Neel transition temperature, doped

In doped semiconductors I is due to direct overlap in transitional-metal oxides the overlap between the d-orbitals is frequentiy via the oxygen ions and is then often called a superexchange interaction. Figure 3.4 shows the kinds of wave function expected. As regards magnitudes, if B 1 eV, 17 10 eV and z=4, kBTN should be 0.01 eV so that 100K, which shows why low Neel temperatures are common. 

Raman investigations show another transition or cross-over at the Neel temperature (x = 6.22). Deconvolution of the apex-phonon width shows the coexistence of four phases in the nonstoichiometric range of 123. It is possible that physical phase separation starts at x = 6.22 with the formation of hole-doped islands in an AF matrix (sect. 3.3.2), the percolation to the superconducting phase taking place at x w 6.40. 

---