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.

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.

Positive-and negative-muon spin rotation and relaxation (μ ± SR) was first used to investigate fluctuations of nuclear magnetic fields in an olivine-type battery material, LiMnPO 4 , in order to clarify the diffusive species, namely, to distinguish between a μ + hopping among interstitial sites and Li + ions diffusing in the LiMnPO 4 lattice. Muon diffusion can only occur in μ + SR, because the implanted μ − forms a stable muonic atom at the lattice site, and therefore any change in linewidth measured with μ − SR must be due to Li + diffusion. Since the two measurements exhibit a similar increase in the field fluctuation rate with temperature above 100 K, it is confirmed that Li + ions are in fact diffusing. The diffusion coefficient of Li + at 300 K and its activation energy were estimated to be 1.4(3) × 10 −10 cm 2 /s and 0.19(3) eV, respectively. Such combined μ ± SR measurements are thus shown to be a suitable tool for detecting ion diffusion in solid-state energy materials.

We report the successful synthesis, crystal structure, and electrical properties of Sr 3 Re 2 O 9 , which contains Re 6+ with the 5d 1 configuration. This compound is isostructural with Ba 3 Re 2 O 9 and shows a first-order structural phase transition at ∼370 K. The low-temperature (LT) phase crystallizes in a hettotype structure of Ba 3 Re 2 O 9 , which is different from that of the LT phase of Sr 3 W 2 O 9 , suggesting that the electronic state of Re 6+ plays an important role in determining the crystal structure of the LT phase. The structural transition is accompanied by a sharp change in the electrical resistivity. This is likely a metal−insulator transition, as suggested by the electronic band calculation and magnetic susceptibility. In the LT phase, the ReO 6 octahedra are rotated in a pseudoa 0 a 0 a + manner in Glazer notation, which corresponds to C-type orbital ordering. Paramagnetic dipole moments were confirmed to exist in the LT phase by muon spin rotation and relaxation measurements. However, the dipole moments shrink greatly because of the strong spin−orbit coupling in the Re ions. Thus, the electronic state of the LT phase corresponds to a Mott insulating state with strong spin−orbit interactions at the Re sites.