How strong the electron-phonon interaction in metals can be?

I’ll show that the dimensionless electron-phonon coupling λ cannot exceed a certain critical value in metals. Increasing λ beyond this value leads to a structural instability accompanied by a metal-insulator transition. This also implies an upper bound on the superconducting Tc in units of the characteristic phonon frequency in conventional superconductors. We’ll compare the upper bounds on λ and Tc with existing experimental data.