A crystal chemical study on the superconductivity and compressibility of LnFePnO (Pn = P, As) superconductors

The covalency fraction increases in an order of Ln-O < Ln-Pn < Fe-Pn, which favors the electron transfer from LnO layer to FePn layer, At the same time, the Ln-O bonds have a higher lattice energy density as well as bulk modulus than the Fe-Pn bonds in the compounds. As a result, either doping F− ions or increase the external pressure leads to an increase in the covalency fraction gradient. This also favors the electron transfer.208