An Alternative Route to Pentavalent Postperovskite

Two different high-pressure and -temperature synthetic routes have been used to produce only the second-known pentavalent CaIrO₃-type oxide. Postperovskite NaOsO₃ has been prepared from GdFeO₃-type perovskite NaOsO₃ at 16 GPa and 1135 K. Furthermore, it has also been synthesized at the considerably lower pressure of 6 GPa and 1100 K from a precursor of hexavalent Na₂OsO₄ and nominally pentavalent KSbO₃-like phases. The latter synthetic pathway offers a new lower-pressure route to the postperovskite form, one that completely foregoes any perovskite precursor or intermediate. This work suggests that postperovskite can be obtained in other compounds and chemistries where generalized rules based on the perovskite structure may not apply or where no perovskite is known. One more obvious consequence of our second route is that perovskite formation may even mask and hinder other less extreme chemical pathways to postperovskite phases

Operando SAXS/WAXS on the a‑P/C as the Anode for Na-Ion Batteries

A complete chemical and morphological analysis of the evolution of battery electrode materials can be achieved combining different and complementary techniques. Operando small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were combined to investigate structural and electrochemical performances of an Na-ion battery, with amorphous red phosphorus in a carbon matrix (a-P/C) as the active anode material in a Swagelok-type cell. The charging process results in the formation of crystalline Na₃P, while during discharging, the anode material returns to the initial a-P/C. From the analysis of the WAXS curves, the formation of crystalline phases appears only at the end of charging. However, SAXS data show that partial reorganization of the material during charging occurs at length scales nonaccessible with conventional X-ray diffraction, corresponding to a real space ordering distance of 4.6 nm. Furthermore, the analysis of the SAXS data shows that the electrode remains dense during charging, while it develops some porosity during the discharge phase. The presented results indicate that the combination of SAXS/WAXS adopted simultaneously, and nondestructively, on a working electrochemical cell can highlight new mechanisms of reactions otherwise undetected. This method can be applied for the study of any other solid electrode material for batteries

Perovskite to Postperovskite Transition in NaFeF₃

The GdFeO₃-type perovskite NaFeF₃ transforms to CaIrO₃-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the <i>p</i>–<i>V</i> data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO₃-type MgSiO₃ at high pressure and temperature. Theoretical studies using density functional theory at the GGA + <i>U</i> level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott–Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets