Synthesis, Structure, and Properties of the Complex Zintl Phase Eu9Zn4.5As9: A Candidate Topological Insulator and Thermoelectric Material

1. Introduction

Zintl phases are intermetallic compounds, characterized by the simultaneous coexistence of covalent and ionic bonding interactions in their structures. Formed from groups 1, 2, or lanthanide metals and the more electronegative early post-transition elements from groups 13–15, the crystal structures of these materials often exhibit unique (poly)anionic frameworks. The charge-balance requirements according to the Zintl-Klemm concept [1] are satisfied if one considers ionic-like interactions between the elements with differing electronegativities. The structural complexity that arises from this formalism is thought of as both the hallmark of Zintl phases and an important element in the design of materials with unique physical properties [2].

Zintl pnictides, the largest group of Zintl phases that have been discovered thus far, arise from combinations of the aforementioned less electronegative metals with the pnictogen elements P, As, Sb, and Bi (Pn hereafter). The unique chemistry of these materials allows them to form a large variety of homoatomic Pn–Pn and heteroatomic Pn–M bonds (M = transition metals and Group 13/14 elements), which result in unique and complex structural motifs [3]. These combinations often yield narrow bandgaps and enhanced transport properties [4,5,6]. As a result, much progress surrounding the study of Zintl pnictides has been dedicated to the study of their potential toward thermoelectric applications [4].

While many Zintl pnictides have already been studied as potential thermoelectrics, the majority of the reports have been dedicated toward the characterization of antimonides [4]. By comparison, Zintl arsenides are less-studied, with only a few publications reporting transport or magnetic properties [7], although computational works have predicted increased thermoelectric performance for several arsenide systems [7,8,9], as well as the experimentally validated interesting physical properties like colossal magnetoresistance, anomalous Hall effect, and superconductivity [10,11,12,13,14,15,16,17].

Herein, we expand the field of Zintl arsenides by reporting, for the first time, the synthesis and the structural characterization of a novel Zintl phase, Eu₉Zn_4.5As₉. We present a detailed analysis of the crystal structure, the first of its kind, as well as its computed electronic band structure. As the chemical formula indicates, this compound belongs to the class of so-called “9–4–9” phases, A₉M_4+xPn₉ (A = Ca, Sr, Eu, Yb; M = Mg, Mn, Zn, Cd; Pn = As, Sb, Bi). This family series is compositionally diverse and comprises several different structural types. Importantly, several “9–4–9” phases are well known as highly efficient thermoelectric materials with potential for dupability and excellent transport properties (Table 1).

2. Materials and Methods

2.1. Synthesis

The ternary arsenide Eu₉Zn_4.5As₉ was originally discovered during attempts to optimize the synthesis of the Zintl arsenide Eu₃ZnAs₃ [34], and later heteroanionic Zintl arsenide oxide, Eu₁₄Zn₅As₁₂O [35]. The following procedure can be utilized to synthesize large needle-like crystals (Figure 1) of the title phase. Europium metal pieces (Edgetech Ind., 99%), zinc shot (Sigma-Aldrich, 99.9%), arsenic granules (ThermoFisher, 99.999%), and tin shot (Thermo Scientific, 99.8%) were combined in a 5:1:3:20 molar ratio and placed in an alumina crucible. Europium pieces were filed prior to use to remove oxidized coatings. All materials were handled inside an argon-filled glove box to prevent degradation (H₂O and O₂ ≤ 1 ppm). The loaded alumina crucible was then sealed within an evacuated fused silica ampoule topped with the quartz wool, which acts as a filter. The reactants were then heated to 1000°C and held for 24 h before being slowly cooled to 650°C and centrifuged to remove metal flux excess. The ampoules were then cracked open inside the glovebox.

This approach leads to the formation of large (≥ 1 mm) needle-like crystals of Eu₉Zn_4.5As₉, which are black in color and very brittle. Crystals of this compound can also be obtained using other approaches, as discussed in Section 3.1. Crystals were found to be air- and moisture-stable for at least several days. Suitable crystals were selected from the reaction product for both single-crystal X-ray diffraction and magnetic studies.

