Die Kristallstrukturen des LiCu2P2 und des Li1.75Cu1.25P2

Computational Investigation of Copper Phosphides as Conversion Anodes for Lithium-Ion Batteries

Using first-principles structure searching with density-functional theory (DFT), we identify a novel Fm3̅m phase of Cu₂P and two low-lying metastable structures, an I4̅3d-Cu₃P phase and a Cm-Cu₃P₁₁ phase. The computed pair distribution function of the novel Cm-Cu₃P₁₁ phase shows its structural similarity to the experimentally identified Cm-Cu₂P₇ phase. The relative stability of all Cu-P phases at finite temperatures is determined by calculating the Gibbs free energy using vibrational effects from phonon modes at 0 K. From this, a finite-temperature convex hull is created, on which Fm3̅m-Cu₂P is dynamically stable and the Cu_3-x P (x < 1) defect phase Cmc2₁-Cu₈P₃ remains metastable (within 20 meV/atom of the convex hull) across a temperature range from 0 to 600 K. Both CuP₂ and Cu₃P exhibit theoretical gravimetric capacities higher than contemporary graphite anodes for Li-ion batteries; the predicted Cu₂P phase has a theoretical gravimetric capacity of 508 mAh/g as a Li-ion battery electrode, greater than both Cu₃P (363 mAh/g) and graphite (372 mAh/g). Cu₂P is also predicted to be both nonmagnetic and metallic, which should promote efficient electron transfer in the anode. Cu₂P's favorable properties as a metallic, high-capacity material suggest its use as a future conversion anode for Li-ion batteries; with a volume expansion of 99% during complete cycling, Cu₂P anodes could be more durable than other conversion anodes in the Cu-P system, with volume expansions greater than 150%. The structures and figures presented in this paper, and the code used to generate them, can be interactively explored online using Binder.

Sc5Pd4Si6- crystal structure and 29Si/45Sc solid state MAS NMR spectroscopic investigations

The silicide Sc₅Pd₄Si₆ was synthesized from the elements by arc-melting. Its structure was refined from single crystal X-ray diffractometer data: Immm, Li₅Cu_3.75P₆ type, a = 397.12(6), b = 945.4(1), c = 1314.4(2) pm, wR₂ = 0.0245, 578 F² values and 29 parameters. The palladium atoms have slightly distorted tetrahedral silicon coordination and a condensation of these PdSi₄ tetrahedra leads to a two-dimensional substructure which is condensed via Si₂ pairs (234 pm Si-Si), forming the [Pd₄Si₆]^δ- polyanionic network. The Sc₅Pd₄Si₆ structure contains three crystallographically independent scandium sites with coordinations Sc1@Pd₄Si₈Sc₆, Sc2@Pd₆Si₆Sc₃ and Sc3@Pd₄Si₆Sc₄. Sc₅Pd₄Si₆ is a Pauli paramagnet with a low susceptibility of 2.9(5) × 10^-5 emu mol^-1 at room temperature. The ²⁹Si MAS-NMR spectrum confirms the presence of two crystallographically distinct sites in a 2 : 1 ratio. Likewise, the three crystallographic scandium sites are well-differentiated by three ⁴⁵Sc MAS NMR signals in the expected 2 : 2 : 1 ratio. Unambiguous assignments could be made based on the comparison of the nuclear electric quadrupolar coupling parameters with predicted values from WIEN2k calculations.

A new pnictide superconductor without iron

LiCu(2)P(2) and LiFeP have been synthesized by conventional solid-state reaction. LiCu(2)P(2) has a crystal structure similar to that of BaFe(2)As(2); LiFeP has the same crystal structure as that of LiFeAs. Resistivity and magnetization measurements reveal that they become superconductive at 3.5 K for LiCu(2)P(2) and 4.1 K for LiFeP.