  In a recent contribution to this journal, it was shown that the transition
temperatures of optimal high-Tc compounds obey the algebraic relation, Tc0 =
kB-1{\beta}/\ell{\zeta}, where \ell is related to the mean spacing between
interacting charges in the layers, {\zeta} is the distance between interacting
electronic layers, {\beta} is a universal constant and kB is Boltzmann's
constant. The equation was derived assuming pairing based on interlayer Coulomb
interactions between physically separated charges. This theory was initially
validated for 31 compounds from five different high-Tc families (within an
accuracy of \pm1.37 K). Herein we report the addition of Fe1+xSe1-y and
Fe1+xSe1-yTey (both optimized under pressure) and AzFe2-xSe2 (for A = K, Rb, or
Cs) to the growing list of Coulomb-mediated superconducting compounds in which
Tc0 is determined by the above equation. Doping in these materials is
accomplished through the introduction of excess Fe and/or Se deficiency, or a
combination of alkali metal and Fe vacancies. Consequently, a very small number
of vacancies or interstitials can induce a superconducting state with a
substantial transition temperature. The confirmation of the above equation for
these Se-based Fe chalcogenides increases to six the number of superconducting
families for which the transition temperature can be accurately predicted.
