Five new ternary indium-arsenides discovered. Synthesis and structural characterization of the Zintl phases Sr3In2As4, Ba3In2As4, Eu3In2As4, Sr5In2As6 and Eu5In2As6

Topotaxial mutual-exchange growth of magnetic Zintl Eu3In2As4 nanowires with axion insulator classification

Due to quasi-one-dimensional confinement, nanowires possess unique electronic properties, which can promote specific device architectures. However, nanowire growth presents paramount challenges, limiting the accessible crystal structures and elemental compositions. Here we demonstrate solid-state topotactic exchange that converts wurtzite InAs nanowires into Zintl Eu3In2As4. Molecular-beam-epitaxy-based in situ evaporation of Eu and As onto InAs nanowires results in the mutual exchange of Eu from the shell and In from the core. Therefore, a single-phase Eu3In2As4 shell grows, which gradually consumes the InAs core. The mutual exchange is supported by the substructure of the As matrix, which is similar across the wurtzite InAs and Zintl Eu3In2As4 and therefore is topotactic. The Eu3In2As4 nanowires undergo an antiferromagnetic transition at a Néel temperature of ~6.5 K. Ab initio calculations confirm the antiferromagnetic ground state and classify Eu3In2As4 as a C2T axion insulator, hosting both chiral hinge modes and unpinned Dirac surface states. The topotactic mutual-exchange nanowire growth will, thus, enable the exploration of intricate magneto-topological states in Eu3In2As4 and potentially in other exotic compounds.

Structural Uniqueness of the [Nb(As5)2]5- Cluster in the Zintl Phase Cs5NbAs10

The structure of the novel Zintl phase, Cs₅NbAs₁₀, is reported for the first time. This compound crystallizes in the monoclinic P2₁/c space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence e32. The most important structural element is the unprecedented [Nb(As₅)₂]^5- cluster anion, formed by a Nb atom enclosed between two As₅ rings. These nonaromatic cyclic species, formally [As₅]^5-, adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As₅]^5- ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As₅)₂]^5- cluster and the [As₅]^5- rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on Cs₅NbAs₁₀ suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.

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.

Synthesis, structural characterization, and electronic structure of the novel Zintl phase Ba2ZnP2

The novel Zintl phase dibarium zinc diphosphide (Ba₂ZnP₂) was synthesized for the first time. This was accomplished using the Pb flux technique, which allowed for the growth of crystals of adequate size for structural determination via single-crystal X-ray diffraction methods. The Ba₂ZnP₂ compound was determined to crystallize in a body-centered orthorhombic space group, Ibam (No. 72). Formally, this crystallographic arrangement belongs to the K₂SiP₂ structure type. Therefore, the structure can be best described as infinite [ZnP₂]^4- polyanionic chains with divalent Ba²⁺ cations located between the chains. All valence electrons are partitioned, which conforms to the Zintl-Klemm concept and suggests that Ba₂ZnP₂ is a valence-precise composition. The electronic band structure of this new compound, computed with the aid of the TB-LMTO-ASA code, shows that Ba₂ZnP₂ is an intrinsic semiconductor with a band gap of ca 0.6 eV.

On the New Oxyarsenides Eu5Zn2As5O and Eu5Cd2As5O

The new quaternary phases Eu5Zn2As5O and Eu5Cd2As5O have been synthesized by metal flux reactions and their structures have been established through single-crystal X-ray diffraction. Both compounds crystallize in the centrosymmetric space group Cmcm (No. 63, Z = 4; Pearson symbol oC52), with unit cell parameters a = 4.3457(11) Å, b = 20.897(5) Å, c = 13.571(3) Å; and a = 4.4597(9) Å, b = 21.112(4) Å, c = 13.848(3) Å, for Eu5Zn2As5O and Eu5Cd2As5O, respectively. The crystal structures include one-dimensional double-strands of corner-shared MAs4 tetrahedra (M = Zn, Cd) and As–As bonds that connect the tetrahedra to form pentagonal channels. Four of the five Eu atoms fill the space between the pentagonal channels and one Eu atom is contained within the channels. An isolated oxide anion O2– is located in a tetrahedral hole formed by four Eu cations. Applying the valence rules and the Zintl concept to rationalize the chemical bonding in Eu5M2As5O (M = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 × [Eu2+] + 2 × [M2+] + 3 × [As3–] + 2 × [As2–] + O2–, which suggests an electron-deficient configuration. The presumed h+ hole is confirmed by electronic band structure calculations, where a fully optimized bonding will be attained if an additional valence electron is added to move the Fermi level up to a narrow band gap (Eu5Zn2As5O) or pseudo-gap (Eu5Cd2As5O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu5M2As5O (M = Zn, Cd), europium is theorized to be in a mixed-valent Eu2+/ Eu3+ state.

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.