O-H stretching vibrations of proton conducting alkaline-earth zirconates

Infrared diffuse reflection spectra of O-H(ν_OH) and O-D(ν_OD) stretching vibration regions are used to characterize proton dissolution sites in high temperature proton conductors (HTPCs) of CaZrO₃, SrZrO₃ and BaZrO₃ doped with trivalent Ga³⁺, In³⁺ and Y³⁺ ions. The number of observed ν_OH band was three, five or six and two, respectively, for the CaZrO₃-, SrZrO₃- and BaZrO₃-based samples. It was inferred that the two lowest bands below 2000 cm^-1 observed for SrZrO₃-based samples were the ν_OH bands for hydrogen bonding of O. . .H. Therefore, the number of proton dissolution sites was concluded to be three for the CaZrO₃-based samples, three or four for the SrZrO₃-based samples and two for the BaZrO₃-based samples. The ν_OH bands were observed at low wave numbers in the order of Sr(Zr,M^III)O_3-δ<Ca(Zr,M ^III)O_3-δ<Ba(Zr,M^III)O _3-δ, i.e., hydrogen bonding strength of the dissolved protons is found to be in the order of Ba(Zr,M^III)O _3-δ<Ca(Zr,M^III)O_3-δ<Sr(Zr, M^III)O_3-δ. The average distances between two oxygen atoms, r_O-O, in the original alkaline-earth zirconates are reported as r_O-O(SrZrO₃)<r_O-O(CaZrO ₃)<r_O-O(BaZrO₃). Strong hydrogen bonding was observed for the alkaline earth zirconate with short r_O-O. It was concluded that the chemical environment of protons dissolved was primarily affected by crystal structure of the original zirconates.