Electronic Structures

Two barium phosphide halides, Ba2P7Br and Ba2P7I, were synthesized and structurally characterized by single crystal X-ray diffraction. Both compounds crystallize in the monoclinic space group P21/m (No. 11) and are isostructural to Ba2P7Cl. The crystal structures of Ba2P7X (X = Cl, Br, I) feature the presence of heptaphosphanortricyclane P73− clusters along with halogen anions and barium cations. According to the Zintl concept, Ba2P7X compounds are electron-balanced semiconductors. Quantum-chemical calculations together with UV-Visible spectroscopy confirm the title compounds are wide bandgap semiconductors. The bonding in the P73− clusters was analyzed by means of electron localization function. The elemental compositions were confirmed using energy dispersive X-ray spectroscopy.


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and have been found in both solution and solid phases [1][2][3][4][5]15]. Heavier analogs of such clusters with As and Sb are also known [16][17][18]. Similar to other intermetallics, the electronic structures of polyphosphides can be rationalized by application of the Zintl concept [19,20]. Zintl theory implies the valence electrons of electropositive atoms are donated to more electronegative atoms. The latter realize their electron octet by forming 2c-2e chemical bonds and by having electron lone pairs. In A 3 P 7 , one electron is donated from each A atom to allow for the formation of the P 7 3− cage [4]. In Ba 3 P 14 , three Ba atoms donate two electrons each to allow for the formation of two P 7 3− cages [3]. An interesting example of a compound combining the P 7 3− Zintl anion with a classical chloride anion was reported by von Schnering and Menge in 1981 [21]. In the Ba 2 P 7 Cl Zintl salt, the total charge of two Ba 2+ cations is compensated by a combination of Cl − and P 7 3− anions. In this work, we expand the family of P 7 3− structures to include the isostructural Ba 2 P 7 Br and Ba 2 P 7 I. We report the synthesis, characterization, and chemical bonding of three Ba 2 P 7 X (X = Cl, Br, I) compounds.

