Pb2AsxP14–x was synthesized by reacting the pnicogens in a lead melt in sealed silica ampoules. A mixture of hydrogen peroxide and glacial acetic acid removed lead from the final product. Pb2AsxP14–x represents the first lead arsenide phosphide adopting a new structure type. Systematic substitution of phosphorus by arsenic leads to the formation of Pb2AsxP14–x with x ~ 3.7, a compound with a two-dimensional arrangement of polypnictide layers, coordinated by Pb2+ cations. Pb2AsxP14–x is structurally related to PbP7 where a three-dimensional polyphosphide network is realized instead. The structure of Pb2As3.7(1)P10.3(1) was determined from single crystal X-ray diffraction data: space group P212121 (no. 19), a = 10.060(1), b = 10.500(1), c = 13.711(2) Å, and V = 1448.3(4) Å3. The structure is discussed relative to PbP7 focusing on the differences in the polyanionic substructures of the two polypnictides.
Phosphorus is an intriguing element with a flexible and interesting structural chemistry . During the past decades, hundreds of binary and ternary phosphides and polyphosphides  were discovered and properties and applications studied thoroughly. The range of materials covers superconductors, catalysts or thermoelectrics. All kinds of electronic structures from metals to large gap semiconductors are known for these materials.
Phosphorus realizes a binary compound with almost every element of the Periodic Table. Exceptions are mercury, and the heavier pnictides antimony, and bismuth. Recently, lead was removed from this list with the discovery of PbP7 [3, 4]. Despite the formation of ternary lead and mercury containing polyphosphides like HgPbP14  and Pb5I2P28  it took almost 30 years to realize a binary lead phosphide. Obviously, the reactivity and tendency of phase formation for the combination of lead and pnictides is not pronounced.
The heavy pnictides crystallize in the well-known rhombohedral A7 structure type, which is a consequence of a small Peierls distortion from an ideal cubic arrangement and the occurrence of lone pairs caused by s–p orbital mixing . Several solid solutions of the heavier pnictides are known like for instance Sb1–xAsx  or Bi1–xSbx  being semiconductors with reasonable charge carrier mobilities. In the case of the As–P system some reports are existing in the literature and a phase diagram has been reported . For a composition of 36 to 47 at% P in As1–xPx an orthorhombic crystal system was postulated. Later on, a solid solution of As1–xPx featuring the A17 structure type of orthorhombic black phosphorus has been reported . Except for SbAs, all solid solutions are characterized by a random distribution of the pnictide atoms within the structure.
In the case of the A17-type solid solution As1–xPx the substitution of P by As leads to a drastic closure of the band gap and nano-structured material has been successfully used to fabricate field effect transistors with an absorption region in the infrared spectrum . PbP7 is a 1.3 eV semiconductor (calculated band gap), which might be tuned to smaller band gaps by an introduction of the heavier homologue As .
Inspired by the first successful synthesis and promising properties of the binary lead phosphide PbP7 we intend to expand the structural chemistry of lead pnictides to the heavier homologue arsenic. Herein, we report on the synthesis and characterization of the first lead phosphide arsenide.
Starting materials for the synthesis of polycrystalline samples of Pb2AsxP14–x were powders of lead (abcr, Karlsruhe, Germany), arsenic (abcr) and pieces of phosphorus (ChemPUR, Karlsruhe, Germany), all with stated purities better than 99%. The elements were mixed in a ratio of Pb to (P, As) of 1:1 in a mortar, subsequently cold-pressed in to pellets of 6 mm diameter and sealed in evacuated quartz tubes. The ratio of P to As was varied between 3:7 and 5:5. Heating of the starting materials took place in a muffle furnace. We applied three ramps each one followed by a holding step: Ramp 1 to 423 K within 48 h, holding period at that temperature for 48 h; Ramp 2: 573 K within 48 h and 48 h holding; Ramp 3: 673 K within 48 h and 48 h holding period. Finally, the samples were cooled to room temperature over a period of 48 h. The lead flux was dissolved in a 1:1 mixture of H2O2 (35%, ACROS, Fisher Scientific, Nidderau, Germany) and glacial acetic acid (96%, VWR International, Darmstadt, Germany) in an ultrasonic bath. Pb2AsxP14–x is slightly less soluble in the H2O2-glacial acetic acid mixture than the lead matrix. Therefore, the maximum treatment of the crude product should not exceed several min. Already after 10 min the crude product is dissolved completely. Due to this behavior, lead and arsenic always remained as side products in the polycrystalline sample and single crystals were of low quality. A representative crystal picture is given in Fig 1. The resulting powder is black and stable in air for several weeks.
