We have been working for sometime on the synthesis at high pressure (P <= 12.5 Gpa) and high temperature (T <= 1400 K) of new materials of the type MSr2RECu2O8 (RE = Rare Earth), which formally derive from YBCO (i.e., CuBa2YCu2O7) by replacing the [Cu-O-4] squares in the basal plane of the structure by [M-O-6] octahedra (M = Ru, Cr or Ir). The adequate formation of these cuprates, as majority phases, can only be performed in a particular and relatively narrow window of P and T, outside which they cannot be obtained pure or even obtained at all. These "optimum conditions" bear a remarkable Gaussian correlation with the rare earth ion size, the rare earth cation being at the center of the unit cell in the YBCO setting, and they do not follow the classic lanthanide contraction so often observed in the chemistry of those elements. Instead, interelectronic repulsion seems to play a major role in fixing the synthesis conditions. Moreover, the position of the Gaussian tip in the pressure-ionic radii space is also dependent on the transition metal that sits in the octahedron, in a way that seems related to the thermodynamic stability of their simpler oxides.