Structural Chemistry, Spin Order, and the Distinction Between the   Cuprate and Pnictide High-Temperature Superconductors
  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. 
S. R. Ovshinsky