2.2 X-ray diffraction
Powder XRD measurements of the remaining Pb2AsxP14–x products were carried out on a STOE Stadi P diffractometer (CuKα1 radiation, λ = 1.54051 Å; germanium monochromator) in transmission using a flatbed sample holder. Analysis of the data was carried out with the program package WinXpow . The resulting diffractograms showed reflections of the title compound together with those of non-dissolved lead and arsenic. Lattice parameters of isolated polycrystalline materials from different batches (molar ratio of P to As of 3:7 and 5:5) ranged from a = 10.078(4) to 10.092(3), b = 10.484(3) to 10.495(2) and c = 13.666 to 13.671 Å, pointing towards a certain but not pronounced phase width.
Several crystals of Pb2AsxP14–x were glued to small quartz fibers using bees wax. Single-crystal intensities data were collected at room temperature using a STOE IPDS-II diffractometer (MoKα radiation; λ = 0.71073 Å; graphite monochromator). Numerical absorption corrections were applied to the data and a structure refinement was performed for each experiment. The collected data sets were corrected for polarization and absorption effects by the aid of the XShape  and Xred  routines. The structure solution and refinement (full-matrix least-squares on F2) has been performed with the Superflip routine , implemented in the program Jana 2006 .
2.3 SEM and EDX data
Semi-quantitative EDX analyses and pictures of crystals remaining after the hydrogenperoxide/glacial acid treatment were carried out by using a JEOL JCM-6000 scanning electron microscope fitted with a JED 2200 detector in high vacuum mode. We also determined the composition of crystals directly mounted on the fibers used for single crystal structure determination. All measurements substantiated a certain phase width for a given starting composition in the solid solutions Pb2AsxP14–x. The maximum arsenic content derived from our analyses was x = 5.8(2) independent of the batch. We could not detect any impurity elements heavier than sodium in our samples. Obviously, the supplied arsenic was not fully incorporated in the structure of Pb2AsxP14–x. For the Pb2As3.7(1)P10.3(1) crystal used for structure determination we found Pb: As: P = 16(2): 14(2): 69(4) in at% (refined composition in at%: 12.5: 23.1(7): 64.4(6)). We need to point out, that the accuracy of the EDX analysis is lower for the direct measurement of the mounted single crystals than for a common metallographic analysis.
3 Results and discussion
3.1 Structure refinements
Single crystals have been isolated from two different batches (molar ratio of P to As of 3:7 and 5:5) and a structure determination was performed for each one. Due to the synthesis conditions stated above, the crystal quality varied drastically, and we found crystals with varying P to As ratio in the range of x = 3.7–4.8, according the sum formula Pb2AsxP14–x. In the following, we report only on the best candidate Pb2As3.7(1)P10.3(1). All relevant crystallographic data of Pb2As3.7(1)P10.3(1) are given in Tables 1–3. In order to verify and substantiate the refined composition, all crystals were subject to quantitative EDX analysis after the single crystal structure determination. Pictures of two crystals representing the minimal and maximal arsenic content (mounted on glass capillaries) are given as Supporting Information available online.
|Unit cell dimensions (single crystal data)|
|Calculated density, g cm−3||4.64|
|Crystal size, μm3||120 × 50 × 50|
|Wave length; λ, Å||MoKα; 0.71069|
|Transm. ratio (max/min)||0.09/0.03|
|Absorption coefficient, mm−1||32.8|
|θ range, deg||2.4–22.2|
|Range in hkl||± 10, ± 10, + 14|
|Total no. reflections||12 529|
|Reflections with I ≥ 3 σ(I)/Rσ||1102/0.115|
|Goodness-of-fit on F2||1.34|
|Inversion twin ratio||0.51(4): 0.49|
|Final indices [I > 2 σ(I)]||0.0450/0.0684|
|R indices R1/wR2 (all data)||0.0926/0.0813|
|Flack parameter x||0.49(4)|
|Largest diff. peak/hole, e Å−3||1.18/–1.24|
aUeq is defined as one third of the trace of the orthogonalized Uij tensor.
aThe anisotropic displacement factor exponent takes the form: –2π2[(ha*)2U11+ … +2hka*b*U12].
