Role of the doping level in localized proton motions in acceptor-doped barium zirconate proton conductors

Abstract

Acceptor-doped barium zirconates are currently receiving considerable interest because of their high proton conductivity at intermediate temperatures, making them applicable as electrolytes in various electrochemical devices, but the mechanism of proton conduction is unclear. Here, we investigate the role of the acceptor-dopant level in the localized proton motions, i.e. proton transfers between oxygens and O-H reorientations, in hydrated samples of the proton conducting, acceptor-doped, perovskites BaZr1-xInxO3-x/2 with x = 0.10 and 0.20, using quasielastic neutron scattering (QENS). Analysis of the QENS spectra reveals that several proton transfer and O-H reorientational motions contribute to the QENS signal, as a consequence of the locally disordered nature of the structure due to the In doping of these materials, and establishes a generic and complex picture of localized proton dynamics in acceptor-doped barium zirconate based proton conductors. A comparison of the QENS results with vibrational spectroscopy data of the same materials, as reported in the literature, suggests a predominance of O-H reorientational motions in the observed dynamics. The highest doping level corresponds to a more distorted structure and faster dynamics, which thus indicates that some degree of structural disorder is favourable for high local proton mobility.

Proton jump diffusion dynamics in hydrated barium zirconates studied by high-resolution neutron backscattering spectroscopy

“…approaching the Q-range in our preceding ToF experiments. [5][6][7] The detriment of NBS in comparison to NSE is the somehow reduced neutron ux on the sample making the characterization of some of the relaxation signals challenging, as we show hereaer.…”

“…Their hydrogen concentration, as determined from neutron diffraction with polarization analysis, are 0.34(3) (10Y:BZO), 0.18(2) (10Sc:BZO), 0.10(1) (10In:BZO), 0.20(1) (20In:BZO), and 0.19(1) (25In:BZO) mol mol Ba À1 . 6,7,10 The NBS experiments were conducted at the IN16B spectrometer of the Institut Laue Langevin (ILL) in Grenoble, France. 11,12 With an incident wavelength of 6.3Å and the Si(111) unpolished analyzer conguration a resolution of 0.9 meV and a maximum Q of 1.9Å À1 were accessed.…”

“…On the other hand, we may consider the rst localized vibrational states leading to pronounced peaks in the vibrational density of states at energies of 10-20 meV (ref. 7) as some characteristic ħu E . In this case the effective mass M approaches the mass of the OH group.…”

“…4 We showed in a series of preceding QENS experiments based on the time-of-ight (ToF) technique, that the response of the local dynamics of protons in Sc-, Y-and In-doped BZO is reminiscent of a jump motion between two and four equivalent sites. [5][6][7] These dynamics correspond to translations of protons between two adjacent oxygens and O-H reorientations, respectively. To access the required momentum transfers (Q) for monitoring the local dynamics the resolution of the spectrometers had to be relaxed to 0.2-0.6 meV.…”

Local structure and vibrational dynamics in indium-doped barium zirconate

“…At sufficiently high temperatures, typically above 300 K, the protons can diffuse throughout the lattice through a two-stage mechanism that involves the transferring (hopping) of a proton from one oxygen to another, neighboring, one and a reorientational motion of the -OH group between such a transfer. [6][7][8][9][10][11][12][13] However, fundamental questions encompassing the defect chemistry of these materials remain, especially in regard to the local coordination environment and dynamics of protons and in general the role of dopant atoms for the proton transport properties.…”