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Today I was making a pastille of Niobium Pentafluoride powder. The procedure includes a 6 ton squeezer, precision wipes, silica gel, filter papers and acetone.

The first suspicious part was that on product details, $NbF_5$ was suppossed to be $5\mu m$ powder. Instead it looked like having hair(forgive my bad terminology but I am not a chemist, but a physicist and I don't know how to describe it).

The procedure is the following.

  1. Acetone and wipe papers$\rightarrow$clean squeezers parts(made of stainless steel)
  2. Powder gets into the squeezer(using a funnel made of the filter paper) and gets pressed
  3. The powder becomes a pastille.
  4. It is stored in a plastic bottle, with Silica gel.

The filter paper is also used to fill the bottle with silica gel(as a funnel) as well as a way to store the pastille while filling the bottle with silica gel.

I have made several pastilles with the exact same recipe(i.e. $B,\;ZnF_2,\;LiF$ etc) and everything went fine. This powder however became very sticky. I have also noticed that while it's stored in the silica gel box, there are some drops inside the bottle.

So I have some questions... First of all why did it became sticky and secondly why the powder wasn't actually a powder?Is it typical for $NbF_5$?

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1 Answer 1

up vote 2 down vote accepted

From the properties block on the Wikipedia page for $\ce{NbF4}$:

Niobium (V) fluoride is hygroscopic - it absorbs water from the atmosphere. This hygroscopicity is your main problem. When the powder absorbs water, it probably is becoming sticky.

The added water is probably also providing the extra mobility the ions need to crystallize from the powder. Niobium (V) fluoride forms monoclinic crystals, which on a very small scale could look like hair or needles.

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$\ce{NbF5}$ also seems to have a tendency to oligomerize/polymerize, as suggested by the tetrameric structure displayed on the Wiki. The only article I found in a quick search that directly mentions $\ce{NbF5}$ polymerization is this (in the abstract). –  Nicolau Saker Neto Feb 7 at 11:45
    
I knew it was hygroscopic, but I didn't know that it can become sticky! Is this typical for hygroscopic powders? I addition, the provider should have have it enclosed on a special container to prevent moisture, isn't that right? Can I request my money back? –  Thanos Feb 7 at 11:58
    
Well, potassium hydroxide is so hygroscopic it will keep on absorbing water until it is a solution. It may be worth contacting the supplier, but I'm not sure how far it will go. –  Ben Norris Feb 7 at 12:02
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