The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202107850.

Bi2MelxV1-xO5.5-(5-l)x/2-δ (BIMEVOX, Me = dopant, l = valency) systems are a family of fast oxide ion conductors that show very high ionic conductivity at low and intermediate temperatures. Despite being...

Grain size can have significant effects on the properties of electroceramics for dielectric, piezoelectric, and ferroelectric applications. Here, we systematically investigate the effect of grain size on the structure and properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 ceramics, an important lead-free piezoelectric ceramic that exhibits both a high piezoelectric coefficient and a high Curie point. Ceramics with average grain sizes ranging from 0.46 to 6.85 μm were prepared using conventional and spark plasma sintering. It was found that the morphotropic phase boundary compositions are composed of two polar structures, rhombohedral and tetragonal, with DC poling inducing an increase in the fraction of the rhombohedral phase. All ceramics show relaxor behavior and their freezing temperature moves to higher temperatures with increasing grain size, although their Burns temperature is independent of grain size. In fine-grained ceramics, which show pronounced relaxor behavior, significant grain size dependency is seen in dielectric, piezoelectric, and ferroelectric properties, which is attributed to the presence of single ferroelectric domains and high concentrations of polar nanoregions. In coarse-grained ceramics, a critical grain size of 2.83 μm yields the highest dielectric permittivity at room temperature, with the piezoelectric coefficient plateauing at this grain size, which can be attributed to the contribution of both polar nanoregions and high domain wall density.

While BIMEVOX systems have attracted attention for their fast oxide-ion conductivity at intermediate temperatures, there are only a limited number of reports concerning their local structure. In this work, both long-range and local structures in the Bi2V1–x Ga x O5.5–x–δ (BIGAVOX, 0.025 ≤ x ≤ 0.40) system are investigated using X-ray powder diffraction (XRD) and a combination of X-ray total scattering, 51V and 71Ga solid-state nuclear magnetic resonance (NMR), and Raman spectroscopy, supported by electrical measurements using a.c. impedance spectroscopy. The three main BIMEVOX polymorphs, α, β, and γ, are observed at room temperature over the compositional ranges 0.025 ≤ x < 0.10, 0.10 ≤ x < 0.20, and 0.20 ≤ x < 0.40, respectively. Above x = 0.10, as more Ga is introduced into the lattice, a general growth of the distance between bismuthate and vanadate layers is observed, indicating increasing ionicity in the interaction between these layers. Ga is found to adopt octahedral and tetrahedral geometries, while V polyhedra include tetrahedral, pentacoordinate, and octahedral geometries. With increasing x-value, as the vacancy concentration increases, more octahedral V polyhedra transform to lower coordinate geometries, resulting in a decrease in the average V–O bond length. Reversible α ↔ β and β ↔ γ phase transitions are observed on heating the x = 0.05 composition, while the β ↔ γ and γ′ ↔ γ phase transitions are observed on heating the x = 0.15 and 0.20 compositions, respectively. The γ-BIGAVOX compositions (x = 0.20 and 0.25) generally show a high conductivity of ∼10–2 S cm–1 at 600 °C.

The BIMEVOXes are among the best oxide ion conductors at low and intermediate temperatures. Their high conductivity is associated with local defect structure. In this work, the local structures of two BIMEVOX compositions, Bi 2 V 0.9 Ge 0.1 O 5.45 and Bi 2 V 0.95 Sn 0.05 O 5.475 , are examined using total neutron and X-ray scattering methods, with both compositions exhibiting the ordered α-phase at 25 °C and the disordered γ-phase at 700 °C. While the diffraction data for the α-phase do not allow for the polar (C2) and nonpolar (C2/m) structures to be readily distinguished, measurements of dielectric permittivity suggest the αphase is weakly ferroelectric in character, consistent with calculations of spontaneous polarization based on a combination of density functional calculations and machine learning methodology. Reverse Monte Carlo (RMC) analysis of total scattering data reveals Ge preferentially adopts tetrahedral geometry at both temperatures, while Sn is found to predominantly adopt octahedral coordination in the α-phase and tetrahedral coordination in the γ-phase. In all cases, V polyhedra are found to consist of tetrahedral, pentacoordinate, and octahedral geometries, as also predicted by the crystallographic analysis and confirmed by 51 V solid state NMR spectroscopy. Although similar long-range structures are observed at room temperature, the oxide ion vacancy distributions were found to be quite different between the two studied compositions, with a nonrandom deficiency in vacancy pairs in the secondnearest shell along the ⟨100⟩ tetragonal direction for BIGEVOX10, compared with a long-distance (>8.0 Å) ordering of equatorial vacancies for BISNVOX05. This is attributed to the differences in the preferred coordination geometries of the substituent cations in the two systems. Impedance spectroscopy measurements reveal both compositions show high conductivity in the order of 10 −1 S cm −1 at 600 °C.