Colossal Magnetoresistance in Mn²⁺ Oxypnictides NdMnAsO_1–<i>x</i>F_<i>x</i>

Colossal magnetoresistance is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO_1–<i>x</i>F_<i>x</i> as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor upon application of a magnetic field. Mn²⁺ oxypnictides are relatively unexplored, and tailored synthesis of novel compounds could result in an array of materials for further investigation and optimization

Chemical Composition of Lithiated Nitrodonickelates Li_{3–<i>xy</i>}Ni_<i>x</i>N: Evidence of the Intermediate Valence of Nickel Ions from Ion Beam Analysis and <i>Ab Initio</i> Calculations

Lamellar lithiated nitridonickelates have been investigated from both experimental and theoretical points of view in a wide range of compositions. In this study, we show that the nickel ion in lamellar lithiated nitridonickelates adopts an intermediate valence close to +1.5. This solid solution can therefore be written Li3–1.5xNixN with 0 ≤ x ≤ 0.68. Attempts to introduce more nickel into these phases systematically lead to the presence of the endmember of the solid solution, Li1.97Ni0.68N, with metallic nickel as an impurity. The LiNiN phase has never been observed, and first-principles calculations suggested that all the structural configurations tested were mechanically unstable

A Comparative Insight of Potassium Vanadates as Positive Electrode Materials for Li Batteries: Influence of the Long-Range and Local Structure

Potassium vanadates with ratio K/V = 1:3, 1:4, and 1:8, prepared by a fast and facile synthesis route, were investigated as positive electrode materials in lithium batteries. KV₃O₈ and K_0.5V₂O₅ have layered structures, while K_0.25V₂O₅ exhibits a tunnel framework isomorphic to that of β-Na_0.33V₂O₅. The Raman spectra of KV₃O₈, K_0.5V₂O₅, and K_0.25V₂O₅ compounds are reported here for the first time, and a detailed comparative analysis distinguishes spectral patterns specific to each structural arrangement. The electrochemical performances of these potassium vanadates toward lithium insertion were investigated. The potassium-richer compound KV₃O₈ shows a good rechargeability in spite of a low discharge capacity of 70 mAh g^–1, while the potassium-poorer bronze K_0.25V₂O₅ exhibits the highest specific capacity of 230 mAh g^–1 but a slow and continuous capacity fade with cycling. We demonstrate that the K_0.5V₂O₅ compound, with its double-sheet V₂O₅ layered framework characterized by a large interlayer spacing of 7.7 Å, is the best candidate as positive electrode for lithium battery among the potassium–vanadium bronzes and oxides. A remarkable specific capacity of 210 mAh g^–1, combined with excellent capacity retention, is achieved

Li_2.0Ni_0.67N, a Promising Negative Electrode Material for Li-Ion Batteries with a Soft Structural Response

The layered lithium nitridonickelate Li_2.0(1)Ni_0.67(2)N has been investigated as a negative electrode in the 0.02–1.25 V vs Li⁺/Li potential window. Its structural and electrochemical properties are reported. Operando XRD experiments upon three successive cycles clearly demonstrate a single-phase behavior in line with the discharge–charge profiles. The reversible breathing of the hexagonal structure, implying a supercell, is fully explained. The Ni²⁺/Ni⁺ redox couple is involved, and the electron transfer is combined with the reversible accommodation of Li⁺ ions in the cationic vacancies. The structural response is fully reversible and minimal, with a maximum volume variation of 2%. As a consequence, a high capacity of 200 mAh g^–1 at C/10 is obtained with an excellent capacity retention, close to 100% even after 100 cycles, which makes Li_2.0(1)Ni_0.67(2)N a promising negative electrode material for Li-ion batteries