Nuclear dynamics in BaZr_0.7Ce_0.2Y_0.1O_3−δ proton conductor as observed by neutron diffraction and Compton scattering

Abstract

Concurrent neutron Compton scattering (NCS) and neutron diffraction experiments at temperatures between 70 K and 300 K have been performed on proton-conducting hydrated BaZr 0.7 Ce 0.2 Y 0.1 O 3−δ (BZCY72) fabricated by spark plasma sintering. A combined neutron data analysis, augmented with density functional theory modelling of lattice dynamics, has enabled, for the first time, a mass-selective appraisal of the combined thermal and nuclear quantum effect on nuclear dynamics and thermodynamic stability of this technologically important proton conducting perovskite oxide. The analysis suggests that the nuclear dynamics in hydrated BZCY72 is a result of a subtle interplay of harmonic, anharmonic and thermal effects, with the increased anharmonic character of the lattice dynamics above the orthorhombic to rhombohedral phase transition at 85 K. The anharmonic effect seems to be most pronounced in the case of oxygen and cerium. The analysis of the proton momentum distribution reveals that the concentration of the hydrogen in the BZCY72 lattice is constant across the orthorhombic to rhombohedral phase transition and further down to the room temperature. Moreover, the average hydrogen concentration obtained from our analysis of the mass-resolved neutron Compton scattering data seems to be commensurate with the total vacancy concentration in the BZCY72 framework. The calculation of the vibrational enthalpy of both phases allows obtaining the value of the enthalpy of the orthorhombic to the rhombohedral phase transition of −3.1±1 kJ mol −1 . Finally, our analysis of the nuclear kinetic energy of the proton obtained from NCS and the oxygen-oxygen distance distributions obtained from ND allows to conclude that BZCY72 in both the orthorhombic and rhombohedral phase at 70 K and 100 K respectively falls into the category of the KDP-type crystals where proton is probably under the influence of a double-well potential and forms hydrogen bonds of moderate strength. The obtained results have important ramifications for this technological important material. ( ) ·· · Materials most commonly used are solid-state solutions of Y-doped BaCeO 3 and BaZrO 3 [8-11]. Both endmembers exhibit outstanding protonic conductivities up to 10 -2 S cm −1 at 450°C [12].between chemical stability and easy fabrication, respectively offered by the end-members, special interest is taken in the composition BaZr 0.7 Ce 0.2 Y 0.1 O 3−δ (BZCY72) [6,13].In perovskite-type proton conductors, protons are not a part of the oxide lattice nor their stoichiometry. Instead, they are present as hydroxide defects, also known as protonic defects which they form together with oxide ions in equilibrium with an ambient stable hydrogen carrier. In most cases, water vapour is absorbed by the surface and dissociates into a hydroxide ion, which fills an oxygen vacancy, while the remaining proton forms a covalent bond with the surface oxygen [14][15][16].The crystal structure of BZCY72 has been precisely worked out by Mather et al employing combined highresolution neutron ...

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