Oxide ion dynamics in hexagonal perovskite mixed conductor Ba 7 Nb 4 MoO 20: a comprehensive ab initio molecular dynamics study

Hexagonal perovskite Ba7Nb4MoO20-related materials are very promising solid electrolytes with high oxide ion conductivity and redox stability, making them potentially applicable in solid oxide fuel cells. Optimizing the properties of this family of materials necessitates atomic-level understanding of the oxide ion dynamics leading to high conductivity. Here we report extensive ab initio molecular dynamics simulations of Ba7Nb4MoO20 investigating oxide ion motions, which allowed the observation of a continuous diffusion pathway for oxide ions in the (ab) plane, but also revealed significant contribution of the oxygen atoms from crystallographic sites located outside this plane, to the long-range dynamics. To probe the timescale of oxide ion diffusion, complementary quasielastic neutron scattering experiments were carried out, and showed that oxide ion dynamics in Ba7Nb4MoO20, even at 950 °C, are too slow to be observable on a nanosecond timescale. Based on the atomic-level understanding of structure–property relationships afforded by this detailed computational study, we propose new materials design strategies with potential to significantly increase oxide ion conductivity in Ba7Nb4MoO20-related hexagonal perovskites, which target the simultaneous increase of the number of oxide ion charge carriers and rotational flexibility of the (Nb/Mo)Ox polyhedra

Oxide Ion Mobility in V- and P-doped Bi2O3-Based Solid Electrolytes: Combining Quasielastic Neutron Scattering with Ab Initio Molecular Dynamics

We report the direct observation of oxide ion dynamics on both nano- and picosecond timescales in the isostructural Bi2O3-derived solid electrolytes Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648 using quasielastic neutron scattering. Comprehensive ab initio molecular dynamics simulations allowed us to reproduce the experimental picosecond timescale data by directly simulating the scattering function at various temperatures. Our analysis of the experimental data in conjunction with the simulations revealed the origin of the picosecond dynamics to be localized motions within the V–O and P–O sublattices, while nanosecond dynamics correspond to the diffusion of the oxide ions in the Bi–O sublattice via vacancy-hopping. This combined approach provides insight into the different oxide ion migration pathways and mechanisms in Bi0.852V0.148O1.648 and Bi0.852P0.148O1.648, with the flexibility of the V coordination environment playing an important role, consistent with the superior conductivity of the vanadate

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O′ anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations

Structural ferroelectric phase transition and polymorphism in 2-aminopyridine dihydrogen phosphate

The structural ferroelectric phase transition in 2-aminopyridine dihydrogen phosphate (2APP) has been studied by single crystal and powder X-ray diffraction between room temperature and 16 K. It has been shown that α-aminopyridine dihydrogen phosphate (α-2APP) undergoes a transition from the centrosymmetric space group C2/c in the paraelectric phase to the polar space group Cc in the ferroelectric phase. This is a second-order phase transition associated with ordering of protons in short O−H···O hydrogen bonds. This system is found to exhibit rich polymorphism: depending on crystallization conditions, three anhydrate forms and one monohydrate can be isolated. 2APP hydrate and γ-2APP have been identified for the first time, and their structures have been solved from single crystal diffraction data

Complex Superstructures of Mo2P4O15

We report structural studies on Mo2P4O15 over the temperature range 16−731 K, which show that it is considerably more complex than revealed by earlier work. Its low-temperature structure has lattice parameters a = 24.1134(6) Å, b = 19.5324(5) Å, c = 25.0854(6) Å, β = 100.015(1)°, and V = 11635.0(5) Å3 at 120 K, containing 441 unique atoms in space group Pn, a remarkably high number for a material with such a simple composition. Mo2P4O15 undergoes a structural phase transition at ∼520 K to a high-temperature phase in space group P1̅, with lattice parameters a = 17.947(3) Å, b = 19.864(3) Å, c = 21.899(3) Å, α = 72.421(3)°, β = 78.174(4)°, γ = 68.315(4)°, and V = 6877.2(19) Å3 at 573 K. The high-temperature structure, with 253 unique atoms, retains much of the low-temperature complexity