and repeatedly rinsed with dry petroleum ether (under N
2) until the adhering I
2 is removed and the petroleum ether stays colorless. The traces of petroleum ether are evaporated in vacuum.