Crystal Structure
Originally, Ba 2 P 7 I was discovered accidentally during attempts to grow large single crystals of BaP 3 utilizing I 2 as a transport reagent. After the crystal structure and composition were established, a sample of Ba 2 P 7 I was synthesized by the reaction of stoichiometric amounts of the elements at 1073 K. The bromine-containing analog was synthesized from barium, red phosphorus, and barium bromide at the same temperature. A summary of pertinent information related to single crystal X-ray diffraction unit cell parameters, data collection, and refinements is provided in Table 1. Energy-dispersive X-ray spectroscopy (EDS) investigations of the selected single crystals confirmed the presence of only the expected Ba, P, and X halogen elements. The new compounds, Ba 2 P 7 Br and Ba 2 P 7 I, are isostructural to the previously reported Ba 2 P 7 Cl ( Figure 1) [21]. The crystal structure of Ba 2 P 7 X can be described as consisting of two distinct ionic layers, [Ba 2 X 2 ] 2+ and [Ba 2 (P 7 ) 2 ] 2− stacked along the [001] direction ( Figure 2a). According to the Zintl concept [19,20], the Ba 2 P 7 X compounds are electron balanced. The barium and halogen atoms realize their electron octet by donating and accepting two and one electrons, respectively. Phosphorous atoms bonded to three other phosphorus atoms should be P 0 , realizing their electron octet by maintaining an electron lone pair. Phosphorous atoms bonded to two other phosphorus atoms should be P 1− , realizing their electron octet by maintaining two electron lone pairs. The P 7 3− clusters contain four three-bonded phosphorus atoms (P1, P2, P5) and three two-bonded P atoms (P3, P4) ( Figure 3a). Thus, the total charge per cluster is −3, (3b-P 0 ) 4 (2b-P 1− ) 3 , and the charge-balanced formula is (Ba 2+ ) 2 (P 7 3− )(X − ). In accordance with this formulation, the halogen atoms of Ba 2 P 7 X are surrounded by barium atoms in the first coordination sphere, while the phosphorus atoms are situated further apart. The P 7 3− units are surrounded by barium atoms coordinating to the faces, edges, and corners of the cluster (Figure 3b). The Ba-P separations for all Ba 2 P 7 X are in the range of 3.3-3.6 Å. The shortest P-X interatomic separations are longer than 3.6 Å.  Comparison of the P 7 3− heptaphosphanortricyclane clusters arrangements in the crystal structures of (a) Ba 2 P 7 X and (b) Ba 3 P 14 [3]. Barium atoms are shown in red; halogen atoms in green; and phosphorous clusters in blue. An interesting feature of the Ba 2 P 7 X crystal structure is the relative arrangement of its P 7 3− fragments. All fragments are arranged parallel to each other in a head-to-head fashion. To the best of our knowledge, no other structures composed of the P 7 3− anions are known to form with such straight linear arrangements of P 7 3− anions. For example, in Ba 3 P 14, the heptaphosphanortricyclane clusters are rotated with respect to each other ( Figure 2b) [3]. Apparently, the presence of [Ba 2 X 2 ] 2+ layers fixes the orientation of the P 7 3− clusters, probably due to the repulsion between negatively charged species.
The lack of halogen atoms in Ba 3 P 14 allows for more structural flexibility, and the P 7 3− clusters are free to face different directions. In Ba 2 P 7 X, the P 7 3− cluster is made up of only five symmetrically unique phosphorous atoms. Bond lengths and angles, and a general view of the P 7 3− clusters are presented in Table 2 and Figure 3, respectively. The heptaphosphanortricyclane of Ba 2 P 7 X has C s local symmetry with a mirror plane containing the P1, P2, and P3 atoms. In Ba 3 P 14 , A 3 M 7 (A = alkali metal), and ammonia solvates of alkali metal phosphides, the P 7 3− heptaphosphanortricyclane clusters have C 1 local symmetry [3,4,15]. clusters appear to be similar to those found in the crystal structures of Ba 3 P 14 and Li 3 P 7 .
The local coordination of the barium and halogen atoms is shown in Figure 4. Five barium atoms form the first coordination sphere of the halogen atoms. Each barium atom is surrounded by 11 neighbors. Four halogen atoms and seven phosphorus atoms form irregular polyhedra around Ba1, while the Ba2 atoms are surrounded by 10 phosphorus atoms and only one halogen atom. As expected, with an increase of the halogen size from chlorine to iodine, the unit cell volume increases ( Table 1). The P 7 3− unit remains nearly the same in all three Ba 2 P 7 X compounds ( Table 2). The shortest Ba-P distance also exhibits a minor variation upon a change in the halogen atom, with the shortest Ba2-P distance remaining the same in all compounds: 3.30-3.31 Å. An increase of the unit cell occurs mainly due to the increase in the shortest Ba-X distances from 3.14 Å (Cl) to 3.32 Å (Br) to 3.47 Å (I). The shortest Ba-X distance is indicated with dashed lines.

