I think this is refering to Goldschmidt's Rule, which is an approximate rule of thumb telling you that substitutions that conserve oxidation state and ionic radius (approximately) introduce least strain in a crystal. This is something of an unwritten philosophy for a synthetic inorganic chemist or materials scientist. Pursuant to Goldschmidt's intuition was the excellent and extensive work done by Shannon and Prewitt on the influence of oxidation state on the ionic radius in a crystal. Shannon-Prewitt tables are a crystal-maker's Bible.
V. M. Goldschmidt was an early geochemist who contributed to the rise of crystallography and crystal chemistry in the mid-1920s.
Now, I'm kinda sure that (Perennial's post) is what the OP wants. Ignore my previous post. Having had Goldschmidt sit in the back of my head for years made me think of his rule almost reflexively. Guess I should have Googled before posting to make sure there wasn't a Schmidt's Rule (I do not recall having seen it before).
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