Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

https://science.sciencemag.org/content/sci/174/4008/493.full.pdf

Intercalation Complexes of Lewis Bases and Layered Sulfides: A Large Class of New Superconductors

Lithium ions in LiFePO4 were observed directly at atomic resolution by an aberration-corrected annular-bright-field scanning transmission electron microscopy technique. In addition, it was found in partially delithiated LiFePO4 that the remaining lithium ions preferably occupy every second layer, along the b axis, analogously to the staging phenomenon observed in some layered intercalation compounds. This new finding challenges previously proposed LiFePO4/FePO4 two-phase separation mechanisms.

Direct Observation of Lithium Staging in Partially Delithiated LiFePO4 at Atomic Resolution

Organic–inorganic hybrid perovskites are promising candidates for the next-generation solar cells. Many efforts have been made to study their structures in the search for a better mechanistic understanding to guide the materials optimization. Here, we investigate the structure instability of the single-crystalline CH3NH3PbI3 (MAPbI3) film by using transmission electron microscopy. We find that MAPbI3 is very sensitive to the electron beam illumination and rapidly decomposes into the hexagonal PbI2. We propose a decomposition pathway, initiated with the loss of iodine ions, resulting in eventual collapse of perovskite structure and its decomposition into PbI2. These findings impose important question on the interpretation of experimental data based on electron diffraction and highlight the need to circumvent material decomposition in future electron microscopy studies. The structural evolution during decomposition process also sheds light on the structure instability of organic–inorganic hybrid perovskites in solar cell applications. Hybrid perovskites are highly promising for photovoltaic applications, but they are prone to decomposition. Here, the authors probe the stability of CH3NH3PbI3 films in a transmission electron microscope, defining the threshold conditions to avoid damage under the electron beam, and describing a decomposition pathway.

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Atomic scale insights into structure instability and decomposition pathway of methylammonium lead iodide perovskite.

Olivine NaFePO4/C microsphere cathode is prepared by a facile aqueous electrochemical displacement method from LiFePO4/C precursor. The NaFePO4/C cathode shows a high discharge capacity of 111 mAh g–1, excellent cycling stability with 90% capacity retention over 240 cycles at 0.1 C, and high rate capacity (46 mAh g–1 at 2 C). The excellent electrochemical performance demonstrates that the aqueous electrochemical displacement method is an effective and promising way to prepare NaFePO4/C material for Na-based energy storage applications. Moreover, the Na2/3FePO4 intermediate is observed for the first time during the Na intercalation process through conventional electrochemical techniques, corroborating an identical two-step phase transition reaction both upon Na intercalation and deintercalation processes. The clarification of the electrochemical reaction mechanism of olivine NaFePO4 could inspire more attention on the investigation of this material for Na ion batteries.

High-Performance Olivine NaFePO4 Microsphere Cathode Synthesized by Aqueous Electrochemical Displacement Method for Sodium Ion Batteries.

Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1 T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2 H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset T CDW s of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions T CDW s usually greatly exceed the superconducting transitions at their low T c values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high- T c cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength ( I c R n ) with the conventional SC, Pb. Hence, the pristine TMDs are highly “unconventional” in comparison with Pb, but are much more “conventional” than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1 T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2 H -TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)1/2, have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high- T c superconductor, Bi2Sr2CaCu2O8+δ and in the organic layered superconductor, κ-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as Nax CoO 2 · y H2O.

Pristine and intercalated transition metal dichalcogenide superconductors

Several layered, transition metal dichalcogenide intercalation complexes with unique superconducting properties have been examined by high-resolution electron microscopy and electron diffraction. Details of the crystalline lattice and of the lattice imperfections have been directly resolved. The results can be correlated with the available x-ray diffraction and chemical data, and they confirm and extend the postulated models.

https://science.sciencemag.org/content/sci/174/4008/498.full.pdf

Charles Hough

Papers

Electron microscopy and diffraction of layered, superconducting intercalation complexes.