Electronic Structure
The electron balanced Zintl salts, Ba 2 P 7 X, are expected to exhibit semiconducting and diamagnetic properties. Band structure calculations were performed to verify this description ( Figure 5). According to the calculations, the Ba 2 P 7 X compounds are wide band-gap semiconductors with indirect bandgaps E g = 1.71 eV (X = Cl), 1.86 eV (X = Br), 1.91 eV (X = I). The results of calculations were verified with UV-Vis spectroscopy (vide infra). In the electronic structure of Ba 2 P 7 X, the top of the valence band is dominated by the contribution of phosphorus 3p orbitals, with both halogen and barium orbitals also contributing to the states near the Fermi level. The contribution of the halogen orbitals to the states near the Fermi level increases in the Cl-Br-I series. The bottom of the conduction band is comprised of phosphorus and barium orbitals with negligible contributions from halogen orbitals. The main contribution of halogen p-orbitals to the valence band is located between −1 and −3 eV. To clarify the bonding in the P 7 3− phosphorus clusters, we applied Electron Localization Function (ELF) analysis [22][23][24][25] to the Ba 2 P 7 I compound ( Figure 6). According to the Zintl concept, single covalent 2e-2c bonds are expected to be present at short P-P interatomic separations. Additionally, one and two electron lone pairs are expected to be located on the two-and three-bonded phosphorus atoms, respectively. The ELF analysis indicates the presence of the expected number of electron lone pairs (Figure 6a). The two bonded phosphorus atoms, P4 and P3, have two electron lone pairs, while the three-bonded P1, P2, and P5 have only one electron lone pair. These results are similar to the results of ELF calculations for less symmetric P 7 3− clusters [15]. Localization domains corresponding to the covalent two centered P-P interactions were found for all of the short interatomic separations (Figure 6b-e). The ELF maxima of the P1-P5 bonds were shifted from the lines connecting the P1 and P5 atoms. The ELF domain for the P5-P5 bond exhibited lower values of ELF (Figure 6c). This localization domain is also shifted from the line connecting the P5 atoms and it is not well separated from the domains corresponding to the electron lone pairs on the P5 atoms (Figure 6c-e). The P1-P5-P5 basal triangle, with a topology close to that of cyclopropane, is the most strained face of the P 7 3− cluster. Angles of ~60° hamper the direct overlap of phosphorus orbitals. This explains the displacement of the bonding maxima from the lines connecting the atoms.
In ELF studies performed for the P 7 3− clusters of the ammonia solvates of alkali metal phosphides, the heptaphosphanortricyclane units are less symmetric, with point group C 1 [15]. These units have almost identical bonds, 2.26-2.30 Å, between phosphorus atoms forming a strained basal triangle. Three well-separated and similar localization domains were found displaced from the line connecting phosphorus atoms [15]. Heptaphosphanortricyclane units in Ba 2 P 7 X have a different geometry ( Table 2) with shorter P1-P5 distances (2.23 Å) and longer P5-P5 distance (2.34 Å) than those distances present in ammonia solvate compounds [15]. This difference is probably due to the presence of a Ba atom coordinated to the P4-P5-P5-P4 square (Figure 3b) [21]. This is reflected in the bonding picture: the localization domains for P1-P5 bonds are well pronounced and remain separated from the lone pair domains, while the localization domain for the P5-P5 bond is less pronounced and is not separated from the domains corresponding to the lone pairs located on the P5 atoms (Figure 6e). No ELF maxima were detected near the shortest Ba-X separations, which are indicated by the dashed lines in Figures 1 and 4. Presumably, all Ba-X interactions are electrostatic cation-anion interactions.

UV-Vis Spectroscopy
Ba 2 P 7 Cl, Ba 2 P 7 Br, and Ba 2 P 7 I are red, orange, and yellow-orange, respectively. Solid state UV-Visible spectroscopy was employed for experimental bandgap determinations.
According to the calculated Tauc plots [26,27], Ba 2 P 7 Cl has indirect optical transitions corresponding to a bandgap of 1.83(2) eV ( Figure 7, Table 3). The calculated bandgap for this compound, 1.71 eV, is in reasonable agreement with the experimental data. Calculations predict the increase of the bandgaps in the Cl-Br-I series ( Figure 5). This increase was observed experimentally and is consistent with the visual color change of the Ba 2 P 7 X analogs from red to yellow. The experimental indirect bandgaps of the bromine and iodine analogs, 1.92(4) eV and 1.89(2) eV, respectively, are approximately 0.1 eV larger than the Ba 2 P 7 Cl bandgap. In our UV-Vis experiments we were not able to detect the small predicted difference between the indirect bandgap values for Ba 2 P 7 Br and Ba 2 P 7 I, when estimated standard deviations are taken into account. A combination of the spectroscopic and calculation approaches indicates the compounds obtained are wide bandgap semiconductors.

