The nephelauxetic effect on the electron binding energy in the 4f^q ground state of lanthanides in compounds

In the construction of a vacuum referred binding energy (VRBE) diagram with the lanthanide 4fq ground states, always a compound independent variation with the number q= 1 to 14 is assumed. Experimental data from thermo-luminescence, intervalence charge transfer bands, and thermo-bleaching studies provide first indications that a minor compound dependence does exist. To explain its origin we will first apply Jørgensen spin pairing theory to reproduce the VRBE in the ground states of the free di- and trivalent lanthanide ions which is equivalent to the 3rd and 4th ionization potentials of the lanthanide atoms. By combining experimental data and calculated trends therein, the relevant Racah E1, Racah E3, and spin orbit coupling ζff parameters for all di-, tri-, and tetravalent free ion lanthanides are derived. Using that as input for the spin pairing theory, the characteristic zigzag shapes in VRBE as function of q, as derived from ionization potentials, are nicely reproduced. Because of the nephelauxetic effect the parameter values are lowered when lanthanides are in compounds. How that reduction affects the VRBE curves will be treated in this work.