The Ba₃Mo_1-xW_xNbO_8.5 system is attracting increasing attention because of its potential application as electrolyte in solid state fuel cells [1-2]. Ba₃Mo_1-xW_xNbO_8.5 compounds feature a hexagonal perovskite structure, composed of edge-sharing octahedra, which alternate to tetrahedra depending on the occupation of oxygen sites. The interest in the system arises also from the peculiar migration mechanism, which appears to involve hopping across partially occupied tetrahedral and octahedral site. This results in a change with temperature of the relative occupancies of O ions in octahedral and tetrahedral sites [1-2]. However, the reliability of the mechanism is still under debate. This is partially due to a wrong crystallographic model reported in the first studies, which missed a split of a cation site [3] that likely hampers the validity of the above migration mechanism. Secondly, Ba₃Mo_1-xW_xNbO_8.5 are sensitive to the oxygen local distribution, i.e. to the thermal history, making problematic comparisons with specimens produced in different conditions, or by different groups.

Here we combine synchrotron and neutron powder diffraction (SPD, NPD) exploiting the most recent crystallographic models. Room temperature NPD revealed that the substitution of W for Mo induces the preferential occupation of octahedral oxygen sites. Temperature-resolved SPD confirmed the enrichment of tetrahedral sites at high temperature. This variation is accompanied by a discontinuity in the SPD line profile, suggesting a reconstructive phase transition, as observed independently by another group [4]. Given the complexity of the system, we exploited the complementary information provided by N- and S-PDFs. The former were employed to focus on the first neighbors, indeed, octahedral (Mo/W/Nb)-O pairs are much longer than the equivalent tetrahedral pairs, and well separated by other metal-oxygen pairs. Whereas the reference structure shows many overlapping signals, the N-PDF provided unambiguous evidence of octahedral sites in W-rich specimens, whose population decreases at high temperature. Other possible interpretations of local PDF involving different coordination [4] are discussed. On the other hand, the limited weight of O ions onto the S-PDFs facilitates the identification of different structural motifs. Room temperature refinements suggested structural inhomogeneities at the local scale. We propose a model where the increase of temperature induces a redistribution of O ions at the nm length scale, leading to a disordered structure more favorable for ion migration [5].