A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Rational Design of Biaxial Tensile Strain for Boosting Electronic and Ionic Conductivities of Na2 MnSiO4 for Rechargeable Sodium-Ion Batteries

Using first‐principles calculations, biaxial tensile (ϵ=2 and 4 %) and compressive (ϵ=−2 and −4 %) straining of Na₂MnSiO₄ lattices resulted into radial distance cut offs of 1.65 and 2 Å, respectively, in the first and second nearest neighbors shell from the center. The Si−O and Mn−O bonds with prominent probability density peaks validated structural stability. Wide‐band gap of 2.35 (ϵ=0 %) and 2.54 eV (ϵ=−4 %), and narrow bandgap of 2.24 eV (ϵ=+4 %) estimated with stronger coupling of p–d σ bond than that of the p–d π bond, mainly contributed from the oxygen p‐state and manganese d‐state. Na⁺‐ion diffusivity was found to be enhanced by three orders of magnitude as the applied biaxial strain changed from compressive to tensile. According to the findings, the rational design of biaxial strain would improve the ionic and electronic conductivity of Na₂MnSiO₄ cathode materials for advanced rechargeable sodium‐ion batteries.

Synthesis, optical and ionic conductivity studies of a lithium cobalt germanate compound

A lithium cobalt germanate compound (Li₂CoGeO₄) was synthesized and studied. The X-ray powder diffraction pattern demonstrated a monoclinic crystal system with the Pn space group. The morphology and composition were done by scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). A vibrational study confirmed the existence of the anion (GeO₄)⁴⁻ and its vibrations. The estimated value of the direct band gap (E_g) was evaluated at 3.45 eV. The measurements of the electrical properties were performed in the frequency interval from 100 Hz to 1 MHz and the temperature range from 553 K to 663 K. The Nyquist plots revealed the existence of a single semicircle in all impedance spectra due to the grain interior effect. The AC electrical conduction in Li₂CoGeO₄ has been explained through several processes, which can be coupled with two different formalisms. The complex electric modulus studies M*(ω) confirmed that the relaxation process is thermally activated.

A lithium cobalt germanate compound (Li₂CoGeO₄) was synthesized and studied.

Synthesis, Structure Determination, and Characterizations of a Polar Salt-Inclusion Scandium Germanate, Rb10Li3Sc4Ge12O36F

A noncentrosymmetric salt-inclusion germanate, Rb₁₀Li₃Sc₄Ge₁₂O₃₆F, was grown through spontaneous crystallization from a LiF-RbF flux. It crystallizes in the polar space group P31c with cell parameters of a = 10.7587(3) Å, c = 21.6691(10) Å, and Z = 2. Its structure features a complex 3D framework composed of helical [Ge₄O₁₂] chains from condensed [GeO₄] tetrahedra running along the c axis, which are interconnected by the [ScO₆] octahedra. Voids of the 3D net are filled with Rb⁺ ions, Li⁺ ions, and isolated trigonal-bipyramidal [Rb₃Li₂F] superalkali clusters. The title compound has a large band gap of 5.6 eV, a moderate powder second-harmonic-generation response of 0.9KDP, and an extremely small birefringence of 0.001, as was further unraveled by theoretical calculations.

Defects, Dopants and Sodium Mobility in Na2MnSiO4

Sodium manganese orthosilicate, Na2MnSiO4, is a promising positive electrode material in rechargeable sodium ion batteries. Atomistic scale simulations are used to study the defects, doping behaviour and sodium migration paths in Na2MnSiO4. The most favourable intrinsic defect type is the cation anti-site (0.44 eV/defect), in which, Na and Mn exchange their positions. The second most favourable defect energy process is found to be the Na Frenkel (1.60 eV/defect) indicating that Na diffusion is assisted by the formation of Na vacancies via the vacancy mechanism. Long range sodium paths via vacancy mechanism were constructed and it is confirmed that the lowest activation energy (0.81 eV) migration path is three dimensional with zig-zag pattern. Subvalent doping by Al on the Si site is energetically favourable suggesting that this defect engineering stratergy to increase the Na content in Na2MnSiO4 warrants experimental verification.