How do good metals go bad?

So-called 'bad metals' are materials right at the border between metal and insulator: tiny changes in chemical composition turn the metal into an insulator - or vice versa. Until now, it seemed that these 'bad metals' could not be explained with conventional theories. But in fact these metals are not that 'bad' after all. Upon closer inspection, their behavior fits in perfectly with what we already knew about metals.

from Latest Science News -- ScienceDaily https://www.sciencedaily.com/releases/2021/03/210315110245.htm