Layered Quaternary Germanides—Synthesis and Crystal and Electronic Structures of AELi2In2Ge2 (AE = Sr, Ba, Eu)

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.

Synthesis, crystal and electronic structure of BaLixCd13–x (x ≈ 2)

A new ternary phase has been synthesized and structurally characterized. BaLi_xCd_13–x (x ≈ 2) adopts the cubic NaZn₁₃ structure type (space group Fm3¯c, Pearson symbol cF112) with unit cell parameter a = 13.5548 (10) Å. Structure refinements from single-crystal X-ray diffraction data demonstrate that the Li atoms are exclusively found at the centers of the Cd₁₂-icosahedra. Since a cubic BaCd₁₃ phase does not exist, and the tetragonal BaCd₁₁ is the most Cd-rich phase in the Ba–Cd system, BaLi_xCd_13–x (x ≈ 2) has to be considered as a true ternary compound. As opposed to the typical electron count of ca. 27e-per formula unit for many known compounds with the NaZn₁₃ structure type, BaLi_xCd_13–x (x ≈ 2) only has ca. 26e-, suggesting that both electronic and geometric factors are at play. Finally, the bonding characteristics of the cubic BaLi_xCd_13–x (x ≈ 2) and tetragonal BaCd₁₁ are investigated using the TB-LMTO-ASA method, showing metallic-like behavior.

Synthesis, crystal and electronic structure of BaLi2Cd2Ge2

A new quaternary germanide has been synthesized and structurally characterized. BaLi2Cd2Ge2 adopts the rhombohedral CaCu4P2 structure type (Pearson code hR7; space group R 3 ‾ m $R&#x203e;{3}m$ , Z = 3) with unit cell parameters a = 4.5929(6) and c = 26.119(5) Å. Structure refinements from single-crystal X-ray diffraction data demonstrate that the layered crystal structure can be regarded as an ordered quaternary variant of the ternary archetype; structural parallels to layered pnictides and binary germanides can also be drawn. The layered crystal structure is characterized by the absence of direct Ge–Ge and Cd–Cd homoatomic bonds, which suggests that BaLi2Cd2Ge2 should be classified as a Zintl phase, according to the formulation (Ba2+)(Li+)2(Cd2+)2(Ge4−)2. Electronic structure calculations show that the Fermi level crosses a distinct peak in the DOS, although the presence of an electronic band gap or a dip in the electronic density of states at the Fermi level is expected based on the electron partitioning.

The Zintl phases AIn2As2 (A = Ca, Sr, Ba): new topological insulators and thermoelectric material candidates

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their surface. The surface states observed in such materials behave as a perfect conductor thereby making them more suited for several cutting-edge technological applications such as spintronic devices. Here, we report the synthesis and structural characterization of the Zintl phases AIn2As2 (A = Ca, Sr, Ba), which could become a new class of TIs. Crystal structure elucidation by single-crystal X-ray diffraction reveals that CaIn2As2 and SrIn2As2 are isostructural and crystallize in the EuIn2P2 structure type (space group P63/mmc, no. 194, Z = 2) with unit cell parameters a = 4.1482(6) Å, c = 17.726(4) Å; and a = 4.2222(6) Å, c = 18.110(3) Å, respectively. Their hexagonal structure is made up of alternating [In2As2]2- layers separated by slabs of A2+ cations. BaIn2As2 on the other hand crystallizes in the monoclinic EuGa2P2 structure type (space group P2/m, no. 10, Z = 4) with unit cell parameters a = 10.2746(11) Å, b = 4.3005(5) Å, c = 13.3317(14) Å and β = 95.569(2)°. This structure is also layered, and it is made up of different type of polyanionic [In2As2]2- units and Ba2+ cations. The valence electron count for all three compounds adheres to the Zintl-Klemm formalism, and all elements achieve closed-shell electronic configurations. Bulk electronic structure calculations indicate the opening of a bandgap Eg ∼ 0.03 eV (CaIn2As2 and Sr2In2As2), and Eg ∼0.21 eV (BaIn2As2) in the absence of strain and spin-orbit coupling (SOC). These results argue in favor of the realization of a nontrivial topological insulator state under the influence of tensile strain and SOC. Preliminary transport properties on BaIn2As2 are suggestive of a degenerate p-type semiconductor-a behavior which is sought after in thermoelectric (TE) materials. Since both TIs and excellent TE materials are known to favor the same material properties such as narrow bandgap, heavy elements, and strong SOC, these three Zintl phases are also projected as candidates TE materials.

