The typical Zintl phases are compounds formed between the alkali or alkaline-earth metals and the metalloids from group 13, 14, or 15 [1]. The chemical bonding in such systems could be rationalized following the Zintl-Klemm concept [2], which assumes that the electropositive elements act as electron donors (becoming spectator cations), while the electronegative counterparts form covalent bonds to obtain closed-shell electronic configurations. In recent years the field has been expanded to include some d-metals, as well as the nominally divalent Eu and Yb from the lanthanide family. Such materials, e.g., Yb₁₄MnSb₁₁ [3], Ca_xYb_1−xZn₂Sb₂ [4], and EuZn₂Sb₂ [5], among others, have received recognition for their potential in thermoelectric energy conversion [6]. There has also been a lasting research interest in cluster-like Zintl ions [7], which is primarily instigated from the fundamental understanding of their bonding characteristics [7,8], as well as the novel properties offered by materials based on cluster assemblies [9-13].

In this article, we report three new Zintl compounds, Cs₂NaAs₇, Cs₄ZnAs₁₄ and Cs₄CdAs₁₄ all of which feature the nortricyclane-like [As₇]³⁻ clusters. In Cs₂NaAs₇ (formally [Cs⁺]₂[Na⁺][As₇]³⁻) the [As₇]³⁻ polyanions are discrete and “solvated” by Cs⁺ and Na⁺ cations, while Cs₄ZnAs₁₄, and Cs₄CdAs₁₄ (formally [Cs⁺]₄[Zn²⁺]{[As₇]³⁻}₂ and [Cs⁺]₄[Cd²⁺]{[As₇]³⁻}₂) are better viewed as containing larger heteroatomic [As₇-Zn-As₇]⁴⁻ and [As₇-Cd-As₇]⁴⁻ units, respectively. The latter can be considered as being made of two [As₇]³⁻ species, bridged by tetrahedrally coordinated Zn and Cd atoms. The crystal structures and the bonding characteristics of the three compounds are presented, alongside with a discussion on the electronic structure of Cs₂NaAs₇, calculated using the LMTO method [14-16].

Cs₂NaAs₇ crystallizes in a monoclinic crystal system with space group P2₁/c (No. 14, Pearson symbol mP40). Important crystallographic information pertaining to the structure is given in Table 1. Although it has identical formula to A₃As₇ (A = Li, Na, K, Rb, Cs) [17-19], it is not isostructural to any of the above-mentioned binary compounds. In Cs₂NaAs₇, which is a true ternary phase, Cs and Na atoms are ordered on different crystallographic sites.

The packing of the [As₇]³⁻ clusters resembles fcc arrangement with Cs2⁺ cations residing in the octahedral holes left between the clusters, and Cs1⁺ and Na⁺ cations in the tetrahedral holes, respectively (Figure 1). Such anion packing (ABCABC) is the same as in the structure of Cs₃As₇ [19], which is distinct from the double hexagonal close packing (ABAC) in K₃As₇ [19] and the hexagonal close packing (ABAB) in Rb₃As₇ [19].

Cs₄ZnAs₁₄ and Cs₄CdAs₁₄ also crystallize with the same monoclinic space group P2₁/c (No. 14), but their structures are different (Table 1 and Figure 2). Notably, despite the same formulae, they are not isostructural to each other, with Cs₄ZnAs₁₄ having a four-times larger repeating unit (Pearson symbol mP304) than Cs₄CdAs₁₄ (Pearson symbol mP76). The detailed descriptions of each structure follow.

