A pressure induced reversal to the 9R perovskite in Ba₃MoNbO_8.5

Ba₃MoNbO_8.5is an oxide ion conductor with an unconventional hybrid crystal structure that is intermediate between the 9R-perovskite (A₃B₃O₉) and the palmierite (A₃B₂O₈). The crystal structure is highly disordered with vacancies distributed across two cation (M(1) and M(2)) and oxygen sites (O(2) and O(3)), with Mo and Nb in variable coordinate environments. M(1)-O(1)-O(2) and M(2)-O(1) sites are associated with the formation of (Mo,Nb)O₆octahedra, whilst tetrahedral units are composed of M(1)-O(1)-O(3) atoms. Upon increasing the temperature, the structure undergoes a change in occupancy in favour of the O(3) site, which results in a change in metal co-ordination as the tetrahedral to octahedral ratio increases. We demonstrate that the structure can also be tuned using externally applied pressure. Variable pressure studies ≤4.8 GPa indicate that densification of the unit cell induces the reverse effect, as the occupancy of the O(2) site increases and the palmierite contribution is suppressed. Our results strongly suggest that by 5.2 GPa the O(3) position will be completely empty as the 9R unit cell stabilises with a network of octahedral MO₆units. Pressure induces a flattening of M(1)O₄tetrahedra in the palmierite layers, as M(1)O₆octahedra become more regular in geometry. Bond valence site energy calculations show that pressure increases the height of all energy barriers to migration along the three-dimensional diffusion pathways, increasing the energy of the dominant pathway from 0.35 to 0.95 eV. The relaxation energy,E₂, disappears above 2.8 GPa, when the average polyhedral distortion (σ(R)) falls below 0.07 Å, indicating the existence of a critical minimum. The bulk modulus of Ba₃MoNbO_8.5is exceptionally low (50(2) GPa) for a layered oxide material and is closer to that of the halide perovskites. These results demonstrate a high degree of flexibility, in terms of the softness of the lattice and variable metal coordination, emphasising the potential for these materials in multi sensory and thin film applications.