Phase diagram of hole-doped cuprates based on ¹⁷O and ⁶³Cu NMR quadrupole splittings

The phase diagram of the superconducting cuprates is often used to show how their electronic properties change as a function of the mean doping level, i.e., the average hole content of the CuO$_2$ plane. In Nuclear Magnetic Resonance (NMR) experiments average doping, as well as the distribution of these holes between planar Cu and O reveals itself through the quadrupole splittings of the $^{63,65}$Cu and $^{17}$O NMR. Here we argue based on all published NMR data available to us in favor a new type of phase diagram that has the planar oxygen quadrupole splitting and with it the planar oxygen hole content as abscissa rather than the average hole content of the CuO$_2$ plane. In such a plot the superconducting domes of the different cuprate families are shifted horizontally according to their maximum critical temperature $T_{\rm c,max}$ set by the chemistry of the parent material, which determines its oxygen hole content. The higher the O hole content the higher $T_{\rm c,max}$ that can be achieved by actual doping. These findings also offer a strategy for finding cuprates with higher $T_{\rm c,max}$.