In the past 10–15 years, intermetallic clathrates have attracted widespread interest, largely due to their demonstrated potential in thermoelectrics development [1,2,3,4]. Having successfully synthesized Rb_7.3Na₁₆Ga₂₀Si₁₁₆ and Cs₈Na₁₆Ga₂₁Si₁₁₅—the first clathrate-II compounds with mixed Ga and Si [5], as well as the first arsenic-based clathrates of type-I Rb₈Zn₁₈As₂₈ and Cs₈Cd₁₈As₂₈ [6], we extended our attention to the A-Ga-Pn systems (A = alkali metal or mixtures of alkali metals, Pn = pnictogen, i.e., group 15 element). Our motivation here was the idea to produce gallium-phosphide and/or gallium-arsenide clathrates (in analogy with the well-known III-V semiconductors, which are isoelectronic with Si and Ge), but so far, all synthetic efforts have failed to provide any evidence for the possible existence of such compounds. Instead, these experiments afforded a number of binary, ternary, and even quaternary compounds, some of which are with novel structures.

With this paper we detail the synthesis and the structural characterization of the new quaternary Zintl compounds RbNa₈Ga₃As₆ and RbNa₈Ga₃P₆, which are isoelectronic and isostructural. They crystallize with a novel orthorhombic structure type, featuring a somewhat unusual building block—[GaPn₃]⁶⁻ trigonal planar units (isosteric with the borate BO₃³⁻, as well as carbonate CO₃²⁻ anions), which are cyclically trimerized into [Ga₃Pn₆]⁹⁻ (isosteric with the metaborate anion B₃O₆³⁻, as well as the 1,3,5-trioxanetrione, a cyclic trimer of carbon dioxide). We note here that while the ternary A-Tr-Pn (Tr = triel i.e., group 13 element) phase diagrams have already been extensively explored, and many A-Tr-Pn compounds have been reported [7,8,9,10,11,12], very little is known about the corresponding A'-A''-Tr-Pn systems, where A' and A'' are two different alkali metals. The newly identified RbNa₈Ga₃As₆ and RbNa₈Ga₃P₆ (formally A₃TrPn₂) underscore the utility of mixtures of chemically and spatially dissimilar cations for the discovery of new compounds with novel structures. On this note, we point out that the prevailing motifs in the structures of most ternary A-Tr-Pn compounds are [TrPn₄]⁹⁻ tetrahedra, either isolated or condensed in different fashions to form diverse polyanionic structures, e.g., and chains in Na₃AlAs₂ [7] and K₃Al₂As₃ [8], respectively; and layers in K₂Al₂Sb₃ [9] and K₃Ga₃As₄ [10], respectively; and 3-D framework in Na₉In₃Bi₆ structure [11]. Planar [Ga₃As₆]⁹⁻, [Ga₃Sb₆]⁹⁻ and [Ga₃Bi₆]⁹⁻ units, analogous to the ones described herein are known only for K₂₀(Ga₃As₆)₂As_0.66 and K₂₀(Ga₃Sb₆)₂Sb_0.66 [12]; and K₂₀(Ga₃Bi₆)₂Bi_0.66 [11].

Reported as well is the new compound Na₁₀NbGaAs₆, which is isoelectronic and isostructural to K₁₀NbInAs₆ [13]. This phase was identified as an inadvertent product from the unwanted reaction of the reaction vessel (niobium) with elemental arsenic; its structure comprises NbAs₄ and GaAs₄ tetrahedra, edge-shared into [NbGaAs₆]¹⁰⁻ dimers, which are isosteric with diborane molecule, B₂H₆.

RbNa₈Ga₃As₆ and RbNa₈Ga₃P₆ both crystallize with the orthorhombic space group Pnma (No. 62, Pearson symbol oP72). The structure contains 18 independent sites in the asymmetric unit—one Rb, eight Na, three Ga, and six pnictogen atoms—all located at special position 4c. The structure can be viewed as being built from [Ga₃Pn₆]⁹⁻ polyanions with alkali metal cations counterbalancing the charges and filling the space among them (Figure 1). The triangularly-shaped [Ga₃Pn₆]⁹⁻ motif could be considered as a cyclic trimer of the [GaPn₃]⁶⁻ trigonal planar units, which form a 6-membered ring by sharing two of the Pn atoms from each [GaPn₃]⁶⁻. Therefore, all three Ga atoms are 3-bonded, while the three Pn atoms on the 6-membered ring are 2-bonded and the three Pn atoms attaching to the ring are 1-bonded. The distances between Ga and the Pn atoms are all within the normal ranges for covalently bound Ga–As and Ga–P. For example, the Ga–As distances range from 2.335 to 2.422 Å; the Ga–P distances range from 2.245 to 2.337 Å (Table 1). These values compare very well with the sums of the corresponding covalent radii (r_Ga = 1.246 Å; r_As = 1.210 Å; r_P = 1.10 Å) [14], as well as with the Ga–Pn distances found in compounds with Ga in similar 3-fold coordination environment of As or P (Table 2), e.g., d_Ga–As = 2.367 ÷ 2.426 Å in Rb₂GaAs₂ [15], d_Ga–As = 2.367 ÷ 2.442 Å in K₂₀(Ga₃As₆)₂As_0.66 [12], and d_Ga–P = 2.226 ÷ 2.392 Å in Rb₃GaP₂ [16]. In general, the distances between Ga and the 2-bonded Pn are just about 0.08 Å longer than those between Ga and the 1-bonded Pn atom, which indicates that there is no appreciable π-bonding in the 6-membered ring. Hence, the formula RbNa₈Ga₃Pn₆ represents a salt-like, electron-balanced Zintl phase [17]. According to the valence rules, the charges could be assigned as Rb⁺(Na⁺)₈(Ga³⁺)₃(Pn^3–)₆ based on oxidation states and counting the Ga and the Pn atoms as tri-valent species; alternatively, based on formal charges, the formula can be broken down to Rb⁺(Na⁺)₈(3b-Ga⁰)₃(2b-Pn^1–)₃(1b-Pn^2–)₃, where 2-bonded and 1-bonded Pn atoms carry different negative charges, consistent with the Zintl formalism.

Three-coordinated Ga (or a triel element in general) is much less common within the A-Tr-Pn compounds than the four-coordinated tetrahedral geometry, which appears to be the prevailing building block among such solids. Examples of borate-like (or carbonate-like) [TrPn₃]⁶⁻ units exist and can be exemplified by [InAs₃]⁶⁻ seen in the K₆InAs₃ structure [18]; [Ga₂P₄]⁶⁻ found in the Rb₃GaP₂ structure [16] can be cited as an example of 4-membered rings formed via condensation of two [GaP₃]⁶⁻ units into [Ga₂P₄]⁶⁻ dimers (isosteric with 1,3-Dioxetanedione). Higher oligomers, such as the cyclic trimers [Ga₃Pn₆]⁹⁻ that present 6-membered rings are known in the K₂₀(Ga₃Pn₆)₂Pn_0.66 structures [12] and in the structures of the title compounds. Since the [TrPn₃]⁶⁻ unit is a planar structural motif, it is not common for it to form structures in higher dimension. A search of the ICSD database only reveals the chain found in the K₂GaP₂ type structure [19]. In this bonding arrangement, 5-memered rings are generated by sharing one common Pn and forming one Pn–Pn bond between every two [GaPn₃]^6– units—these 5-membered rings are further connected to form an infinite chain using the third Pn atom as a linker (Figure 2).

As we already mentioned, there are nine crystallographically different cations in the RbNa₈Ga₃Pn₆ structure and they have subtly different coordination environments. The large Rb cation is coordinated to six pnictogen atoms that form trigonal prism, with Rb–Pn distances falling in the range from 3.549 to 3.698 Å for Rb–As, and from 3.487 to 3.623 Å for Rb–P (Table 1). Two Ga atoms are found at a little longer distance, enlarging the Rb first coordination sphere to the shape of a quadrilateral prism (Figure 3). Three different local environments could be found for Na: Na1 and Na2 are found in a tetrahedral surrounding of four pnictogen atoms; square pyramids formed by five next-nearest pnictogen atoms are observed for Na3, Na5, Na6, Na7, and Na8—notice that the bases of the square-pyramids are capped by two Ga atoms, enlarging the coordination number to 7; and a hexagonal prism for Na4, where each hexagonal base is formed by three Ga and three Pn atoms. The Na–Pn distances fall into a very wide range due to the different coordination numbers (CN) for Na, with the shortest Na–Pn distance noted for 4-coordinated Na, and the longest Na–Pn distance found for the 12-coordinated Na (Table 1).

As already noted, the [Ga₃Pn₆]⁹⁻ 6-membered motif is not without a precedent, having been reported for some time already in K₂₀(Ga₃Pn₆)₂Pn_0.66 (defect Ho₆Ni₂₀P₁₃ structure type, Pearson symbol hP39) [11,12]. However, in the latter hexagonal structure, in addition to the [Ga₃Pn₆]⁹⁻ polyanions, there are isolated Pn³⁻ anions (note the partial occupancy of this site). Apparently, a single cation like K⁺ cannot pack well enough with the [Ga₃Pn₆]⁹⁻ polyanions, requiring some subtle rearrangements to accommodate the counterbalancing Pn³⁻ anions. Following the notion of describing the structure from the standpoint of cation packing, in analogy with what has already been presented for K₂₀(Ga₃Pn₆)₂Pn_0.66 [12], one can see that the cation arrangements in both cases are actually very similar. From Figure 4, it can be seen that in both structures, apart from the partially occupied Pn atoms in the K₂₀(Ga₃Pn₆)₂Pn_0.66 structure, which reside in large hexagonal channels, all Ga and Pn atoms are at the centers of trigonal prisms formed by the cations. Nine such trigonal prisms are conjoined, forming a much larger prism that encloses the entire [Ga₃Pn₆]⁹⁻ unit. The difference is that in K₂₀(Ga₃Pn₆)₂Pn_0.66, these larger trigonal prisms can be noted as discrete components, while in RbNa₈Ga₃Pn₆ (=Rb₂Na₁₆(Ga₃Pn₆)₂, i.e., two cations less per formula) they further connect with each other along the a axis by sharing Rb atoms (Figure 4). It seems that the much bigger Rb brings the necessary distortion to the larger trigonal prisms, which leads to a slightly different packing of the cations compared with K₂₀(Ga₃Pn₆)₂Pn_0.66.

Na₁₀NbGaAs₆ crystallizes with the monoclinic space group P2₁/n (No. 14, Pearson symbol mP36), and is found to be isoelectronic and isostructural to the Zintl compound K₁₀NbInAs₆ [13]. The structure is composed of isolated dimers of edge-shared tetrahedra (NbAs₄ or GaAs₄), which are isosteric with diborane, B₂H₆. These dimeric units are separated by the alkali metal cations (Figure 5). Note that there is only one tetrahedral site in the structure, which is co-occupied by Ga and Nb in equal amounts. In the earlier paper on K₁₀NbInAs₆, a diamagnetic response from the magnetic susceptibility of the compound can be inferred, suggesting the Nb to be assigned as Nb⁵⁺ (i.e., d⁰ closed-shell species) [13]. This argument allows for the structure to be rationalized readily as (K⁺)₁₀Nb⁵⁺In³⁺(As^3–)₆. Clearly, the same approach will be applicable to Na₁₀NbGaAs₆ (e.g., (Na⁺)₁₀Nb⁵⁺Ga³⁺(As^3–)₆), which means that it can be classified as a Zintl compound with a transition metal [13,20].

The Nb/Ga–;As distances fall in the range of 2.4855(6) to 2.5645(6) Å, and all of the As-Nb/Ga-As angles are close to the ideal tetrahedral angle 109°28' (Table 3). An examination of the structures of some compounds with ordered GaAs₄ and NbAs₄ tetrahedra reveals similar lengths for the Ga–As and Nb–As bonds. For example, the Ga–As bonds range from 2.444 to 2.556 Å in K₃Ga₃As₄ [9], and from 2.435 to 2.599 Å in Na₂Ga₂As₃ [21], respectively. The Nb–As distances, as reported for K₃₈Nb₇As₂₄ [22] fall in the range of 2.444–2.571 Å; very similar values for d_Nb-As are seen in Cs₉Nb₂As₆ (2.464–2.592 Å) [22]. Since the established ranges for the Nb–As and Ga–As distances are similar, refinements of the thermal ellipsoids in Na₁₀NbGaAs₆ do not show any abnormal behavior—in fact they are virtually spherical. This is not the case for K₁₀NbInAs₆, where the significant difference between the lengths of the Nb–As and In–As bonds gives rise to characteristic features in their thermal ellipsoids—the thermal ellipsoids of the bridging As atoms are elongated tangentially to the central square (Nb/In-As-Nb/In-As). This observation has been considered as a key evidence of these dimers being heteroatomic [(As)₂Nb(µ-As)₂In(As)₂] species, and not an equimolar mixture of [(As)₂Nb(µ-As)₂Nb(As)₂] and [(As)₂In(µ-As)₂In(As)₂] dimers [13].

All the manipulations involving alkali metals were performed either inside an argon-filled glove box or under vacuum. The starting materials were elemental Rb, Na, Ga, As, and P, either from Alfa Aesar or Aldrich with the stated purity higher than 99.9%. Crystals of RbNa₈Ga₃As₆ were first identified from a reaction starting with Na/Rb/Ga/As with a molar ratio of 16/8/80/56, which was originally attempted for type II clathrate Na₁₆Rb₈Ga₈₀As₅₆. The elements with a total mass of ca. 500 mg were loaded into a niobium ampoule, which was subsequently arc-welded under high purity Ar and then jacketed in a fused silica tube under vacuum. The reaction mixture was heated up to 550 °C, and equilibrated for one week before it was slowly cooled to room temperature. This experiment resulted in black, small crystals of RbNa₈Ga₃As₆, which were found to be extremely air-sensitive and had to be handled with great care. After the composition was established, the reaction was repeated with the correct stoichiometry. However, besides RbNa₈Ga₃As₆, Na₁₀NbGaAs₆, which formed from a side reaction with the Nb container, was also identified. This observation indicated that Nb containers were not well suited for such reactions; more expensive Ta or Mo tubes should be considered. RbNa₈Ga₃P₆ could be synthesized from the same setup as well as the same heat treatment, with the coexistence of another known Zintl compound, Na₆GaP₃ [23].

The crystal structures of the title compounds were established using single-crystal X-ray diffraction with a Bruker SMART CCD-based diffractometer (monochromated Mo Kα₁ radiation). Crystals with suitable dimensions (<100 µm) were mounted on glass fiber with Paratone-N oil, and then quickly transferred to the goniometer of the diffractometer. A cold nitrogen stream (200(2) K) was used to keep the crystals at low temperature as well as to protect them from being oxidized. Full spheres of data were collected in four batch runs with a frame width of 0.4° for ω and θ. Integration of the intensity data was done with the SAINT program [24], and semi-empirical absorption correction based on equivalents was applied with the SADABS code [25]. The structures were solved by direct method and refined to convergence by full matrix least squares on F² using the SHELXTL package [26]. The unit cell axes and the atomic coordinates of RbNa₈Ga₃Pn₆ were standardized with the aid of the Structure TIDY [27] in the last refinement cycles.

The structure refinements for Na₁₀NbGaAs₆ required some specific attention. In this structure, the tetrahedrally coordinated site was originally assigned as Ga, which is a common coordination environment for it. However, the thermal displacement parameter in this situation was unreasonably small. Based on the discussion on K₁₀NbInAs₆ structure, the model that this site was co-occupied by Ga and Nb was considered, where Nb was from the reaction vessel. Such refinement lead to nearly half and half occupancy of Ga and Nb, i.e., 49% Ga and 51% Nb. Thus, the ratio was fixed at 1:1 in the final refinement cycle, which resulted in R₁ values as low as 0.0263 and reasonable thermal ellipsoids.

Selected crystal data and refinement parameters are given in Table 4; important bond distances and angles are listed in Table 1,Table 2,Table 3. CIFs have also been deposited with the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247-808-666; Email: crysdata@fiz.karlsruhe.de)—depository numbers CSD-423946 for RbNa₈Ga₃As₆, CSD-423947 for RbNa₈Ga₃P₆, and CSD-423948 for Na₁₀NbGaAs₆.

Quaternary compounds RbNa₈Ga₃Pn₆ (Pn = P, As) and Na₁₀NbGaAs₆ have been synthesized and characterized. These findings suggest that although many ternary compounds of the alkali and alkaline-earth metals with the triels and the pnictogen elements have already been discovered, still little is known about the corresponding systems with two types of cations. Such seemingly trivial approaches can be the key for the synthesis of many more new compounds with novel structures.