A review to delineate the chemistry and physics for honeycomb layered oxides.

We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high-temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No k_{z} dispersion is observed along the nodal direction, whereas a significant antinodal k_{z} dispersion is identified for La-based cuprates. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La_{2-x}Sr_{x}CuO_{4} cannot be assigned to the van Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.

The magnetic ground states in highly ordered double perovskites LaSr1−xCaxNiReO6 (x = 0.0, 0.5, 1.0) are studied in view of the Goodenough-Kanamori rules of superexchange interactions in this paper. In LaSrNiReO6, Ni and Re sublattices are found to exhibit curious magnetic states separately, but no long range magnetic ordering is achieved. The magnetic transition at ~255 K is identified with the independent Re sublattice magnetic ordering. Interestingly, the sublattice interactions are tuned by modifying the Ni-O-Re bond angles through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction data reveals long range ferrimagnetic ordering of the Ni and Re magnetic sublattices along the crystallographic b-axis. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration.

In the quest for developing novel and efficient batteries, a great interest has been raised for sustainable K-based honeycomb layer oxide materials, both for their application in energy devices as well as for their fundamental material properties. A key issue in the realization of efficient batteries based on such compounds, is to understand the K-ion diffusion mechanism. However, investigation of potassium-ion (K$$^+$$ + ) dynamics in materials using e.g. NMR and related techniques has so far been very challenging, due to its inherently weak nuclear magnetic moment, in contrast to other alkali ions such as lithium and sodium. Spin-polarised muons, having a high gyromagnetic ratio, make the muon spin rotation and relaxation ($$\mu ^+$$ μ + SR) technique ideal for probing ions dynamics in these types of energy materials. Here we present a study of the low-temperature magnetic properties as well as K$$^+$$ + dynamics in honeycomb layered oxide material $${\hbox {K}_2\hbox {Ni}_2\hbox {TeO}_6}$$ K 2 Ni 2 TeO 6  using mainly the $$\mu ^+$$ μ + SR technique. Our low-temperature $$\mu ^+$$ μ + SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at $$T_{\mathrm{N}}\approx 27$$ T N ≈ 27  K. Further $${\mu}^{+}$$ μ + SR studies performed at higher temperatures reveal that potassium ions (K$$^+$$ + ) become mobile above 200 K and the activation energy for the diffusion process is obtained as $$E_{\mathrm{a}}=121 (13)$$ E a = 121 ( 13 )  meV. This is the first time that K$$^+$$ + dynamics in potassium-based battery materials has been measured using $$\mu ^+$$ μ + SR. Assisted by high-resolution neutron diffraction, the temperature dependence of the K-ion self diffusion constant is also extracted. Finally our results also reveal that K-ion diffusion occurs predominantly at the surface of the powder particles. This opens future possibilities for potentially improving ion diffusion as well as K-ion battery device performance using nano-structuring and surface coatings of the particles.

In times where research focuses on the use of organic polymers as a base for complex organic electronic applications and improving device efficiencies, degradation is still less intensively addressed in fundamental studies. Hence, advanced neutron scattering methods are applied to investigate a model system for organic electronics composed of the widely used conductive polymer blend poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) together with nanocellulose as flexible reinforcing template material. In particular, the impact of relative humidity (RH) on the nanostructure evolution is studied in detail. The implications are discussed from a device performance point of view and the changing nanostructure is correlated with macroscale physical properties such as conductivity. The first humidification (95% RH) leads to an irreversible decrease of conductivity. After the first humidification cycle, however, the conductivity can be reversibly regained when returning to low humidity values (5% RH), which is important for device manufacturing. This finding can directly contribute to an improved usability of emerging organic electronics in daily live.