CAUTION! Arsenic and arsenic-containing compounds should be handled in proper personal protective equipment. Reactions must be conducted in well-ventilated areas, as reaction temperatures above the sublimation point of arsenic (614°C) can produce dangerous byproducts. Arsenic tends to hydrolyze and create arsane gas, which is highly toxic; crucibles should not be immediately cleaned with water. Slow oxidation under the fume hood is advised.

2.2. Structural Characterization

Structural characterization of the Eu₉Zn_4.5As₉ phase was carried out using single-crystal X-ray diffraction (SCXRD) methods. Data were collected using a Bruker D8 Venture DUO diffractometer (Bruker, Billerica, MA, USA) equipped with a Photon III C14 detector and Ag Kα radiation microfocus tube (λ = 0.56086 Å). Small black needle-like crystals were selected from the reaction product and mounted on a MiTiGen loop attached to the goniometer. A constant cold (100 K) nitrogen flow was applied to the crystal during data collection to protect the material from potential degradation due to contact with air and moisture. After collection, the data were processed using the SAINT and SADABS programs (version 2.03) from the APEX4 software suite [36,37]. The crystal structure was solved using intrinsic phasing with SHELXT (version 2018/2) and further refined via full-matrix least-squares on F² with SHELXL (version 2018/3) [38,39]. Olex2 (version 1.5) was used as a GUI for structure determination [40]. Finally, atomic coordinates were standardized using the STRUCTURE TIDY software [41]. Selected crystallographic data, including data collection details, atomic coordinates, and bond distances, can be found in Table 2, Table 3 and Table 4.

2.3. Electronic Structure Calculations

Electronic structure calculations were performed by employing the density functional theory (DFT) using the Vienna Ab Initio Simulation Package (VASP) (version 6.4.3) [42]. The Perdew–Burke–Ernzerhof (PBE) exchange–correlation energy with the generalized gradient approximation (GGA) was employed for calculations [43]. Both core and valence electrons were treated with the projector-augmented wave method, with the Eu_2 pseudopotential used to consider Eu in the +2 configuration [44]. Structure optimization was performed prior to calculating the density of states and band structures. A Gamma-centered mesh of 8 × 3 × 1 points in the Brillouin zone generated by VASPKIT and an energy cutoff of 430 eV with a convergence tolerance of 1×10^–6 eV/atom was used for self-consistent field calculations [45].

As disorder cannot be accounted for in VASP, a disorder-free model had to be designed in order to perform the calculations. With a goal of preserving charge-balanced composition, the split As1A/As1B and Zn4A/Zn4B positions were consolidated into single, fully occupied sites (As1 and Zn4) by averaging the refined atomic coordinates. Similarly, with both the Eu1 and Eu2 8d sites modeled as 50% occupied, we returned to the 4c fully occupied sites located on the mirror plane with coordinates listed in the footnote to Table 3. This model was again refined with SHELXL to ensure the accuracy of the newly applied coordinates. Finally, the half-occupied Zn sites (Zn1, Zn3, and Zn5) had to be addressed. As this disorder could not be easily resolved by treating these sites as fully occupied, the symmetry of the model was reduced from Pnma to Pmn2₁ in order to split each Zn position (Wyckoff 4c) into two distinct positions (Wyckoff 2a) with the help of the ISODISTORT tool [46,47]. From there, half of the newly split Zn1, Zn3, and Zn5 sites were removed, and the remaining atoms were set to fully occupied. This method produced both a structurally comparable and charge-balanced model suitable for calculations.

2.4. Magnetic Properties Measurements

Magnetic susceptibility was measured using a Quantum Design PPMS DynaCool system equipped with a 9 T Vibrating Sample Magnetometer (VSM). A suitable single crystal of Eu₉Zn_4.5As₉ was selected and checked with the SCXRD method to confirm the identity of the sample prior to property studies. The crystal was then cleaned of flux residue and washed with hexane. It was then mounted on the sample holder and fixed using GE varnish glue. Temperature-dependent measurements were obtained with a magnetic field of 1 kOe applied along the b-axis over a temperature range from 1.8 K to 300 K under both zero-field-cooling (ZFC) and field-cooling (FC) conditions. Magnetic isotherms were measured at 2 K, 6.5 K, 11 K, 15.5 K, and 20 K with applied magnetic fields varying from 0 Oe to 90 kOe.

3. Results and Discussion

3.1. Synthesis

As was mentioned in Section 2.1, the initial goal of the experiment that led us to the discovery of Eu₉Zn_4.5As₉ was to optimize the synthetic conditions for the growth of single crystals of the heteroanionic Eu₁₄Zn₅As₁₂O [35] Zintl arsenide oxide. The attempt instead led to the formation of a new, previously unreported compound, later identified as the title phase. Eu₉Zn_4.5As₉ was also identified from our efforts to obtain single-phase Eu₃ZnAs₃ [34]. In both cases, Eu₉Zn_4.5As₉ was observed as a minor phase. To obtain the title compound in higher yields, several other synthetic methods were employed. Here, we caution that direct reactions of the elements in sealed Nb tubes are not suitable for high-temperature work with As[48], although many previously reported “9–4–9” compounds (Table 1) can be synthesized by this method.

Since there have been successful reports using lead or antimony molten fluxes to grow large single crystals [32,33], our subsequent efforts involved the former. Pb-flux, indeed, also yielded Eu₉Zn_4.5As₉ crystals over various component ratios and at temperatures ranging from 900° C to 1100° C. However, in most cases, the reactions afforded multiphase products with tiny crystals of the title phase, suitable for SCXRD studies but not for property studies. By comparison, crystals obtained by the tin-flux method at similar conditions reached lengths of up to 3 mm, making them suitable for property characterization.

Another synthetic challenge worth mentioning here is the heavily populated Eu–Zn–As compositional space with already six reported ternary phases, EuZn₂As₂ [49], Eu₂Zn₂As₃ [50], Eu₁₁Zn₆As₁₂ [51], Eu₃ZnAs₃ [34], Eu₂₁Zn₄As₁₈ [52,53], Eu₁₄Zn_1+xAs₁₁ [54], alongside multiple binaries (Figure 1). The discovered compound falls between Eu₁₁Zn₆As₁₂ [51] and Eu₃ZnAs₃ phases [34], which explains the presence of the latter as common impurity phases.

Interestingly, most of the reported ternary compounds in the Eu–Zn–As phase diagram possess unusual thermoelectric, electrical, and magnetic properties, as was exemplified for EuZn₂As₂ [55,56,57,58,59], Eu₂Zn₂As₃ [50], Eu₁₁Zn₆As₁₂ [51], Eu₂₁Zn₄As₁₈ [60], and Eu₁₄Zn_1+xAs₁₁ [61]. Arguably, further investigation of this compound will be warranted, which may yield the discovery of new and interesting physical properties.

3.2. Crystal Structure and Bonding

Formally, the structure of Eu₉Zn_4.5As₉ belongs to the Ca₉Mn_4+xSb₉ structure type (space group Pnma, Table 1) [32], although it has its own nuances, specifically, it is more disordered. In light of this, the structural description here will be brief, focusing on differences from the archetype.

Being only the second Zintl arsenide (apart from Ca₉Zn_4+xAs₉) belonging to the 9–4–9 family (Table 1), the title Eu₉Zn_4.5As₉ phase exemplifies a rare example of so-called 9–4–9 “supercell” [32,33] with a doubled unit cell volume, compared to the majority of the reported A₉M_4+xPn₉ compositions (A = Ca, Sr, Ba, Eu, Yb; M = Mn, Zn, Cd; Pn = Sb, Bi, Table 1). Interestingly, recently discovered Mg-bearing Sr₉Mg_4+xPn₉ (Pn = Sb, Bi) phases [33] are also isostructural to the “supercell” structure presented herein.

As can be expected, the unit cell volume of the Eu₉Zn_4.5As₉ phase (~2276 ų) is slightly larger than that of the only isostructural arsenide analog, Ca₉Zn_4+xAs₉ (~2255 ų) [32]. This difference is due to the larger atomic radius of Eu (1.98 Å) compared to that of Ca (1.76 Å) [62]. As a general rule, the majority of A₉M_4+xPn₉ compounds favor smaller cations, such as Ca or Yb, and, in fewer instances, larger Sr and Eu have been found to form phases with these structures (Table 1). On this note, Eu₉Zn_4.5As₉ exemplifies the first Eu-bearing 9–4–9 Zintl arsenide.

The schematic view of the orthorhombic crystal structure of Eu₉Zn_4.5As₉ is shown in Figure 2a. The imaginary, disorderless structure can be described with the Wyckoff sequence c²⁴, with 9 crystallographically independent Eu sites, 6 Zn sites, and 9 As sites all occupying special 4c positions. However, if all atomic positions are considered with full site occupancy factors (SOF), the resulting composition of (Eu²⁺)₉(Zn²⁺)₆(As⁻³)₉ would significantly depart from being charge-balanced. On the other hand, the common trait of the structural chemistry of 9–4–9 compounds is the presence of partially occupied M sites (M = Mg, Mn, Zn, Cd), as inferred from the non-stoichiometric A₉M_4+xPn₉ (though close to charge-balanced) compositions, quantified by the “x” value.

The Eu₉Zn_4.5As₉ structure is not an exception, and it contains three Zn sites (Zn1, Zn3, and Zn5) with fractional SOFs of 0.485(3), 0.486(3), and 0.499(3), respectively, should they be refined unconstrained. Since all these values are very close to 0.5 (within 5σ), we have fixed their occupancies to 50%. Assuming divalent Eu (which is confirmed by our magnetic property studies, vide infra), this model yields the charge-balanced (Eu²⁺)₉(Zn²⁺)_4.5(As⁻³)₉ composition.

We do not, however, exclude the possibility that occupancies of these three Zn sites may vary from sample to sample, leading to Eu₉Zn_4+xAs₉ compositions in which the x value slightly deviates from 0.5, akin to previously reported Sr₉Mn_4.45(1)Bi₉ and Ca₉Mn_4.46(1)As₉ [32]. Despite the predicted (semi)metallic behavior of the title compound, as discussed in Section 3.3, these small differences may slightly alter charge-carrier concentration and transport properties, similarly to the previously reported studies of thermoelectric properties of several other 9–4–9s [22,25,30,33,63,64]. In this context, the isovalent and aliovalent doping of these interstitial atoms can be viewed as an efficient mechanism to alter electrical transport properties and, therefore, potentially increase thermoelectric performance.

In addition, two Eu sites, Eu1 and Eu2, were modeled as split 8d sites, each with SOF of 0.5, due to the offset from the ac mirror plane. The 50% occupancy of Zn1 and Zn5 sites correlates with the split on Eu1 site, as shown in Figure 3b, in analogy to the previous reports on Sr₉Mn_4.45(1)Bi₉ and Ca₉Mn_4.46(1)As₉ [32]. The split on the Eu2 site, although less pronounced (as can be judged from the deviation of the y coordinate from ¼, Table 3), is not observed in these previously reported isostructural compounds for Sr2 and Ca2, making the title phase more disordered. This split, however, correlates with the concomitant split of Zn4 and As1 sites as illustrated in Figure 3c. Both sites were refined initially anisotropically and showed significantly elongated thermal ellipsoids, so after the modeled “split”, their occupancies were found to be very close to 0.5 (Table 3), comparable with the previous reports on Sr₉Mn_4.45(1)Bi₉ and Ca₉Mn_4.46(1)As₉. The correlation between positional disorder at the As1 and Eu1/Eu2 sites is also apparent, assuming that the As1 atom is octahedrally coordinated by Eu atoms, as shown in Figure 3c.

The elongation of the thermal parameter on As1 and consequent split to As1A and As1B positions is necessary to complete the tetrahedral coordination of partially occupied Zn4B, Zn1, and Zn5 (Figure 2c), avoiding unrealistically long interatomic Zn–As distances (Table 4). Such disorder is quite common for Zintl pnictides, and the nearly identical local structure arrangement was recently reported for the Eu₁₄Zn₅As₁₂O phase [35].

Due to the lack of homoatomic As–As bonds, the anionic substructure of Eu₉Zn_4.5As₉ is exclusively composed of Zn-centered polyhedra (Figure 2b). As shown in previous works, the unique crystal chemistry of Zn-bearing Zintl phases mainly originates from the versatility of Zn atoms to coordinate pnictogens in trigonal-planar and tetrahedral fashion [10,34,35,54,65,66,67,68]. In most cases, a partially occupied Zn site is associated with trigonal planar coordination, whereas fully occupied atoms tend to be tetrahedrally coordinated. While this is not always the case, as was recently shown for the Sr₃ZnAs₃ phase [34], the occupancy of metal often dictates the degree of protrusion from the As₃ plane. In Figure 2c, the local coordination environments of all Zn sites are presented. Only for the fully occupied Zn2 and Zn6 atoms, the longest Zn–As contact does not exceed 2.76 Å, which is somewhat longer than the sum of covalent radii of Zn and As (~2.41 Å) [62], yet still indicates a sizable interaction. These two atoms also protrude the most from the As₃ plane (~0.76 Å for Zn2 and 0.68 Å for Zn6), whereas the protrusion does not exceed ~0.56 Å for the partially occupied Zn atoms. The corresponding bond lengths fall in a wide range (Table 4), with all values being comparable to the previous reports [49,50,51,52,53,54].

The corner- and edge-sharing arrangement of these [ZnAs₄] and [ZnAs₃] units (Figure 2c) composes the anionic substructure of Eu₉Zn_4.5As₉, which can be described as a series of [Zn₉As₁₈]³⁶⁻ layers stacked along the c-direction (Figure 2a,b) and was discussed in a detailed manner in the previous works [32,33]. Apparently, the 2D dimensionality of this assembly can be partially broken due to multiple partially occupied Zn and As sites, as exemplified in Figure 4b, which represents the disordered-free model used for the electronic structure calculations, vide infra. This needs to be considered when drawing structure-property relationships in the context of electronic and thermal transport. For instance, extensive disorder may reduce the lattice thermal conductivity, whereas doping with cations that prefer tetrahedral coordination, such as Al³⁺, Mn²⁺, or In³⁺, may prevent the dimensionality reduction and positively affect transport properties. These results will be presented in future studies.

At last, we would like to briefly discuss the complex arrangement of the lattice made of magnetic Eu²⁺ cations (Figure 4a). It is mainly composed of three main building units, typical for Zintl pnictides: (i) trigonal [Eu₆] prisms, both empty and filled with As anions, (ii) distorted [Eu₆] octahedra, that host disordered As1 atoms, and (iii) empty [As₄] tetrahedra. There are several relatively close Eu–Eu contacts (shorter than 4.0 Å), which can be an indication of complex magnetic interactions and, potentially, anisotropy of the magnetic response.

3.3. Electronic Structure

As discussed in the experimental section, a supercell model had to be constructed in order to address partially occupied sites when performing calculations on Eu₉Zn_4.5As₉. This model is schematically represented in Figure 4b. Figure 5 shows the calculated density of states (DOS) and band structure for the title phase in the energy range of –5 to 2 eV. The Fermi level is set at 0 eV and indicated by the dashed line. A small pseudogap is observed in the DOS plot at the Fermi level.

Following the earlier discussion, the Eu₉Zn_4.5As₉ formula is charge-balanced, and one should expect the bulk material to be a semiconductor, possibly with a narrow bandgap. From general considerations, viz., the electronegativity differences between the respective elements, many other Eu, Zn, and As-based materials should fall within the realm of semiconducting behavior. Yet, these expectations are only partially corroborated by the electronic structure calculations for Eu₉Zn_4.5As₉, where the gapless DOS plot indicates semimetallic behavior. A possible explanation for it is the partial covalency of the Eu–As interactions, which is not captured by the fully ionic (Eu²⁺)₉(Zn²⁺)_4.5(As^3–)₉ description, Evidence in support of this conjecture are the partial DOS curves, which show that Eu–p and Eu–d orbitals make up the majority of states below the Fermi level, with a lesser contribution from Zn–p and As–p states. The total and partial DOS plots exhibit features similar to those of other reported ternary Zintl pnictides [53].

Also of note is the fact that in the band structure of Eu₉Zn_4.5As₉, along the Y–Γ path (Figure 5c), the bands come extremely close to one another without crossing to form Dirac-like linearized points. This band feature has been found in other ternary topological insulating Zintl phases, such as EuAuSb [69], EuCd₂As₂ [70], and CaIn₂As₂ [11], which could indicate Eu₉Zn_4.5As₉ as a possible nontrivial topological material. Computational studies exploring the role of spin-orbit coupling (SOC) and symmetry analyses, as well as more in-depth physical property measurements, could shed further light on the title phase’s electronic structure, however these studies are beyond the scope of this work and will not be addressed here.

3.4. Magnetic Properties

Temperature-dependent magnetic susceptibility of single-crystal Eu₉Zn_4.5As₉ was measured under a magnetic field of 1 kOe applied along the b axis using both zero-field-cooled (ZFC) and field-cooled (FC) protocols (Figure 6a). At high temperatures, the magnetic susceptibility of Eu₉Zn_4.5As₉ is well described by Curie–Weiss behavior (1)

χ(T) = C/(T − θ_CW)

(1)

where C represents the Curie constant (emu·mol⁻¹·K⁻¹), given by C = N_Aµ_eff²/3k_B (N_A is Avogadro’s number, µ_eff is the effective magnetic moment, k_B is the Boltzmann constant), and θ_CW denotes the Curie-Weiss temperature [71]. A fit to the susceptibility data over the temperature range 50–300 K yields an effective magnetic moment µ_eff = 7.9 µ_B and Weiss constant θ_CW = −2.15 K. The extracted effective moment is in excellent agreement with the theoretical free-ion value µ_th = 7.94 µ_B expected for divalent Eu²⁺ ions with electronic configuration [Xe] 4f⁷, total angular momentum J=7/2 , and Landé factor g=2 [72]. The magnetization measurement further shows a saturation moment consistent with the theoretical value µ_sat = g J µ_B = 7.0 µ_B per Eu²⁺ ion (Figure 6b).

Upon cooling, the magnetic susceptibility of Eu₉Zn_4.5As₉ exhibits a pronounced anomaly at the Néel temperature T_N = 7.3 K (Figure 6a, inset), marking the onset of antiferromagnetic order. Below T_N, a weak feature appears in the field-cooled susceptibility data near 4.9 K, accompanied by a small ZFC–FC bifurcation. Previously reported Eu-bearing 9–4–9 phases, Eu₉Cd_4.2Bi₉ [26] and Eu₉Cd_4.45Sb₉ [32], exhibit similar antiferromagnetic transitions at T_N = 11 K and 10 K, respectively. Multiple other Eu-bearing Zintl arsenides also exhibit the antiferromagnetic transition, which is a common trait for Zintl chemistry of Eu [30,73,74,75,76,77,78,79,80,81,82,83]. In contrast, the Yb-containing representatives of this family display simple paramagnetic behavior along the low-temperature range [26,27].

4. Conclusions

In this work, we have introduced another member of the heavily populated Eu–Zn–As system—a new Eu-bearing Zintl arsenide, Eu₉Zn_4.5As₉. This compound has a complex, heavily disordered crystal structure, although the disorder is necessary to achieve a charge-balanced composition. Electronic structure calculations predict that this compound will likely behave like a (semi)metal, which, together with the presence of a complex framework of magnetic Eu²⁺ species, could lead to interesting physical properties, such as colossal magnetoresistance, complex magnetic interactions, or other quantum phenomena typical of other Zintl phases with such features. In addition, this compound can be thoroughly doped with iso-and aliovalent metal cations, such as Cd²⁺, Mn²⁺, Mg²⁺, Al³⁺, Ga³⁺, and In³⁺, which may open the possibility for excellent thermoelectric performance, which will be studied in the following works.