51�������

Abstract:

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Abstract:

Oxide ions in transition metal oxide cathodes can store charge at high voltage offering a route towards higher energy density batteries. However, upon charging these cathodes, the oxidized oxide ions condense to form molecular O2 trapped in the material. Consequently, the discharge voltage is much lower than charge, leading to undesirable voltage hysteresis. Here we capture the nature of the electron holes on O2− before O2 formation by exploiting the suppressed transition metal rearrangement in ribbon-ordered Na0.6[Li0.2Mn0.8]O2. We show that the electron holes formed are delocalized across the oxide ions coordinated to two Mn (O–Mn2) arranged in ribbons in the transition metal layers. Furthermore, we track these delocalized hole states as they gradually localize in the structure in the form of trapped molecular O2 over a period of days. Establishing the nature of hole states on oxide ions is important if truly reversible high-voltage O-redox cathodes are to be realized.

Abstract:

Sr2NiO2Cu2Se2, comprising alternating [Sr2NiO2]2+and [Cu2Se2]2-layers, is reported. Powder neutron diffraction shows that the Ni2+ions, which are in a highly elongated NiO4Se2environment with D4hsymmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below 160 K in a 2a × 2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μBper Ni2+ion. The adoption of the high-spin configuration for this d8cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr2NiO2Cu2S2, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni2+, and accordingly, there is no evidence for magnetic moment on the Ni2+ions. Examination of the solid solution Sr2NiO2Cu2(Se1-xSx)2shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr2NiO2Cu2Se2up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni2+coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr2NiO2Cu2Se2and Sr2NiO2Cu2S2, suggest that simple high-spin and low-spin models for Ni2+may not be entirely appropriate and point to further complexities in these compounds.

Abstract:

Sr₂NiO₂Cu₂Se₂, comprising alternating [Sr₂NiO₂]²⁺ and [Cu₂Se₂]^2– layers, is reported. Powder neutron diffraction shows that the Ni²⁺ ions, which are in a highly elongated NiO₄Se₂ environment with D_4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μ_B per Ni²⁺ ion. The adoption of the high-spin configuration for this d⁸ cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr₂NiO₂Cu₂S₂, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni²⁺, and accordingly, there is no evidence for magnetic moment on the Ni²⁺ ions. Examination of the solid solution Sr₂NiO₂Cu₂(Se_1–xS_x)₂ shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr₂NiO₂Cu₂Se₂ up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni²⁺ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr₂NiO₂Cu₂Se₂ and Sr₂NiO₂Cu₂S₂, suggest that simple high-spin and low-spin models for Ni²⁺ may not be entirely appropriate and point to further complexities in these compounds.