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

Infrared diffuse reflection spectra of O–H(νOH) and O–D(νOD) 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 νOH 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 νOH 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 νOH bands were observed at low wave numbers in the order of Sr(Zr,MIII)O3−δ<Ca(Zr,MIII)O3−δ<Ba(Zr,MIII)O3−δ, i.e., hydrogen bonding strength of the dissolved protons is found to be in the order of Ba(Zr,MIII)O3−δ<Ca(Zr,MIII)O3−δ<Sr(Zr,MIII)O3−δ. The average distances between two oxygen atoms, rO–O, in the original alkaline-earth zirconates are reported as rO–O(SrZrO3)<rO–O(CaZrO3)<rO–O(BaZrO3). Strong hydrogen bonding was observed for the alkaline earth zirconate with short rO–O. It was concluded that the chemical environment of protons dissolved was primarily affected by crystal structure of the original zirconates.