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

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.

Detailed understanding of charge diffusion processes in a lithium-ion battery is crucial to enable its systematic improvement. Experimental investigation of diffusion at the interface between active particles and the electrolyte is challenging but warrants investigation as it can introduce resistances that, for example, limit the charge and discharge rates. Here, we show an approach to study diffusion at interfaces using muon spin spectroscopy. By performing measurements on LiFePO4 platelets with different sizes, we determine how diffusion through the LiFePO4 (010) interface differs from that in the center of the particle (i.e., bulk diffusion). We perform ab initio calculations to aid the understanding of the results and show the relevance of our interfacial diffusion measurement to electrochemical performance through cyclic voltammetry measurements. These results indicate that surface engineering can be used to improve the performance of lithium-ion batteries.

We report a muon spin rotation (μ + SR) study of the magnetic properties of the double perovskite compound LaSrNiReO 6. Using the unique length and time scales of the μ + SR technique, we successfully clarify the magnetic ground state of LaSrNiReO 6 , which was previously deemed as a spin glass state. Instead, our μ + SR results point toward a long-range dynamically ordered ground state below T C = 23 K, for which a static limit is foreseen at T = 0. Furthermore, between 23 K < T 300 K, three different magnetic phases are identified: a dense (23 K < T < 75 K), a dilute (75 K T 250 K), and a paramagnetic (T > 250 K) state. Our results reveal how two separate yet intertwined magnetic lattices interact within the unique double perovskite structure and the importance of using complementary experimental techniques to obtain a complete understanding of the microscopic magnetic properties of complex materials.

Inverse trirutile MnTeO was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P4/mnm) to a monoclinic phase (P2/c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO octahedra alternate with more regular TeO octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn-Teller origin, is driven by a single primary mode.

The crystal and magnetic properties of honeycomb BiMnTeO 6 have been studied between 1.5 K and 300 K using synchrotron X-rays and neutron scattering experiments. Commensurate magnetic ordering is observed below T N = 10 K, and corresponds to a non collinear spin arrangement, with spins tilted away from the anisotropy easy-axis set by the Jahn-Teller distortion of the MnO 6 octahedra. Inelastic scattering experiments show two main nearly localised magnetic excitations, which can be well described by an exchange Hamiltonian involving weak Mn-Mn magnetic interactions and a crystal field Hamiltonian characterizing the strong easy-axis anisotropy associated with the d z2 orbital ordering of Mn 3+ . The crystal field levels can be accurately calculated, taking into account a transverse molecular field imposed by the magnetic ordering of the neighbouring atoms below T N . This makes of BiMnTeO 6 an interesting example of a multi-axis Ising system in a self-imposed transverse magnetic field.

Two-dimensional (2D) triangular lattice antiferromagnets (2D-TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2-dimensional transition metal dichalcogenide LiCrTe$$_2$$ 2 , being a 2D-TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation ($$\mu ^+$$ μ + SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe$$_2$$ 2 . From high-resolution NPD the magnetic spin order at base-temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the ab-plane and AFM coupling along the c-axis. The value if the ordered magnetic Cr moment is established as $$\mu _{\textrm{Cr}}= 2.36~\mu _{\textrm{B}}$$ μ Cr = 2.36 μ B . From detailed $$\mu ^+$$ μ + SR measurements we observe an AFM ordering temperature $$T_{\textrm{N}}\approx 125$$ T N ≈ 125  K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From $$\mu ^+$$ μ + SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe$$_2$$ 2 in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high-resolution synchrotron X-ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon–phonon coupling, similar to what has been previously observed in the closely related compound LiCrO$$_2$$ 2 .