Electronic stabilization by occupational disorder in the ternary bismuthide Li3-x-yInxBi (x ≃ 0.14, y ≃ 0.29)

A ternary derivative of Li₃Bi with the composition Li_3-x-yIn_xBi (x ≃ 0.14, y ≃ 0.29) was produced by a mixed In+Bi flux approach. The crystal structure adopts the space group Fd-3m (No. 227), with a = 13.337 (4) Å, and can be viewed as a 2 × 2 × 2 superstructure of the parent Li₃Bi phase, resulting from a partial ordering of Li and In in the tetrahedral voids of the Bi fcc packing. In addition to the Li/In substitutional disorder, partial occupation of some Li sites is observed. The Li deficiency develops to reduce the total electron count in the system, counteracting thereby the electron doping introduced by the In substitution. First-principles calculations confirm the electronic rationale of the observed disorder.

Exploration of Multi-Component Vanadium and Titanium Pnictides Using Flux Growth and Conventional High-Temperature Methods

The flux growth method was successfully employed to synthesize millimeter-sized single crystals of the ternary barium vanadium pnictides Ba₅V₁₂As_19+x (x ≈ 0.02) and Ba₅V₁₂Sb_19+x (x ≈ 0.36), using molten Pb and Sb, respectively. Both compositions crystallize in space group P 4 ¯ 3m and adopt a structure similar to those of the barium titanium pnictides Ba₅Ti₁₂ Pn _19+x (Pn = Sb, Bi), yet with a subtly different disorder, involving the pnictogen and barium atoms. Attempts to obtain an arsenide analog of Ba₅Ti₁₂ Pn _19+x using a Pb flux technique yielded binary arsenides. High-temperature treatment of the elements Ba, Ti, and As in Nb or Ta tubes resulted in side reactions with the crucible materials and produced two isostructural compositions Ba₈Ti_13-x M _x As₂₁ (M = Nb, Ta; x ≈ 4), representing a new structure type. The latter structure displays fcc-type metal clusters comprised of statistically distributed Ti and M atoms (M = Nb, Ta) with multi-center and two-center bonding within the clusters, as suggested by our first-principle calculations.

Multifaceted Sn-Sn bonding in the solid state. Synthesis and structural characterization of four new Ca-Li-Sn compounds

Four novel ternary phases have been prepared in the system Ca-Li-Sn using both the metal flux method and conventional high-temperature synthesis. Each of the obtained compositions represents its own (new) structure type, and the structures feature distinct polyanionic Sn units. Ca₄LiSn₆ (space group Pbcm, Pearson symbol oP44) accommodates infinite chains, made up of cyclopentane-like [Sn₅]-rings, which are bridged by Sn atoms. The Sn atoms in this structure are two- and three-bonded. The anionic substructure of Ca₉Li_6+xSn_13-x (x≈ 0.28, space group C2/m, Pearson symbol mS56) displays extensive mixing of Li and Sn and combination of single-bonded and hypervalent interactions between the Sn atoms. Hypervalent bonding is also pronounced in the structure of the third compound, Ca₂LiSn₃ (space group Pmm2, Pearson symbol oP18) with quasi-two-dimensional polyanionic subunits and a variety of coordination environments of the Sn atoms. One-dimensional [Sn₁₀]-chains with an intricate topology of cis- and trans-Sn-Sn bonds exist in the structure of Ca_9-xLi₂Sn₁₀ (x≈ 0.16, space group C2/m, Pearson symbol mS42), and the Sn-Sn bonding in this case demonstrates the characteristics of an intermediate between single- and double- bond-order. The peculiarities of the bonding are discussed in the context of the Zintl approach, which captures the essence of the main chemical interactions. The electronic structures of all four compounds have also been analyzed in full detail using first-principles calculations.