TO(Γ) mode resonances in the rare-earth nitrides

Far infrared measurements of the absorption into the IR-active TO(Γ) mode are reported for a selection of the rare-earth mononitrides. The frequencies harden as anticipated by ≈15% as the lattice constant shrinks by ≈6% from SmN to LuN, though they are typically ∼25% softer than the LSDA+U prediction published a decade ago. The data are in much closer agreement with our computation based on more recent software for two in the series: GdN and LuN. The resonances show surprisingly heavy damping, diminishing from the lighter to heavier rare earth elements.