“…Another peak at 1,622 cm −1 or 1,573 cm −1 is assigned to the bending mode of H-OH adsorbed on surface. 24 And two strong peaks at 1,488 and 1,394 cm −1 are possibly due to the oxygen ions' stretching vibrations and the bending mode of O-H. 25 , 26 When protons are injected into the lattice and interact with oxygen ions, the vibrational frequency of oxygen ions changes, making the double peaks degenerate and shift to a single peak with a shoulder. And the bands at around 686 cm −1 are assigned to the bend vibration of Eu- O -H. 27 Other signals at 1,065 cm −1 , 1,003 cm −1 , and 857 cm −1 may be assigned to C-O, C-H, and CH 2 , due to Eu2O3 easily absorbing carbon dioxide from the environment.…”