The data sets revealed primitive orthorhombic lattices and the only systematic extinctions h00 with h = 2n, 0k0 with k = 2n and 00l with l = 2n led to the space group P212121 (No. 19). The calculated Flack parameter was close to 0.5 and the structure was subsequently refined as an inversion twin. Twin volumes are 0.51(4) and 0.49. Platon  was used to check for higher metric symmetry and no other space group than the given one was suggested. A check of precession plots did not show any hints for a symmetry reduction We observed 16 electron density maxima, which we assigned to two lead and 14 pnictide sites due to bond length considerations and differences in electron density. A separate refinement of the occupancy parameters led to mixed P/As occupancies for most pnictide sites. The site occupancy factors for all pnictide sites were restricted to 1. Due to the low crystal quality, an extinction correction was not necessary. The refined atomic positions, displacement parameters, and interatomic distances of Pb2As3.7(1)P10.3(1) are given in Tables 1–4.
|P3/As3 (iii)||2.832(5) 2×|
|P11/As11:||P13/As13 (vi)||2.281(11) 4×|
aSymmetry operations: (i) –x+1/2, –y+2, z–1/2; (ii) –x+1, y–1/2, –z+1/2; (iii) –x, y–1/2, –z+1/2; (iv) x–1, y, z; (v) –x+1, y+1/2, –z+1/2; (vi) –x, y+1/2, –z+1/2.
Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: firstname.lastname@example.org, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-431033 (Pb2As3.7(1)P10.3(1)).
3.2 Crystal chemistry
Recently, the discovery and examination of two-dimensional monolayer materials of heavy group 15 elements lead to huge interest in the development and optimization of elemental pnicogen based and related materials . Related to graphene, mono-layer and multi-layer materials of phosphorus, arsenic and antimony have been either prepared or predicted theoretically [20–24]. Heavy doping of such materials and the search for binary and ternary derivatives is of interest due to the expected improvement of physical properties like charge carrier mobility, thermoelectric behavior or optoelectronic properties [12, 20, 22]. Examples of black phosphorus-related binary materials are BaP3 [25, 26], SnP3  or PbP7 [3, 4]. They adopt substructures, which are structurally correlated with black phosphorus and gray arsenic by a replacement of selected atoms in the element.
The discovery of PbP7 [3, 4] marks a milestone in solid state phosphorus chemistry because the formation of a binary lead phosphide had been tried for decades without success. PbP7 and black phosphorus are structurally related in such a way that the polyphosphide substructure of PbP7 can be regarded as a partially substituted and strongly corrugated derivative of black phosphorus and gray arsenic. Formally, Pb2+ replaces some P atoms of a black phosphorus monolayer and coordinates either the resulting 2-bonded P atoms or lone pairs of neighboring P atoms pointing towards their direction.
Starting from PbP7 we intended to replace P by As following Vegard’s rule for isotypic structures. As stated in the Introduction, such a Vegard-like behavior was observed for the solid solution related to black phosphorus and black arsenic . Surprisingly, neither an isotypic structure nor a Vegard-like behavior was observed. In the following we discuss similarities and differences of the two lead pnictides Pb2As3.7(1)P10.3(1) and PbP7.
The structure of Pb2AsxP14–x (exemplarily discussed for Pb2As3.7(1)P10.3(1)) contains 16 crystallographically independent sites, two lead and 14 pnictide (Pn) ones. All Pn sites are mixed-occupied by P and As in various ratios (Fig 1b). Pb2As3.7(1)P10.3(1) consists of strongly corrugated Pn layers located parallel to . In Fig 1, a section of the crystal structure, the polypnictide substructure and the coordination of the two crystallographically independent lead positions are given.
