Why are Pd and Pt completely soluble in Cu, but Ag only partly in Cu when forming a substitutional solid solution? If we look at the Hume-Rothery rules, they all are within the 15% atomic radius factor with respect to Cu and all have the same crystal structure:
The last two rules are:
Then Pd and Pt have an electronegativity difference of 0.3 while Ag has 0.
Pd and Pt have same valency while Ag has one less.
I would have expected that Ag would form a complete solid solution while Pd and Pt would only partly form a solid solution with Cu because difference of 0.3 in EN is quite large, because the valence difference between Ag and Cu is only 1 electron and because of the words: 'Other factors being equal,' it sounds like the other rules are more important.
To sum up, my thought process was:
Pd, Pt and Ag meet rule 1 and 2. Pd and Pt don't meet rule 3, so only form partly solid solutions. Then Ag meets rule 3 and rule 4 is less important and yet it is still close, so Ag will completely dissolve in Cu.
Why is this thought process wrong? If a difference in EN of 0.3 isn't big, what is then the border? Like 15% is the border for rule 1?