As already mentioned, the structure of Cs2NaAs7 features isolated nortricyclane-like (or birdcagelike) [As₇]³⁻ clusters, with Cs⁺ and Na⁺ cations filling the space between them. The As–As distances (Table 2) could be grouped into three sets based on their length: set A, 2.4799(17)-2.5139(19) Å, which comprises the longest bonds, found between the As atoms of the triangular base (As3, As5, As6); the intermediate set B, which accounts for the bonds between the apical As (As2) and the three As atoms at the “waist” of the cluster (As1, As4, As7), ranging from 2.4042(18) to 2.4237(17) Å; and set C, which encompasses the shortest As–As contacts (2.3411(18)-2.348(2) Å), observed between the three basal As atoms and the three atoms in the waist (Figure 3(a)). Such distribution of bond distances seems to be a signature of all [As₇]³⁻ cluster anions, as it is noted in the binary intermetallic compounds A₃As₇ (A = Li, Na, K, Rb, Cs) [17-19], as well as the compounds [Li(NH₃)₄]₃As₇·NH₃ [20], [Rb(18-crown-6)]₃As₇·8NH₃ [20], and Cs₃As₇·6NH₃ [20], which are crystallized from liquid ammonia solutions. The bond angles are also characteristic, with α, β, γ, and δ (Figure 3(a)) close to 101°, 99°, 105°, and 60°, respectively. These metric parameters are comparable with the corresponding angles in the above-mentioned compounds [17-20]. Following the Zintl-Klemm concept [2], the formal charges in the [As₇]³⁻ clusters can be assigned as (3b-As⁰)₄(2b-As⁻)₃, and the Cs₂NaAs₇ formula can be readily rationalized as [Cs⁺]₂[Na⁺][As₇]3⁻ Electronic band structure calculations confirm this reasoning.

Cs⁴ZnAs₁₄ crystallizes with an unusually large unit cell for an intermetallic compound (V = 10309.9(14) ų) and the structure is devoid of any disorder. The structure contains four crystallographically independent [ZnAs₁₄]⁴⁻ polyanions, which are made up of tetrahedral Zn atoms, bridging two [As₇]³⁻ clusters (Figure 3(b)). The structural complexity is apparently caused by the intricate orientation and packing of the [ZnAs₁₄]⁴⁻ units. Interestingly, all the As atoms bonded to Zn are the 2-bonded As atoms at the waist of the [As7]^3– “birdcage”, wherein they could be assigned as negatively charged (the remaining As atoms are 3-bonded, and thus, with a formal charge 0). The same bonding features have also been revealed for the cluster compounds which have the similar structural motif, e.g., [K(2,2,2-crypt)]₃[In(P₇)₂]·3.5py [21], [K(2,2,2-crypt)]₄ZnE₁₄(E = P, As) [22], and [K(2,2,2-crypt)]₄CdP₁₄·6py [22]. The Zn–As bond distances fall in the range 2.514 to 2.598 Å, comparing well with the sum of their covalent radii [23], as well as with the corresponding distances found in other Zintl compounds with Zn-As bonding such as Ba₂ZnAs₂ [24], Eu₁₁Zn₆As₁₂ [25], and RbZn₄As₃ [26].

Another interesting observation is the geometric distortion of the [As₇]³⁻ moiety of the [ZnAs₁₄]⁴⁻ anion compared to the isolated [As₇]³⁻. Although the nortricyclane-like topology essentially remains intact, the “bidentate” coordination of the [As₇]³⁻ ligand to the Zn atom induces some changes. For example, with a reference to the [Zn(1)As₁₄]⁴⁻ species, the α angles involving the chelating As atoms significantly decrease (∠As4-As5-As12 = 94.256° and ∠As20-As29-As24 = 94.193°), concomitant with the slight increase of the bond distances (0.02-0.04 Å). On the other hand, the distances between the apical As and the bridging As atoms that are not bonded to Zn decrease (As5−As8 = 2.379 Å, As29−As54 = 2.365 Å) to a degree that they fall in the range of distances for the shortest set C (Table 3). Similar distortion is observed for all the eight [As₇]^3– subunits, which again, are not symmetry-equivalent to each other. Examples in which the insertion of transition metals leads to more significant changes to the [Pn₇]^3– cluster are also known from the organometallic literature. For instance, in the [E₇M(CO)3]³⁻ clusters (E = P, As, Sb and M = Cr, Mo, W) [27,28], the nortricyclane-like E₇³⁻ fragments transformed into norbornadiene-like clusters due to the four bonds between the transition metal and two of the bridging and two of the basal pnictogens. More profound change has been noted in [Sb₇Ni₃(CO)₃]^3– [29], where the Sb₇^3– is bonded to three Ni atoms, forming a 10-vertex nido cluster.

Cs₄CdAs₁₄ crystallizes with the same space group as Cs₄ZnAs₁₄, however, with only one [CdAs₁₄]⁴⁻ polyanion in the asymmetric unit. The Cd atoms are tetrahedrally bonded to the As atoms from two adjacent [As₇]³⁻ clusters, with average Cd–As distance of 2.757 Å, which is just slightly larger than the sum of their covalent radii (r_Cd = 1.38 Å; r_As = 1.21 Å) [23]. In both Cs₄ZnAs₁₄ and Cs₄CdAs₁₄, the [ZnAs₁₄]^4– and [CdAs₁₄]^4– anions form a simple cubic array (very distorted in the case of Cs₄ZnAs₁₄). Compared with the ZnAs₄ tetrahedra in the analogous [ZnAs₁₄]⁴⁻ motif of Cs₄ZnAs₁₄, the CdAs₄ tetrahedra are more distorted. This can be seen from an inspection of the As-Cd-As angles, which deviate more from the ideal 109.5° tetrahedral angle. The [As₇]^3– units experience similar geometric distortion due to the “bidentate” coordination to Cd; however, comparison of the corresponding distances (Table 3) and angles reveals that the distortion in [CdAs₁₄]⁴⁻ is less pronounced. This is understandable since the longer Cd–As distances would naturally allow more flexibility.

Following the above discussion, the electron count in both Cs₄ZnAs₁₄ and Cs₄CdAs₁₄ could be assigned in the same way: (Cs⁺)₄(4b-Zn²⁻)(3b-As⁰)₁₂(2b-As⁻)₂ and (Cs⁺)₄(4b-Cd²⁻)(3b-As⁰)₁₂(2b-As⁻)₂, which is in consistent with the Zintl formalism [2]. Exaggerating the ionicity of the Zn–As and Cd–As interactions, the formula units can also be broken down as [Cs⁺]₄[Zn²⁺]{[As₇]³⁻}₂ and [Cs⁺]₄[Cd²⁺]{[As₇]³⁻}₂, respectively.

The electronic structure of Cs₂NaAs₇ was calculated using the LMTO method [14-16] and the plot of the total and partial density of states is shown in Figure 4(a). A band-gap in the order of 1.6 eV is noticeable from the plot, suggesting that Cs2NaAs7 should be an intrinsic semiconductor. Note that the LMTO method usually underestimates the band-gap, this value actually agrees very well with the dark red color of the crystals. Due to the low symmetry and very complex structures of Cs₄ZnAs₁₄ and Cs₄CdAs₁₄, the electronic structures for neither were carried out; the appearance of their crystals also suggested semiconducting behavior with a band-gap in the visible range.

Analyzing the electronic structure of Cs₂NaAs₇, one can readily see that the states below the Fermi level in the range from −5 to 0 eV (set as the Fermi level) are predominately contributed from As p orbitals, with only a small admixture of states from the alkali metals, in agreement with the Zintl formalism. In the vicinity of the Fermi level, however, the contributions of the three different types of As atoms differ very much, as seen from Figure 4(b). The apical As atom has almost negligible partial DOS, while the contribution from the As atoms at the bridging position (waist of the cluster) is significant in this energy window. Such characteristics of the electronic structure should indicate different chemical and physical properties of As atoms at different positions, i.e., chemical reactivity.

All manipulations involving the alkali metals were performed either inside an argon-filled glove box with controlled moisture/oxygen levels or under vacuum. Crystals of Cs₂NaAs₇, Cs₄ZnAs₁₄, and Cs₄CdAs₁₄ were all identified from reactions originally aimed at the pnictide clathrates Cs₈Zn₁₈As₂₈, Cs₈Cd₁₈As₂₈ or the mixed-cation variant Na₆Cs₂Zn₁₈As₂₈. In the typical reactions, the elements (Na/Cs/Ca/Zn/Cd/As, purchased from either Alfa Aesar or Aldrich with the stated purity higher than 99.9%, used as received) with the desired stoichiometric ratio were loaded into niobium ampoules. The niobium ampoules were arc-welded under high purity Ar and then jacketed within fused silica tubes, which were subsequently flame-sealed under vacuum. The reaction mixtures were heated up to 500 °C in a programmable furnace, and equilibrated for 1 week before being slowly cooled to room temperature. The dark red crystals of Cs₂NaAs₇ were initially identified as a byproduct of a reaction with starting elements Na, Cs, Zn, and As, carried out as described above. The major product was CsZn₄As₃ [26], with Zn₃As₂ [30] and Cs₂NaAs₇ as side products. Cs₂NaAs₇ can be made as phase pure material from stoichiometric mixture without Zn.

