synthesis of PGE compounds (and some others), but what to do about osmium?

[Brian Joy]

I don’t think I’ve ever described a large set of synthesis experiments that I undertook in 2014.  I grew a variety of compounds in open-ended silica glass tubing (one end melted shut with a torch) using B₂O₃ as a flux and liquid “encapsulent.”  I've attached the paper from which I got the idea.  The B₂O₃ easily wets the surfaces of the reactants and glass tubing and at least mostly prevents interaction with the atmosphere.  The major advantage of this technique is that it doesn’t require the tube to be sealed under vacuum.  B₂O₃ is hygroscopic and reacts eventually to form boric acid; it must be dehydrated prior to use by heating to ~400°C for a few hours (melting point is ~450°C).  The B₂O₃ is simply added to the open capsule along with the reactants; the volume of B₂O₃ added should be relatively large compared to the reactants.  Upon completion of a synthesis run, the B₂O₃ can simply be dissolved/softened in water.  Using this technique, metals can easily be reacted with Sb, Bi, Se, or Te.  (I’ve tried As, but I don’t recommend it.)

I undertook the project because I obtained some bottles of Ru, Rh, Pd, Ir, and Pt in “sponge” form.  When I mounted the sponge itself, I found that the particle size was so tiny and the amount of void space so large that the metals simply weren’t useable in this form.  So I reacted them!  It’s one of my favorite things to do!  In addition, I often analyze for PGE, and so I need reliable standard materials for them.

Because the system in the tube is somewhat “open,” there can be some difficulties with mass fractions changing due to slow escape of a “volatile” component such as Se or Te.  For this reason, it may be necessary to rely on saturation in a given component element to produce a stoichiometric compound.  For instance, Ag₂Se is stoichiometric when the system is saturated in Se.  Further, it’s important not to exceed the boiling or sublimation point of any of the component elements or the desired compound; for the example of Ag₂Se, the boiling point of Se is 685°C.  Also, I should note that, not only is Ag₂Se easy to grow, but, unlike Ag₂S, it holds up well under the beam.

Among some of the useful, stoichiometric compounds I grew were these:
RuSb₂
RhSb₂
PdSb₂
PdTe₂ (merenskyite)
PdBi₂ (froodite – subject to oxidation over time)
IrSb₂
IrTe₂ (shuangfengite)
PtSb (stumpflite)
PtSb₂ (geversite)
PtTe₂ (moncheite)
AuTe₂ (calaverite)
AuSb₂ (aurostibite)
Ag₂Se (naumannite)
Ag₂Te (hessite)
Ag₃AuTe₂ (petzite)
PbSe (clausthalite)
PbTe (altaite)

Notice that Os is conspicuously missing, as I had no loose sponge available.  Further, I don’t know to what extent it would react to produce OsO₄, which is volatile and deadly poisonous in small amounts in air.  Because of this, I’ve continued to use some very poor already-mounted Os sponge as a standard.  It really doesn’t work that well.

So now I finally get to the question I want to ask, which is this:  What do others of you use as an osmium standard?  Osmium produces some natural compounds, such as osarsite (OsAsS), omeiite (OsAs₂), and erlichmanite (OsS₂) and it is often present in significant quantity in laurite (nominally RuS₂).  Has anyone come across a well-characterized, homogeneous Os-bearing compound of some sort that might be useful as a standard?

[crystalgrower]

Hi Brian, there is a dealer doing business online as Element Sales.  He has a custom setup to melt metals including Os in vacuum and his products are free of oxidation  and inclusions.  Users should ask him to quote his  price for his scraps as they are longish pieces off the fused buttons.   In my direct experience, reasonable price and at least 4N quality.  He keeps the certificates of analysis for his raw materials. 

[Probeman]

You can get osmium metal as "pieces or shot" from ESPI (just down the road from us in Oregon!):

https://www.espimetals.com/index.php/msds/225-Osmium

[Brian Joy]

Hi John and Irene,

Thanks for the suggestions.  I wonder how well Os metal polishes, as it is very hard -- Mohs hardness = 7; then again, I've polished rhenium before.  Does it develop a film of OsO₄ over time?

Brian

[crystalgrower]

Brian, the Element Sales Os polished very well using normal procedures.  It  did not appear to form a carbide by sitting in 20 year old mounts.  Don't know about beam damage  but you need somebody who does a lot of PGE mineral work to say if that happens.

If Os is free of O2 when mounted it remains free of all kinds of crap that would show up by SEM.  I saw a lot that  had been fused (wrong) to solidify it, it had cracks of OsOx (x<<4) running through it.   

FYI I was reading through my stored papers.  Have to find the reference again but some work has been published using pure Os as the calibrator for trace minerals.