What Happens When Silver Contacts Pressed Sulphur Tablets?

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Discussion Overview

The discussion revolves around the chemical interaction between silver and pressed sulphur tablets, specifically focusing on the formation of silver sulfide (Ag2S) and the diffusion processes involved. Participants explore theoretical aspects, experimental observations, and the interpretation of X-ray diffraction (XRD) data related to this reaction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that Ag2S forms when silver contacts sulphur tablets, leading to observable black circles, but questions the subsequent diffusion processes of silver and sulphur.
  • Another participant suggests that while atomic radii can provide a crude estimate of diffusion likelihood, the actual dynamics are influenced by multiple factors, including lattice energies and interface characteristics.
  • Experimental methods such as Auger Electron Spectroscopy and Secondary Ion Mass Spectroscopy are proposed as more reliable means to determine the diffusing species, although they are noted to be expensive and less common than SEM and XRD.
  • Concerns are raised about the quality of the XRD data obtained, with one participant expressing difficulties in identifying Ag2S and other phases due to poor data quality.
  • A comparison is made to the Cu-Cu2O-O2 interface, questioning whether the size of oxygen as a diatomic molecule affects diffusion in a similar manner as in the silver-sulphur interaction.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the diffusion mechanisms and the interpretation of experimental data. There is no consensus on the specifics of the diffusion processes or the reliability of the XRD results.

Contextual Notes

Participants acknowledge limitations in their experimental methods and data quality, particularly regarding the depth of analysis in XRD and the identification of phases present in the samples.

osskall
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I was thinking about the reaction of silver and sulphur.

I've read that when copper is oxidised by oxygen, after the formation of the first film of Cu2O, it is actually the copper atoms diffusing out of the bulk copper through the Cu2O layer to the air, where they are oxidised.

Does anyone know what happens when silver is in contact with pressed sulphur tablets? I guess Ag2S forms instantaneously (we did this in a lab session and saw indeed black circles, but we didn't manage to interpret the X-ray diffractograms in a way that made sense), but what happens then? Can we exspect silver to diffuse through the Ag2S layer, or will sulphur diffuse through Ag2S towards silver and oxidise it? Of course, I can put a tablet on a silver plate and leave it for a year and see if the reaction has stopped because of complete formation of an Ag2S film, or if the silver plate seems porous or hollow (or vice versa the sulphur tablet). Could I deduce from ionic radiuses and silver sulphide's structure which species should diffuse through which medium?
 
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You can make a guess from the atomic radii (S is much smaller and lighter than Ag, so is more likely to diffuse across) and the crystal structure of Ag2S...but it would only be a very crude guess. The dynamics are more complex because they depend on lattice energies, atomic masses, the nature of the Ag - Ag2S - S interfaces and much more. So, my advice would be to not try to deduce the mechanism simply from such data.

The best way to determine what - if at all - was the diffusing species, is through experimental methods. Something like Auger Electron Spectroscopy, or Secondary Ion Mass Spectroscopy should work very well for this, but such instruments are often expensive, and hence uncommon. SEM and XRD are more common, and will give you pretty good info as well.

What was the problem with the XRD data ? I'm assuming you took powder (from different depths) diffractograms. Were you unable to identify the Ag2S phase itself ? I know that Ag2S is distinctly crystalline, with some kind of cubic structure - unlike say, Al2O3, which under some conditions is quite amorphous. Could you see peaks that corresponded to Ag (by comparing with the diffractogram from deep within the Ag plate) , S (likewise) and Ag2S (from some XRD database) ? I can't see what exactly the problem here might have been.
 
Gokul43201 said:
What was the problem with the XRD data ? I'm assuming you took powder (from different depths) diffractograms.

No, much more crude than so.
I really can't see how I would succeed in accessing different depths, scraping a powder is one thing.
But, no, we just put the silver plate with the black Ø~0.5 cm spot in the diffractometer and looked for suitable PDF files. Some peaks could possibly be explained by some elementary sulphur phases but the data were too bad to draw any safe conclusion. I recall that one time (I think I did the XRD analysis twice) (different samples) I maybe found Ag2S among other peaks that I could not explain.

I was just thinking about the Cu-Cu2O-O2 interface, where copper actually diffuses out to the oxygen. Should't the radius of oxygen also be smaller than that of Cu?
 
It certainly is, if you consider only oxygen atoms. But oxygen exists as a diatomic molecule. The size of the molecule is only marginally smaller than that of a Cu atom. Still, this shows why you can't simply go by the size.