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?