Structures and electron distributions of α-Co2SiO4 and α-Ni2SiO4 (olivine structure)

Stabilization of Lithium Transition Metal Silicates in the Olivine Structure

While olivine LiFePO₄ shows amongst the best electrochemical properties of Li-ion positive electrodes with respect to rate behavior owing to facile Li⁺ migration pathways in the framework, replacing the [PO₄]^3- polyanion with a silicate [SiO₄]^4- moiety in olivine is desirable. This could allow additional alkali content and hence electron transfer, and increase the capacity. Herein we explore the possibility of a strategy toward new cathode materials and demonstrate the first stabilization of a lithium transition metal silicate (as a pure silicate) in the olivine structure type. Using LiInSiO₄ and LiScSiO₄ as the parent materials, transition metal (Mn, Fe, Co) substitutions on the In/Sc site were investigated by computational modeling via atomic scale simulation. Transition metal substitution was found to be only favorable for Co, a finding confirmed by the successful solid state synthesis of olivine Li_xIn_yCo_2-x-ySiO₄. Stabilization of the structure was achieved by entropy provided by cation disorder.

Structural chemistry of A 2 MX 4 compounds (X = O, F) with isolated tetrahedral anions: search for the densest structure types

The packing density of various structures is important not only for understanding and the prediction of high-pressure phase transitions, but also because of its reported correlation with thermodynamic stability. Plotting the cube root of formula volume against the cation radii (R) for nine morphotropic series with isolated tetrahedral anions, A 2 MO4 (M = Si, Ge, S, Se, Cr, Mn, Mo, W) and A 2BeF4, permits the comparison of packing densities for 13 structure types (about 80 individual compounds and several solid solutions) stable at (or near) ambient temperature. The spinel type is the densest. The next densest types are those of K2MoO4, Tl2CrO4, β-Ca2SiO4, β-K2SO4, Ag2CrO4 and Sr2GeO4. In three series (M = Ge, Mo, W) the densest type comes with somewhat intermediate values of R, and not the largest, in contrast to the classical homology rule. Another contradiction with traditional views is that some of the densest phases have abnormally low overall binding energies. The correlation between packing density and coordination number (CN) is better when CN of A counts entire MX 4 groups rather than individual X atoms; many, but not all, A 2 MX 4 structures have binary A 2 M analogues (of course, A and M are not necessarily the same in these structure types). The most frequent arrangement of A around M is of the Ni2In type: a (distorted) pentacapped trigonal prism.