High-Temperature Neutron and X‑ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate

Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na_2.5Fe_1.75(SO₄)₃ (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied high-temperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo-)­alluaudite system, forming emerging functional materials group for electrochemical applications

Structural Origin of the Anisotropic and Isotropic Thermal Expansion of K₂NiF₄‑Type LaSrAlO₄ and Sr₂TiO₄

K₂NiF₄<i>-</i>type LaSrAlO₄ and Sr₂TiO₄ exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO₄ and Sr₂TiO₄ have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the <i>c</i>-axis (α_<i>c</i>) being higher than that along the <i>a</i>-axis (α_<i>a</i>) of LaSrAlO₄ [α_<i>c</i> = 1.882(4)­α_<i>a</i>] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α­(Al–O2) being higher than that between Al and equatorial oxygen O1 α­(Al–O1) [α­(Al–O2) = 2.41(18)­α­(Al–O1)]. The higher α­(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr₂TiO₄, the Ti–O2 interatomic distance, <i>d</i>(Ti–O2), is equal to that of Ti–O1, <i>d</i>(Ti–O1) [<i>d</i>(Ti–O2) = 1.0194(15)<i>d</i>(Ti–O1)], relative to LaSrAlO₄ [<i>d</i>(Al–O2) = 1.0932(9)<i>d</i>(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr₂TiO₄ than LaSrAlO₄. These results indicate that the anisotropic thermal expansion of K₂NiF₄-type oxides, <i>A</i>₂<i>B</i>O₄, is strongly influenced by the anisotropy of <i>B</i>–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance <i>d</i>(<i>B</i>–O2)/<i>d</i>(<i>B</i>–O1) of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compared with <i>A</i>₂²⁺<i>B</i>⁴⁺O₄, <i>A</i>₂^2.5+<i>B</i>³⁺O₄ compounds have higher α­(<i>B</i>–O2), and <i>A</i>₂²⁺<i>B</i>⁴⁺O₄ materials exhibit smaller α­(<i>B</i>–O2), leading to the anisotropic thermal expansion of <i>A</i>₂^2.5+<i>B</i>³⁺O₄ and isotropic thermal expansion of <i>A</i>₂²⁺<i>B</i>⁴⁺O₄. The “true” thermal expansion without the chemical expansion of <i>A</i>₂<i>B</i>O₄ is higher than that of <i>AB</i>O₃ with a similar composition

A‑Site and B‑Site Charge Orderings in an <i>s–d</i> Level Controlled Perovskite Oxide PbCoO₃

Perovskite PbCoO₃ synthesized at 12 GPa was found to have an unusual charge distribution of Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with charge orderings in both the A and B sites of perovskite ABO₃. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ quadruple perovskite structure. It is shown that the average valence distribution of Pb^3.5+Co^2.5+O₃ between Pb³⁺Cr³⁺O₃ and Pb⁴⁺Ni²⁺O₃ can be stabilized by tuning the energy levels of Pb 6<i>s</i> and transition metal 3<i>d</i> orbitals