two chemical potential axes. The projection scheme is illustrated for Cu 2 HgGeTe 4 in Figure . While the projection of the stability region is fixed for ternary materials, the shape of the projected stability region for quaternary materials can vary with the chemical potential of the fourth species. For example, the projection of the phase stability region of Cu 2 HgGeTe 4 when ∆µ Te = 0 eV (Figure ) is different from the projection when ∆µ Te = 0.2 eV (Figure ).