Electronic structure of the kagome staircase compounds Ni₃V ₂O₈ and Co₃V₂O₈

The electronic structure of the kagome staircase compounds, Ni ₃V₂O₈ and Co₃V₂O ₈, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering. Comparison between the two compounds, and with first-principles band structure calculations and crystal-field multiplet models, provides unique insight into the electronic structure of the two materials. Whereas the location of the narrow (Ni,Co) d bands is predicted to be close to E_F, we experimentally find they lie deeper in the occupied O 2p and unoccupied V 3d manifolds, and determine their energy via measured charge-transfer excitations. Additionally, we find evidence for a dd excitation at 1.5 eV in Ni₃V₂O₈, suggesting the V d states may be weakly occupied in this compound, contrary to Co₃V₂O₈. Good agreement is found between the crystal-field dd excitations observed in the experiment and those predicted by atomic multiplet theory.