Significant oxide-ion conduction has been reported in the composition , a cationic deficient palmierite-type derivative of the 7H hexagonal perovskite polytype1. This material exhibits stacking of isolated tetrahedra (palmierite layers) and corner-sharing octahedra (perovskite layers) units along the crystallographic c-axis. Pronounced oxide-ion disorder and the resulting distribution of octahedral and tetrahedral units in the palmierite layers are prevalent2,3. This site disorder has so far been linked to the high oxide-ion conductivity of the base composition. Therefore, this behaviour calls for the investigation of the influence of chemical substitution upon this material.

Herein, we studied the series of Vanadium-modified , using a combination of neutron and X-ray diffraction, solid-state Nuclear Magnetic Resonance spectroscopy (NMR), and bond-valence site energy calculations, in order to understand the structure-property relations essential for their enhanced oxide-ion conduction.

A combined neutron and X-ray Rietveld refinement evidence the presence of both oxide-ion disorder and cationic split sites. At high-temperature, an oxide-ion rearrangement indicates that tetrahedralunits are the prevalent geometry in the palmieirite-layers for the base composition. Contrastingly, the composition (x = 0.3) shows no oxide-ion disorder in the palmieirite layer already at room temperature; thus, indicating the presence of only tetrahedra units in its palmierite layers on an average scale, and mimicking the high-temperature structure upon V-substitution. Based on MAS NMR spectroscopy data, we propose this observation is related to the preferred occupation of tetrahedral sites by .

The bond-valence site energy calculation on the average structure of series depicts that the dominant oxide-ion migration pathways are along the palmieirite-layers. And these oxide-ion migration pathways have two energy barriers that correlate to the occupancies of the average crystallographic oxygen sites in the palmierite layers. These results are contrasted to the disorder in oxide-ion sublattice and ion dynamics found from solid-state NMR studies.

 

1.​García-González, E., Parras, M. & González-Calbet, J. M. Crystal Structure of an Unusual Polytype: 7H-Ba 7 Nb 4 MoO 20. Chem. Mater. 11, 433–437 (1999).

2.​Fop, S. et al. High oxide ion and proton conductivity in a disordered hexagonal perovskite. Nat. Mater. 19, 752–757 (2020).

3.​Yashima, M. et al. High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides. Nat. Commun. 12, 556 (2021).