Advancing Heteroanionicity in Zintl Phases: Crystal Structures, Thermoelectric and Magnetic Properties of Two Quaternary Semiconducting Arsenide Oxides, Eu8Zn2As6O and Eu14Zn5As12O.

Two novel quaternary oxyarsenides, Eu8Zn2As6O and Eu14Zn5As12O, were synthesized through metal flux reactions, and their crystal structures were established by single-crystal X-ray diffraction methods. Eu8Zn2As6O crystallizes in the orthorhombic space group Pbca, featuring polyanionic ribbons composed of corner-shared triangular [ZnAs3] units, running along the [100] direction. The structure of Eu14Zn5As12O crystallizes in the monoclinic space group P2/m and its anionic substructure can be described as an infinite "ribbonlike" chain comprised of [ZnAs3] trigonal-planar units, although the structural complexity here is greater and also amplified by disorder on multiple crystallographic positions. In both structures, the O2- anion occupies an octahedral void with six neighboring Eu2+ cations. Formal electron counting, electronic structure calculations, and transport properties reveal the charge-balanced semiconducting nature of these heteroanionic Zintl phases. High-temperature thermoelectric transport properties measurements on Eu14Zn5As12O reveal relatively high resistivity (ρ500K = 8 Ω·cm) and Seebeck coefficient values (S500K = 220 µV K-1), along with a low concentration and mobility of holes as the dominant charge-carriers (n500K = 8.0 × 1017 cm-3, µ500K = 6.4 cm2/V s). Magnetic studies indicate the presence of divalent Eu2+ species in Eu14Zn5As12O and complex magnetic ordering, with two transitions observed at T1 = 21.6 K and T2 = 9 K.

Novel ternary Zintl phosphide halides Ba3P5X (X = Cl, Br) with 1D helical phosphorus chains: synthesis, crystal and electronic structure.

Black single crystals of two novel ternary phosphide halides, Ba3P5Cl and Ba3P5Br, were grown using molten metal Pb-flux high-temperature reactions. These compounds were structurally characterized with the aid of the single-crystal X-ray diffraction (SCXRD) method at 100(2) K. The SCXRD shows that both compounds are isostructural and adopt a new structure type (space group R3̄c, No. 167, Z = 6) with unit cell parameters a = 14.9481(16) Å, c = 7.3954(11) Å and a = 15.045(4) Å, c = 7.537(3) Å for Ba3P5Cl and Ba3P5Br, respectively. Cl- and Br- anions are octahedrally coordinated by Ba2+ cations, thus composing a face-sharing 1D infinite chain 1∞[XBa3]5+ running along the [001] direction. Moreover, the crystal structures feature peculiar one-dimensional disordered infinite helical chains of 1∞P-, composed of partially occupied phosphorous atoms, each being a superposition of three symmetrical copies of the ordered phosphorus chain, with continuity along the c-axis. Ba3P5X (X = Cl, Br) compounds are charge-balanced heteroanionic Zintl phases according to the charge-partitioning scheme (Ba2+)3[P-]5X-. The presumed semiconducting behavior of both compounds corroborates well with the results of the electronic structure calculations performed with the aid of the TB-LMTO-ASA code.

Synthesis, crystal and electronic structure of BaLi x Cd13-x (x ≈ 2).

A new ternary phase has been synthesized and structurally characterized. BaLi x Cd13-x (x ≈ 2) adopts the cubic NaZn13 structure type (space group Fm 3 ¯ 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 x Cd13-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 x Cd13-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 x Cd13-x (x ≈ 2) and tetragonal BaCd11 are investigated using the TB-LMTO-ASA method, showing metallic-like behavior.

Synthesis and structural characterization of the new Zintl phases Eu10Mn6Bi12 and Yb10Zn6Sb12.

Two new ternary compounds, Eu10Mn6Bi12 and Yb10Zn6Sb12, were synthesized and structurally characterized. The synthesis was achieved either through reactions in sealed niobium tubes or in alumina crucibles by combining the elements in excess molten Sb. Their structures were elucidated using single-crystal X-ray diffraction, and they were determined to crystallize in the orthorhombic space group Cmmm (no. 65) with the Eu10Cd6Bi12 structure type. Akin to the archetype phase, both Mn and Zn sites contain about 25% of vacancies. The anionic substructure of the title phases can be described as [M6Pn12] (M = Zn, Mn; Pn = Sb, Bi) double layers composed of the corner and edge-sharing [MPn4] tetrahedra, linked by [Pn2]4- dumbbells. Eu2+/Yb2+ cations fill the space between the layers, with the valence electron counts adhering closely to the Zintl-Klemm rules, i.e., both Eu10Mn6Bi12 and Yb10Zn6Sb12 are expected to be valence-precise compounds. Analysis of the electronic structure and transport properties of Yb10Zn6Sb12 indicate semimetallic behavior with relatively low Seebeck coefficient and resistivity that slightly decreases as a function of temperature.

Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca2CdSb2 with a Non-centrosymmetric Monoclinic Structure.

The Zintl phase Ca2CdSb2 was found to be dimorphic. Besides the orthorhombic Ca2CdSb2 (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, Ca2CdSb2 (-m), and its Lu-doped variant Ca2-xLuxCdSb2 (x ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group Cm (Z = 30). The electrical resistivity ρ(T) measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration nH from 7.15 × 1018 to 2.30 × 1019 cm-3 at 300 K. The Seebeck coefficient S(T) of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 µV/K at 600 K for Ca2CdSb2 (-m) and Ca2-xLuxCdSb2, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 Å3 drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 µW/cm K2 and an estimated thermoelectric figure of merit zT of ∼0.5 for Ca2-xLuxCdSb2 at 600 K. Differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.

Materials design, synthesis, and transport properties of disordered rare-earth Zintl bismuthides with the anti-Th3P4 structure type.

The synthesis, structural elucidation, and transport properties of the extended series Ca4-xRExBi3 (RE = Y, La-Nd, Sm, Gd-Tm, and Lu; x ≈ 1) and Ca4-xRExBi3-δSbδ (RE = La, Ho, Er, and Lu; x ≈ 1, δ ≈ 1.5) are presented. Structural elucidation is based on single-crystal X-ray diffraction data and confirms the chemical drive of Ca4Bi3 with the cubic anti-Th3P4 structure type (space group I4̄3d, no. 220, Z = 4) into a Zintl phase by the introduction of trivalent rare-earth atoms. The structure features complex bonding, heavy elements, and electron count akin to that of valence-precise semiconductors, making it an ideal target for thermoelectrics development. Introducing crystallographic site disorders at the cation site for the Ca4-xRExBi3 phase and both the cation and anion sites for the Ca4-xRExBi3-δSbδ phase brings about additional desirable characteristics for thermoelectric materials in the context of tuning knobs for lowering thermal conductivity. Electronic structure calculations of idealized Ca3YBi3 and Ca3LaBi3 compounds indicate the opening of indirect bandgaps at the Fermi level with magnitudes Eg = 0.38 eV and 0.57 eV, respectively. The electrical resistivity ρ(T) of some of the investigated phases measured on single crystals evolve in a metallic manner with magnitudes of order 1.4 mΩ cm near 500 K, thus supporting the notion of a degenerate semiconducting state, with the temperature dependence of the Seebeck coefficient α(T) suggesting the p-type behavior. The low electrical resistivity and the realization of a degenerate semiconducting state in the title phases present a window of opportunity for optimizing their carrier concentrations for enhanced thermoelectric performance.

Computational design of thermoelectric alloys through optimization of transport and dopability.

Alloying is a common technique to optimize the functional properties of materials for thermoelectrics, photovoltaics, energy storage etc. Designing thermoelectric (TE) alloys is especially challenging because it is a multi-property optimization problem, where the properties that contribute to high TE performance are interdependent. In this work, we develop a computational framework that combines first-principles calculations with alloy and point defect modeling to identify alloy compositions that optimize the electronic, thermal, and defect properties. We apply this framework to design n-type Ba2(1-x)Sr2xCdP2 Zintl thermoelectric alloys. Our predictions of the crystallographic properties such as lattice parameters and site disorder are validated with experiments. To optimize the conduction band electronic structure, we perform band unfolding to sketch the effective band structures of alloys and find a range of compositions that facilitate band convergence and minimize alloy scattering of electrons. We assess the n-type dopability of the alloys by extending the standard approach for computing point defect energetics in ordered structures. Through the application of this framework, we identify an optimal alloy composition range with the desired electronic and thermal transport properties, and n-type dopability. Such a computational framework can also be used to design alloys for other functional applications beyond TE.

Two Polymorphs of BaZn2P2: Crystal Structures, Phase Transition, and Transport Properties.

The novel α-BaZn2P2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu2S2 structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With ß-BaZn2P2 being previously identified as belonging to the ThCr2Si2 family and with the precedent of structural phase transitions between the α-BaCu2S2 type and the ThCr2Si2 type, the potential for the pattern to be extended to the two different structural forms of BaZn2P2 was explored. Thermal analysis suggests that a first-order phase transition occurs at ∼1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal ß-BaZn2P2, the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn2P2 is a p-type, narrow-gap semiconductor with a direct bandgap of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the ß-BaZn2P2 phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 µV/K to 184 µV/K at 600 K. The electrical resistivity (ρ) of α-BaZn2P2 is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.

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 ß = 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.

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.

Complex Structural Disorder in the Zintl Phases Yb10MnSb9 and Yb21Mn4Sb18.

