RESUMO
The magnetic phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ([Formula: see text]SR). The weak pressure dependence for the [Formula: see text] compounds suggests that the rich phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of x at ambient pressure may not solely be attributed to chemical pressure effects. The [Formula: see text] compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at [Formula: see text] kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of magnetic domains is formed above [Formula: see text] kbar where they co-exist with a magnetically disordered state. These domains are likely to be ferromagnetic islands (FMI) and consist of an high- (FMI-[Formula: see text]) and low-temperature (FMI-[Formula: see text]) region, respectively, separated by a phase boundary at [Formula: see text] K. This kind of co-existence is unusual and is originating from a coupling between lattice and magnetic degrees of freedoms.
RESUMO
Sodium transition metal oxides with a layered structure are one of the most widely studied cathode materials for Na+-ion batteries. Since the mobility of Na+ in such cathode materials is a key factor that governs the performance of material, electrochemical and muon spin rotation and relaxation techniques are here used to reveal the Na+-ion mobility in a P2-type Na0.5MgxNi0.17-xMn0.83O2 (x = 0, 0.02, 0.05 and 0.07) cathode material. Combining electrochemical techniques such as galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique with µ+SR, we have successfully extracted both self-diffusion and chemical-diffusion under a potential gradient, which are essential to understand the electrode material from an atomic-scale viewpoint. The results indicate that a small amount of Mg substitution has strong effects on the cycling performance and the Na+ mobility. Amongst the tested cathode systems, it was found that the composition with a Mg content of x = 0.02 resulted in the best cycling stability and highest Na+ mobility based on electrochemical and µ+SR results. The current study clearly shows that for developing a new generation of sustainable energy-storage devices, it is crucial to study and understand both the structure as well as dynamics of ions in the material on an atomic level.
RESUMO
The crystal structure and magnetic properties of the cubic spinel MgFeMnO4 were studied by using a series of in-house techniques along with large-scale neutron diffraction and muon spin rotation spectroscopy in the temperature range between 1.5 and 500 K. The detailed crystal structure is successfully refined by using a cubic spinel structure described by the space group Fd3Ì m. Cations within tetrahedral A and octahedral B sites of the spinel were found to be in a disordered state. The extracted fractional site occupancies confirm the presence of antisite defects, which are of importance for the electrochemical performance of MgFeMnO4 and related battery materials. Neutron diffraction and muon spin spectroscopy reveal a ferrimagnetic order below TC = 394.2 K, having a collinear spin arrangement with antiparallel spins at the A and B sites, respectively. Our findings provide new and improved understanding of the fundamental properties of the ferrispinel materials and of their potential applications within future spintronics and battery devices.
RESUMO
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[Formula: see text]) 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 ([Formula: see text]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[Formula: see text] dynamics in honeycomb layered oxide material [Formula: see text] using mainly the [Formula: see text]SR technique. Our low-temperature [Formula: see text]SR results together with complementary magnetic susceptibility measurements find an antiferromagnetic transition at [Formula: see text] K. Further [Formula: see text]SR studies performed at higher temperatures reveal that potassium ions (K[Formula: see text]) become mobile above 200 K and the activation energy for the diffusion process is obtained as [Formula: see text] meV. This is the first time that K[Formula: see text] dynamics in potassium-based battery materials has been measured using [Formula: see text]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.
RESUMO
Anomalous successive structural transitions in layered 1T-CrSe2 with an unusual Cr4+ valency were investigated by synchrotron X-ray diffraction. 1T-CrSe2 exhibits dramatic structural changes in in-plane Cr-Cr and interlayer Se-Se distances, which originate from two interactions: (i) in-plane Cr-Cr interactions derived from Peierls-like trimerization instabilities on the orbitally assisted one-dimensional chains and (ii) interlayer Se-Se interactions through p-p hybridization. As a result, 1T-CrSe2 has the unexpected ground state of an antiferromagnetic metal with multiple Cr linear trimers with three-center-two-electron σ bonds. Interestingly, partial substitution of Se for S atoms in 1T-CrSe2 changes the ground state from an antiferromagnetic metal to an insulator without long-range magnetic ordering, which is due to the weakening of interlayer interactions between anions. The unique low-temperature structures and electronic states of this system are determined by the competition and cooperation of in-plane Cr-Cr and interlayer Se-Se interactions.
RESUMO
The K2Cr8O16 compound belongs to a series of quasi-1D compounds with intriguing magnetic properties that are stabilized through a high-pressure synthesis technique. In this study, a muon spin rotation, relaxation and resonance (µ+SR) technique is used to investigate the pressure dependent magnetic properties up to 25 kbar. µ+SR allows for measurements in true zero applied field and hereby access the true intrinsic material properties. As a result, a refined temperature/pressure phase diagram is presented revealing a novel low temperature/high pressure (pC1 = 21 kbar) transition from a ferromagnetic insulating to a high-pressure antiferromagnetic insulator. Finally, the current study also indicates the possible presence of a quantum critical point at pC2 ~ 33 kbar where the magnetic order in K2Cr8O16 is expected to be fully suppressed even at T = 0 K.
