FULL MULTIPLE-SCATTERING AND CRYSTAL-FIELD MULTIPLET CALCULATIONS PERFORMED ON THE SPIN TRANSITION FE-II(PHEN)(2)(NCS)(2) COMPLEX AT THE IRON K(2,3) AND L(2,3) X-RAY-ABSORPTION EDGES

The pseudooctahedral Fe-II(phen)(2)(NCS)(2) (where phen = 1,10-phenanthroline) complex is well-known for its changing spin state under external perturbations like temperature. In this paper, a detailed X-ray absorption study of the thermally induced spin transition at the iron K and L(2,3) edges is presented. The XANES features at both edges are extremely sensitive to the spin state. But insofar as the spin transition is associated simultaneously with a change in the magnetic state, S = 2 reversible arrow S = 0, and a modification of the atomic arrangement, the attribution of strong changes in XANES spectra is not obvious. In order to sort out the nature of these modifications, we have performed calculations of the X-ray absorption cross sections at both edges. In the framework of the multiple scattering theory, we have calculated the iron K edge spectra of both spin states, and by using the multiplet picture in the intermediate crystal field framework, we have simulated the iron L(2,3) edge spectra of both spin states. We show that the main XANES changes at the K edge are related to the structural modifications around the iron, whereas those at the L(2,3) edges are due to the change of the occupancy of the 3d levels split by the pseudooctahedral crystal field. Furthermore, we show that multiplet calculations give precise information about the ground state of iron(II) in O-h symmetry. We have determined the 10Dq crystal field strengths in both spin states and compared them with those determined by optical spectroscopy. In the low-spin state, both visible and X-ray spectroscopies give comparable values, but we observe a large discrepancy in the high-spin state. Analyzing the LS terms of the ground state and their energies for both spin states, we discuss, in the case of the 3d(6) iron ion, the effect of the 10Dq values introduced in the multiplet calculations in reproducing experimental spectra and explaining the discrepancy between 10Dq values extracted from the two spectroscopies for the high-spin state.