A systematic investigation of the ternary system Yb-Mn-Sb led to the discovery of the novel phase Yb10MnSb9. Its crystal structure was characterized by single-crystal X-ray diffraction and found to be complex and highly disordered. The average Yb10MnSb9 structure can be considered to represent a defect modification of the Ca10LiMgSb9 type and to crystallize in the tetragonal P42/mnm space group (No. 136) with four formula units per cell. The structural disorder can be associated with both occupational and positional effects on several Yb and Mn sites. Similar traits were observed for the structure of the recently reported Yb21Mn4Sb18 phase (monoclinic space group C2/c, No. 15), which was reevaluated as part of this study as well. In both structures, distorted Sb6 octahedra centered by Yb atoms and Sb4 tetrahedra centered by Mn atoms form disordered fragments, which appear as the hallmark of the structural chemistry in this system. Discussion along the lines of how difficult, and important, it is to distinguish Yb10MnSb9 from the compositionally similar binary Yb11Sb10 and ternary Yb14MnSb11 compounds is also presented. Preliminary transport measurements for polycrystalline Yb10MnSb9 indicate high values of the Seebeck coefficient, approaching 210 µV K-1 at 600 K, and a semiconducting behavior with a room-temperature resistivity of 114 mΩ cm.

Caught in Action. The Late Rare Earths Thulium and Lutetium Substituting Aluminum Atoms in the Structure of Ca14AlBi11.

Described are two unprecedented cases, where the rare earth metals Tm and Lu partially substitute Al atoms in the structure of the Zintl phase Ca14AlBi11. These are the first examples within this large family where lanthanides replace the atoms of a main group element. Such crystal chemistry has never been observed before in other materials with the same structure. The uniqueness of this finding is also amplified by the fact that the rare earth metal atoms in the crystal structure are tetrahedrally coordinated, which is another remarkable trait of the new Tm- and Lu-substituted compounds.

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.

Synthesis and structural characterization of the type-I clathrates K8AlxSn46-x and Rb8AlxSn46-x (x ≃ 6.4-9.7).

Exploratory studies in the systems A-Al-Sn (A = K and Rb) yielded the clathrates K8AlxSn46-x (potassium aluminium stannide) and Rb8AlxSn46-x (rubidium aluminium stannide), both with the cubic type-I structure (space group Pm-3n, No. 223; a ≃ 12.0 Å). The Al:Sn ratio is close to the idealized A8Al8Sn38 composition and it is shown that it can be varied slightly, in the range of ca ±1.5, depending on the experimental conditions. Both the (Sn,Al)20 and the (Sn,Al)24 cages in the structure are fully occupied by the guest alkali metal atoms, i.e. K or Rb. The A8Al8Sn38 formula has a valence electron count that obeys the valence rules and represents an intrinsic semiconductor, while the experimentally determined compositions A8Al8±xSn38∓x suggest the synthesized materials to be nearly charge-balanced Zintl phases, i.e. they are likely to behave as heavily doped p- or n-type semiconductors.

From the Ternary Phase Ca14Zn1+δSb11 (δ ≈ 0.4) to the Quaternary Solid Solutions Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd, x ≈ 0.9). A Tale of Electron Doping via Rare-Earth Metal Substitutions and the Concomitant Structural Transformations.

The ternary compound Ca14Zn1.37(1)Sb11 and its six rare-earth metal substituted derivatives Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd; x ≈ 0.90 ± 0.06) have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. All compounds formally crystallize in the tetragonal Ca14AlSb11 structure type (space group I41/ acd, No. 142, Z = 8). The crystal structure of Ca14Zn1.37(1)Sb11 subtly differs from the structure of the remaining six, as well as from the structure of the archetype, due to the presence of a partially occupied interstitial Zn position. The extra zinc atom is needed in this structure to alleviate the unfavorable number of valence electrons in the imaginary Ca14ZnSb11. Electron doping, via substitution of RE3+ ions on Ca2+ sites, is shown as an alternative route to achieve electron balance in these Zn-based analogs of the Ca14AlSb11 structure, which does not require the incorporation of interstitial atoms. Electrical resistivity measurements done on single-crystalline samples are in agreement with the notion that Ca14- xRE xZnSb11 moieties behave as either bad metals or heavily doped semiconductors. Magnetization measurements show Curie-Weiss paramagnetic behavior related to the local-moment magnetism of the RE3+ ions.

Niobium-bearing arsenides and germanides from elemental mixtures not involving niobium: a new twist to an old problem in solid-state synthesis.

Three isostructural transition-metal arsenides and germanides, namely niobium nickel arsenide, Nb0.92(1)NiAs, niobium cobalt arsenide, NbCoAs, and niobium nickel germanide, NbNiGe, were obtained as inadvertent side products of high-temperature reactions in sealed niobium containers. In addition to reporting for the very first time the structures of the title compounds, refined from single-crystal X-ray diffraction data, this article also serves as a reminder that niobium containers may not be suitable for the synthesis of ternary arsenides and germanides by traditional high-temperature reactions. Synthetic work involving alkali or alkaline-earth metals, transition or early post-transition metals, and elements from groups 14 or 15 under such conditions may yield Nb-containing products, which at times could be the major products of such reactions.