Phase behaviour, thermal expansion and compressibility of SnMo2O8

Exploring negative thermal expansion materials with bulk framework structures and their relevant scaling relationships through multi-step machine learning

Discovering new negative thermal expansion (NTE) materials is a great challenge in experiment. Meanwhile, the machine learning (ML) method can be another approach to explore NTE materials using the existing material databases. Herein, we adopt the multi-step ML method with efficient data augmentation and cross-validation to identify around 1000 materials, including oxides, fluorides, and cyanides, with bulk framework structures as new potential NTE candidate materials from ICSD and other databases. Their corresponding coefficients of negative thermal expansion (CNTE) and temperature ranges are also well predicted. Among them, about 57 materials are predicted to have an NTE probability of 100%. Some predicted NTE materials were tested by the first-principles calculations with quasi-harmonic approximation (QHA), which indicates that the ML results are in good agreement with the first principles calculation results. Based on the comprehensive analysis of the existing and predicted NTE materials, we established three universal relationships of CNTE with an average electronegativity, porosity, and temperature range. From these, we also identified some important critical values characterizing the NTE property, which can serve as an important criterion for designing new NTE materials.

Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A₃OhTd₂O_7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO_2.5 compositions relative to that in the perovskite ABO₃ or brownmillerite A₂B₂O₅ structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O^2- migration. The low- and high-temperature average and local structures of Ba₃YGa₂O₇ (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P112₁/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O^2- ions was identified. Ionic conductivities of Ba₃YGa₂O_7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba₃YGa₂O_7.5 retaining proton conductivity at high temperatures. Ba_2.9La_0.1YGa₂O_7.55 and Ba₃YGa_1.9Ti_0.1O_7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.