Revealing the effect of interstitial oxygen on the low-energy crystal electric field excitations of Pr3+ in 214 -nickelates

We report an inelastic neutron scattering study (INS) on the low-energy crystal electric field (CEF) excitations of ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ single crystals at various temperatures. The observed low-$E$ CEF level of the O-doped sample $(x=0,\phantom{\rule{4pt}{0ex}}\ensuremath{\delta}\ensuremath{\approx}0.24)$ at $\ensuremath{\sim}5.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ appears at significantly lower energy than that of the Sr-doped sample $(x=0.5,\phantom{\rule{4pt}{0ex}}\ensuremath{\delta}=0.0)$ at $\ensuremath{\sim}8.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$. Applying the point charge (PC) model calculation, this has been interpreted as an effect of the interstitial oxygen via lowering the local symmetry and modifying the CEF environment of the central rare earth ${\mathrm{Pr}}^{3+}$ $(^{3}H_{4})$ ions.