Electronic structure and local environment of substitutional V (super 3+) in grossular garnet Ca (sub 3) Al (sub 2) (SiO (sub 4) ) (sub 3) ; K-edge X-ray absorption spectroscopy and first-principles modeling

The V K-edge spectrum of a V-bearing grossular garnet (tsavorite) single crystal has been measured by high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS). First-principles calculations based on the density functional theory (DFT) reproduce the experimental spectra well, including the angular dependence of the pre-edge. The latter arises from pure electric quadrupole transitions since vanadium is incorporated in a centrosymmetric site, which inhibits the electric dipole transitions. An interpretation of the spectral features is given using a monoelectronic description of the transitions involved in the X-ray absorption process. The assignment of experimental pre-edge peaks in terms of 1s-3d transitions makes it possible to determine the magnitude of the 3d-splitting, which is similar to the 10 Dq crystal-field value determined from optical absorption spectroscopy. DFT calculations also indicate a full structural relaxation around V (super 3+) substituting Al (super 3+) in the garnet structure. The angular dependence only causes weak variations of the pre-edge intensity, which supports the use of V-bearing grossular garnet as a reference compound for octahedral V (super 3+) .