Phase Transitions, Chemical Expansion, and Deuteron Sites in the BaZr_0.7Ce_0.2Y_0.1O_3−δ Proton Conductor

Abstract

The crystal structure of the technologically relevant, high-temperature proton conductor BaZr0.7Ce0.2Y0.1O3- (BZCY72) has been studied by high-resolution neutron powder diffraction performed on a deuterated sample in the temperature range 10-1173 K, complemented with synchrotron X-ray diffraction in the range RT-1173 K. A volume discontinuity on heating indicates a first-order phase transition from orthorhombic (space group Imma) to rhombohedral symmetry ( 3 Rc ) between 85 and 150 K. A further transition to cubic symmetry ( 3 Pm m ) takes place at ~ 570 K, indicated to be second order from the temperature dependence of the octahedral tilt angle. The stability field of the cubic phase was extended on cooling in the dehydrated state to 85 K. Expansion/contraction of the unit-cell volume on heating in low vacuum and air, respectively observed by neutron diffraction and synchrotron X-ray diffraction, was described with a point-defect model involving the temperature dependence of the water content and thermal expansion. Isotropic strain in the hydrated state is apparent on analysis of the broadening of the neutron-diffraction reflections during heating and cooling cycles. Rietveld refinement of the low-temperature neutron data and Fourier nuclear-density maps were employed to locate the deuterium position at a distance of ~ 0.90 Å from the bonding oxygen at 10 K.

Nuclear dynamics in BaZr0.7Ce0.2Y0.1O3−δ proton conductor as observed by neutron diffraction and Compton scattering

“…Due to the disordered nature of BZCY72 structure [17], the supercell approach was adopted in modelling, as detailed in supplementary material. The supercell approach has enabled the calculation of both NCS-specific and thermodynamic observables.…”

“…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 diffraction and synchrotron x-ray diffraction [17]. The perovskite oxide (ABO 3 ) has a 12coordinated Ba cation on the A site and a 6-coordinated Zr, Ce or Y on the M site with the latter building the characteristic MO 6 octahedra.…”

“…The protonic defects are located at the corners of the octahedra. The O-H distance was experimentally determined in the range from 0.76-0.96 [17].It is generally believed that protons migrate within perovskites via the Grotthuss mechanism [19, 20], a combination of rotational diffusion of the protonic defect and transfer of the proton from oxygen to the nearest neighbour. From quantum molecular dynamics (MD) simulations [21] and quasi-elastic neutron scattering (QENS) experiments [22, 23] the reorientation process is characterised as fast (∼1 ps) with low activation energy.…”

“…The crystal structure of BZCY72 has been precisely worked out by Mather et al employing combined highresolution neutron diffraction and synchrotron x-ray diffraction [17]. The perovskite oxide (ABO 3 ) has a 12coordinated Ba cation on the A site and a 6-coordinated Zr, Ce or Y on the M site with the latter building the characteristic MO 6 octahedra.…”

“…The protonic defects are located at the corners of the octahedra. The O-H distance was experimentally determined in the range from 0.76-0.96 [17].…”