From an analogous reaction with starting elements Na, Cs, Cd, and As, Cs₄CdAs₁₄ was initially identified, together with Cs₈Cd₁₈As₂₈ [31], NaCd₄As₃ [26], and CdAs₂ [32]. The differentiation from the other phases was possible due to the red color and transparent habit of the crystals. Cs₄ZnAs₁₄ was originally obtained from a reaction loaded as Ca₁₆Cs₈Zn_58.67As_77.33, and subjected to the above-described heat treatment. The dark red crystals in this reaction were identified as Cs₄ZnAs₁₄, between the black crystals of CaZn₂As₂ [33] and the silvery ones of Zn₃As₂ [30]. After the structures and the compositions were established from single-crystal diffraction work, Cs₄CdAs₁₄ and Cs₄ZnAs₁₄ were synthesized in high yield from the corresponding stoichiometric reactions.

All three compounds degraded quickly upon exposure to air.

The crystal structures of the title compounds were established using single-crystal X-ray diffraction. The diffraction data were collected on a Bruker SMART CCD-based diffractometer using monochromated Mo Kα₁ radiation. Crystals were cut to suitable dimensions (<100 μm) under a microscope and then mounted on glass fiber with Paratone-N oil. The fiber was then quickly transferred to the goniometer of the diffractometer, where a cold nitrogen stream (200(2) K) was used to harden the oil in order to protect the crystals from being oxidized. Full spheres of data were collected in four batch runs with a frame width of 0.3° for ω and θ. Integration of the intensity data was done with SAINT program [34], and semi-empirical absorption correction based on equivalents was applied with the SADABS code [35]. The structures were solved by direct method and refined to convergence by full matrix least squares on F² using the SHELXTL package [36]. The atomic coordinates were standardized with the aid of the Structure TIDY [37]. In the last refinement cycles, all atoms were treated with anisotropic displacement parameters. Tables with selected refinement parameters and the atomic coordinates and equivalent isotropic displacement parameters are submitted as electronic supplementary information. Selected crystal data and refinement parameters are given in Table 1; important bond distances and angles are listed in Table 2 and Table 3. CIFs have also been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247-808-666; e-mail: crysdata@fiz.karlsruhe.de)—depository numbers CSD-423080 for Cs₄NaAs₇, CSD-423081 for Cs₄CdAs₁₄, and CSD-423082 for Cs₄CdAs₁₄.

The Stuttgart TB-LMTO 4.7 code [38], which employs the tight-binding linear muffin-tin orbital (TB-LMTO) method [14-16], was used to calculate the band structures of Cs₂NaAs₇. In this program, local density approximation (LDA) was used to treat exchange and correlation [39]. All relativistic effects except for spin-orbital coupling were taken into account by scalar relativistic approximation [40]. Due to the limited computing resources, Cs₄CdAs₁₄ and Cs₄ZnAs₁₄ (very complex and large structures) could not be treated. For the calculation of Cs₂NaAs₇, the basis set included the 6s, 6p, 5d and 4f orbitals for Cs, 3s, 3p and 3d orbitals for Na, and 4s, 4p and 4d orbitals for As. The 6p, 5d and 4f orbitals of Cs, 3p and 3d orbitals of Na, and 4d orbital of As were treated with the downfolding technique [41]. The total and partial density of states were plotted with the Fermi level set as a reference at 0 eV.

Three new Zintl phases Cs₂NaAs₇, Cs₄ZnAs₁₄ and Cs₄CdAs₁₄ have been synthesized via high temperature reactions. The structure of Cs₂NaAs₇ features the nortricyclane-like cluster [As₇]³⁻, while the structures of the latter two contain very large [ZnAs₁₄]⁴⁻ and [CdAs₁₄]⁴⁻ anions, which are composed of two [As₇]³⁻ units, linked by 4-coordinated Zn or Cd. Band structure calculations confirm the intrinsic semiconductor with a size of band-gap consistent with the dark red color of the crystals.