Natural linear dichroism at the Fe 2p absorption edge of gillespite

We observed a strong polarization dependence in the double peak structures at Fe2p absorption edges of BaFeSi4O10, which agrees with atomic multiplet calculations for a d6 5A1 ground state in square planar symmetry. The quadrupole moment is obtained from the sum rule for the integrated intensity of the linear dichroism. The analysis of the branching ratio and the details of the spectral shape give evidence confirming that the pressure induced structural phase transition in gillespite has a crystal field origin rather than being due to a high to low spin transition.