Applicability of the Zintl Concept to Understanding the Crystal Chemistry of Lithium-Rich Germanides and Stannides

With this contribution, we take a new, critical look at the structures of the binary phases Li5Ge2 and Li5Sn2. Both are isostructural (centrosymmetric space group R3̅m, no. 166), and in their structures, all germanium (tin) atoms are dimerized. Application of the valence rules will require the allocation of six additional valence electrons per [Ge2] or [Sn2] unit considering single covalent bonds, akin to those in the dihalogen molecules. Alternatively, four additional valence electrons per [Ge2] or [Sn2] anion will be needed if homoatomic double bonds exist, in an analogy with dioxygen. Therefore, five lithium atoms in one formula unit cannot provide the exact number of electrons, leaving open questions as to what is the nature of the chemical bonding within these moieties. Additionally, by means of single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, and neutron powder diffraction, we established that the Li and Sn atoms in Li5Sn2 are partially disordered, i.e., the actual chemical formula of this compound is Li5-xSn2+x (0 < x < 0.1). The convoluted atomic bonding in the case where tin atoms partially displace lithium atoms results in the formation of larger covalently bonded fragments. Our first-principle calculations suggest that such disorder leads to electron doping. Contrary to that, both experimental and computational findings indicate that in the Li5Ge2 structure, the [Ge2] dimers are slightly oxidized, i.e., hole-doped, as a result of approximately 30% vacancies on a Li site, i.e., the actual chemical formula of this compound is Li5-xGe2 (x ≈ 0.3).

Yet Another Case of Lithium Metal Atoms and Germanium Atoms Sharing Chemistry in the Solid State: Synthesis and Structural Characterization of Ba2 LiGe3

Several Ba-Li-Ge ternary phases are known and structurally characterized, including the title compound Ba2 LiGe3 . Its structure is reported to contain [Ge6 ]10- anions that exhibit delocalized bonding with a Hückel-like aromatic character. The Ge atoms are in the same plane with the Li atoms, and if both types of atoms are considered as covalently bonded, [LiGe3 ]4- honeycomb-like layers will result. The latter are separated by slabs of Ba2+ cations. However, based on the systematic work detailed herein, it is necessary to re-evaluate the phase as Ba2 Li1-x Ge3+x (x<0.05). Although small, the homogeneity range is clearly demonstrated in the gradual change of the unit cell for four independent samples. Subsequent characterization by single-crystal X-ray diffraction methods shows that the Ba2 Li1-x Ge3+x structure, responds to the varied number of valence electrons and the changes are most pronounced for the refined lengths of the Li-Ge and Ge-Ge bonds. Indirectly, the changes in the Ge-Li/Ge distances within layers affect the stacking too, and these changes can be correlated to the variation of the c-cell parameter. Chemical bonding analysis based on TB-LMTO-ASA level calculations affirms the notion for covalent character of the Ge-Ge bonds; the Ba-Ge and Li-Ge interactions also show some degree of covalency.

Lithium metal atoms fill vacancies in the germanium network of a type-I clathrate: synthesis and structural characterization of Ba8Li5Ge41

Clathrate phases with crystal structures exhibiting complex disorder have been the subject of many prior studies. Here we report syntheses, crystal and electronic structure, and chemical bonding analysis of a Li-substituted Ge-based clathrate phase with the refined chemical formula Ba8Li5.0(1)Ge41.0, which is a rare example of ternary clathrate-I where alkali metal atoms substitute framework Ge atoms. Two different synthesis methods to grow single crystals of the new clathrate phase are presented, in addition to the classical approach towards polycrystalline materials by combining pure elements in desired stoichiometric ratios. Structure elucidations for samples from different batches were carried out by single-crystal and powder X-ray diffraction methods. The ternary Ba8Li5.0(1)Ge41.0 phase crystallizes in the cubic type-I clathrate structure (space group Pm3̄n no. 223, a ≈ 10.80 Å), with the unit cell being substantially larger compared to the binary phase Ba8Ge43 (Ba8□3Ge43, a ≈ 10.63 Å). The expansion of the unit cell is the result of the Li atoms filling vacancies and substituting atoms in the Ge framework, with Li and Ge co-occupying one crystallographic (6c) site. As such, the Li atoms are situated in four-fold coordination environment surrounded by equidistant Ge atoms. Analysis of chemical bonding applying the electron density/electron localizability approach reveals ionic interaction of barium with the Li-Ge framework, while the lithium-germanium bonds are strongly polar covalent.

Metallic alloys at the edge of complexity: structural aspects, chemical bonding and physical properties

Complex metallic alloys belong to the vast family of intermetallic compounds and are hallmarked by extremely large unit cells and, in many cases, extensive crystallographic disorder. Early studies of complex intermetallics were focusing on the elucidation of their crystal structures and classification of the underlying building principles. More recently, ab initio computational analysis and detailed examination of the physical properties have become feasible and opened new perspectives for these materials. The present review paper provides a summary of the literature data on the reported compositions with exceptional structural complexity and their properties, and highlights the factors leading to the emergence of their crystal structures and the methods of characterization and systematization of these compounds.

Controlling superstructural ordering in the clathrate-I Ba8M16P30 (M = Cu, Zn) through the formation of metal-metal bonds

Order-disorder-order phase transitions in the clathrate-I Ba8Cu16P30 were induced and controlled by aliovalent substitutions of Zn into the framework. Unaltered Ba8Cu16P30 crystallizes in an ordered orthorhombic (Pbcn) clathrate-I superstructure that maintains complete segregation of metal and phosphorus atoms over 23 different crystallographic positions in the clathrate framework. The driving force for the formation of this Pbcn superstructure is the avoidance of Cu-Cu bonds. This superstructure is preserved upon aliovalent substitution of Zn for Cu in Ba8Cu16-x Zn x P30 with 0 < x < 1.6 (10% Zn/Mtotal), but vanishes at greater substitution concentrations. Higher Zn concentrations (up to 35% Zn/Mtotal) resulted in the additional substitution of Zn for P in Ba8M16+y P30-y (M = Cu, Zn) with 0 ≤ y ≤ 1. This causes the formation of Cu-Zn bonds in the framework, leading to a collapse of the orthorhombic superstructure into the more common cubic subcell of clathrate-I (Pm3n). In the resulting cubic phases, each clathrate framework position is jointly occupied by three different elements: Cu, Zn, and P. Detailed structural characterization of the Ba-Cu-Zn-P clathrates-I via single crystal X-ray diffraction, joint synchrotron X-ray and neutron powder diffractions, pair distribution function analysis, electron diffraction and high-resolution electron microscopy, along with elemental analysis, indicates that local ordering is present in the cubic clathrate framework, suggesting the evolution of Cu-Zn bonds. For the compounds with the highest Zn content, a disorder-order transformation is detected due to the formation of another superstructure with trigonal symmetry and Cu-Zn bonds in the clathrate-I framework. It is shown that small changes in the composition, synthesis, and crystal structure have significant impacts on the structural and transport properties of Zn-substituted Ba8Cu16P30.

A type-II clathrate with a Li-Ge framework

Na

Type-I silicon clathrates containing lithium

The intermetallic phase [Li