Now, that is a good introduction, perhaps, to Mössbauer spectroscopy, but what you’ll realise, Brian, is having said that, that it is a solid state… you need a crystalline solid, perhaps at liquid helium temperature, certainly liquid nitrogen temperature, whereas most of my other work had been done on liquids. But this new interest in solids revived my interests in what I had been doing in Melbourne many years earlier. 

That was, you recall, on nonstoichiometry, and the effect in cuprous oxide, which had cuprous and cupric irons in the lattice. Copper, unfortunately, is not a Mössbauer element, but I realised that this would be a very good technique in principle for looking at the structure of defects in the solid – whether it was an interstitial site, whether it was a vacant site – a lattice vacancy, what the structure was around the vacancy, is it a vacancy cluster, what the actual energy and oxidation states were, of elements, when this was confused.

For example, if you have a mixed iron sulphide, is it ferrous and ferric, in what proportions? Are there other elements there as well? Something which you can’t get from simple chemical experiments on the solid, but Mössbauer would give a clear answer to.