  Recently, an anomalous generation of multiple plasmons with large spectral
weight transfer in the visible to ultraviolet range (energies below the band
gap) has been experimentally observed in the insulating-like phase of
oxygen-rich strontium niobium oxides (SrNbO$_{3+\delta}$). Here, we investigate
the ground state and dielectric properties of SrNbO$_{3+\delta}$ as a function
of $\delta$ by means of extensive first principle calculations. We find that in
the random phase approximation by taking into account the local field effects
(LFEs), our calculations are able to reproduce both the unconventional multiple
generations of plasmons and spectral weight transfers, consistent with
experimental data. Interestingly, these unconventional plasmons can be tuned by
oxygen stoichiometry as well as microscopic superstructure. This unusual
predominance of LFEs in this class of materials is ascribed to the strong
electronic inhomogeneity and high polarizability and paves a new path to induce
multiple plasmons in the untapped visible to ultraviolet ranges of
insulating-like oxides.
