  In the cuprate and iron-pnictide systems, valence changes induce
high-temperature superconductivity while the local structural chemistry and
local spin order both independently generate the attractive interactions
responsible for the high transition temperature. We argue that together they
favor d-wave singlet superconductivity in the cuprates but s-wave singlet in
the pnictides. This difference arises from the existence of a large on-site
repulsion between carriers in the cuprates largely absent in the pnictides.
Fluorine is responsible for raising Tc significantly in some pnictides and in
the cuprates to 155K-168K, the highest achieved at ambient pressure. We propose
an experimental procedure for finding and fabricating the fluorinated cuprate
phase having that exceptional property.