Within these polypnictide layers, ribbons of condensed (Pn)6 rings in chair conformation (Fig 1b) are running along the a axis. Pn bridges or Pb2+ cations (Pb1) connect these ribbons to neighboring ribbons forming a strongly corrugated polypnictide net. Only Pb2 is coordinating this net in the third dimension.
In Pb2As3.7(1)P10.3(1), the bond distances of 2.233–2.384 Å within the (Pn)6 rings are longer than an average P–P single bond in black phosphorus (2.228 Å ), which can be explained by the greater atomic radius of arsenic. Two out of fourteen Pn sites are fully filled by phosphorus, each coordinated to Pb1 and Pb2. Obviously, lead tends to be preferably bonded to P instead of As. All other Pn sites are mixed occupied with variable amounts of P and As. Pn3 to Pn14 are forming the (P/As)6 ring strands. Pn1, Pn2 and Pb1 in a ratio of (Pn1+Pn2) to Pb of 2:1 are at the bridging positions to neighboring (Pn)6 ring strands in 1,4 position. The stereochemistry of the bridging atoms determines the dimensionality of the whole structure. In Fig 2 the stereochemistry of the (Pn)6 ring fragments for PbP7  and Pb2As3.7(1)P10.3(1) is illustrated in detail.
In 1-position either two trans-Pn or one trans-standing Pb1 is realized while in 4-position the opposite is true (Fig 2). Two cis-standing Pn and one cis-Pb can be found. Exactly this stereochemistry of the bridging atoms is responsible for the two-dimensional arrangement of the polypnictide substructure. Two of such (Pn)6 ring strand units are attached to each other sharing the bridging atoms. In PbP7, a three-dimensional polypnictide substructure is realized with a different stereochemistry for the bridging sites. Herein, alternating Pb, cis-P and trans-P sites are present. Due to the opposite orientation of the bridging P sites in PbP7, a three-dimensional connectivity for the anionic substructure results. At this point the stereochemistry of such (Pn)6 strands in black phosphorus and grey arsenic should be recalled. In black phosphorus, such strands are only trans-connected while in grey arsenic cis-connection is realized. Therefore, PbP7 and Pb2As3.7(1)P10.3(1) are derivatives of both element structures but with different stereochemical substitution patterns.
P and Pb are connecting the neighboring Pn layers. In the case of Pb2As3.7(1)P10.3(1) only Pb2 bridges the two-dimensional polypnictide substructures.
For better comparison, in Fig 3 we show structure sections of black phosphorus, PbP7, and Pb2AsxP14–x. The close relationship becomes obvious if the polypnictide substructure is compared. Choosing the right projection for each structure, the resulting polypnictide substructure seems to be almost identical. To illustrate the differences we draw three layers of black P and the respective polypnictide substructures of PbP7 and Pb2AsxP14–x in different colors (turquoise, blue and green). By cutting 3-bonded P atoms out of the black phosphorus layers, 2-bonded ones with a formal charge of –1 result. These 2-bonded P atoms and some lone pairs of neighboring P atoms are coordinated by Pb2+.
Within a Zintl-precise formulation four of fourteen Pn atoms have two (oxidation number –1) and ten have three (oxidation number 0) Pn neighbors. Exactly two Pb2+ cations are needed for charge balance.
If we formulate a stacking sequence for the polypnictide substructure in PbP7 a sequence A A A is realized while in Pb2As3.7(1)P10.3(1) the sequence is A B A. The B layer turned by 180° relative to A and a slight translation are present. In Fig 3, a vertical line illustrates the translation. This small translation in combination with A B stacking renders an inversion center impossible substantiating the non-centrosymmetric space group.
4 Supporting information
Pictures of two crystals representing the minimal and maximal arsenic content are given as Supporting Information available online (DOI: 10.1515/znb-2016-0048).
Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.
This work was supported by the Deutsche Forschungsgemeinschaft. F.B. thanks the TUM Graduate School for support.
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