Experimental Section
All samples of Ba 2 P 7 Cl, Ba 2 P 7 Br, and Ba 2 P 7 I were prepared via solid-state reactions in evacuated and sealed silica ampoules. The samples were prepared in an Argon-filled glovebox with p(O 2 ) and p(H 2 O) < 1 ppm. The starting materials: metallic barium (Sigma Aldrich, St. Louis, MO, USA, 99.9%), iodine (Alfa Aesar, Ward Hill, MA, USA, 99.9985%), and red phosphorous (Alfa Aesar, Ward Hill, MA, USA, 99%) were used as received. Barium bromide and barium chloride (Alfa Aesar, Ward Hill, MA, USA, 99.998% and 99%, respectively) were heated at 573 K overnight under an inert atmosphere to remove traces of water.
A single phase sample of Ba 2 P 7 I was synthesized by annealing stoichiometric amounts of the elements at 1073 K for 140 h. Samples of Ba 2 P 7 Cl and Ba 2 P 7 Br were prepared using metallic barium, phosphorous, and barium chloride or barium bromide in a 3:14:1 ratio under similar annealing conditions. In agreement with the previous report [21], Ba 2 P 7 Cl crystals have a red color. Ba 2 P 7 Br and Ba 2 P 7 I crystals exhibit orange and orange-yellow colors, respectively. The color of all Ba 2 P 7 X lightened upon grinding. Similar to other compounds containing the P 7 3− phosphorus cluster, Ba 2 P 7 X degrades in air over time, and appears to quickly decompose in water, so samples were stored in an Argon-filled glovebox. The samples were characterized by X-ray powder diffraction (XRD) using a Bruker D8 Advance diffractometer employing Cu K α radiation. Elemental analysis of the selected single crystals was carried out on a Hitachi S4100T scanning electron microscope (SEM) with energy-dispersive X-ray (EDX) microanalysis (Oxford INCA energy). Sample analysis confirmed the presence of only Ba, P, and X (X = Cl, Br, I). Solid-state UV-Visible spectroscopy (Thermo Scientific Evolution 220 Spectrometer) was employed for experimental bandgap determinations. For UV-Vis measurements, solid samples were ground into powders and gently pressed onto rough filter paper to form a thick layer of sample. Attempts to dissolve samples of Ba 2 P 7 X for solution-based UV-Vis measurements were unsuccessful. Samples of Ba 2 P 7 X were water sensitive, turning to amorphous yellow material when immersed. No visible color change was observed for immersion in ethanol, but as samples dried they turned brown.
Single crystal X-ray diffraction experiments were carried out at 90 K using a Bruker AXS SMART diffractometer with an APEX-II CCD detector and Mo K α radiation. The data sets were recorded as ω-scans at a 0.3° stepwidth and integrated with the Bruker SAINT software package [28]. A multiscan absorption correction was applied. The solution and refinement of crystal structures were carried out using the SHELX suite of programs [29]. The structures were solved in the P2 1 /m space group and the final refinement was performed using anisotropic atomic displacement parameters for all atoms.

Conclusions
Two new compounds, Ba 2 P 7 Br and Ba 2 P 7 I, have been synthesized and their crystal structures have been determined by means of single crystal X-ray diffraction. In the crystal structure of Ba 2 P 7 X (X = Cl, Br, I) negatively charged layers of heptaphosphanortricyclane P 7 3− clusters surrounded by barium cations are stacked along the [001] direction, alternating with positively charged Ba 2 X 2 layers. According to the Zintl count and quantum-chemical calculations, Ba 2 P 7 X are electron-balanced semiconductors with wide bandgaps. UV-Vis spectroscopy confirms the semiconducting nature of Ba 2 P 7 X compounds. Analysis of the chemical bonding in P 7 3− clusters by means of electron localization function indicates their bonding generally follows the Zintl counting scheme. However, for the basal triangle of phosphorus atoms, the ELF localization domains are displaced from the shortest interatomic separations.