Portion of spectra surrounding the Hf II line at 3399.8 Å for two stars in our sample. Black dots represent the observed spectra. The red solid lines represent the best fit, with their respective [Hf/Fe] shown on the right. Blue dash-dotted lines show synthetic spectra without Hf. Golden dashed lines represent synthetic spectra when N is removed. Grey dotted lines correspond to the synthetic spectra without either N or Hf. Orange shaded areas display a ±0.3 dex interval around the best fit. The N abundances shown are from Paper II.