75As NQR signature of the isoelectronic nature of ruthenium for iron substitution in LaFe1-xRuxAsO

We have investigated the effects of the isovalent ruthenium substitution
in LaFe1−xRuxAsO, by extensive 75As NQR (nuclear
quadrupole resonance) measurements, supported by DFT (density
functional theory) calculations, in order to characterize both
the lattice and electronic structure details. The evidence for five
different local configurations around the arsenic site emerges
upon increasing ruthenium for iron substitution. DFT calculations
confirm the attribution of the measured electric field
gradients (EFGs) to ruthenium atom occupancies (0, 1, 2, 3, and
4) on the nearest-neighbour sites of arsenic. It is found that the
low-frequency (Ru-free) NQR peak remains almost unaffected
upon ruthenium substitution, providing an experimental confirmation
that ruthenium does not introduce delocalized carriers
in the iron plane.