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Article

Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data

by
Matthias Weil
†
Division of Structural Chemistry, Institute for Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria
†
Dedicated to the late Ekkehard Tillmanns. He will be remembered as a human and scientific role model.
Minerals 2021, 11(11), 1156; https://doi.org/10.3390/min11111156
Submission received: 21 September 2021 / Revised: 5 October 2021 / Accepted: 14 October 2021 / Published: 20 October 2021
(This article belongs to the Special Issue Mineral-Related Oxo-Salts: Synthesis and Structural Crystallography)
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Abstract

:
Single-crystals of lead(II) oxoarsenates(V) were grown from the melt (Pb2As2O7), from solution (Pb(H2AsO4)2 and Pb5(AsO4)3OH), and under hydrothermal conditions (NaPb4(AsO4)3). Crystal structure refinements from single-crystal X-ray diffraction data revealed isotypism for both Pb2As2O7 and Pb(H2AsO4)2 with the corresponding barium and phosphate phases. A quantitative comparison of the crystal structures showed a high similarity for the isotypic M2X2O7 structures (M = Pb, Ba; X = As, P), whereas for the M(H2XO4)2 structures only the pair Pb(H2AsO4)2 and Pb(H2PO4)2 is similar, but not Ba(H2AsO4)2. Pb5(AsO4)3OH adopts the apatite structure type in space group P63/m, with the hydroxyl group disordered around Wyckoff position 2 b (0, 0, 0) in the channels of the structure. NaPb4(AsO4)3 represents a lacunar apatite with two of the three metal positions occupationally disordered by Pb and Na. In contrast to a previous X-ray powder study of NaPb4(AsO4)3 that reported an apatite-type structure in space group P63/m, the current single-crystal data clearly revealed a symmetry reduction to space group P 3 ¯ . Hence, NaPb4(AsO4)3 is the first lacunar apatite that comprises only tetrahedral anions and adopts the belovite structure type.

1. Introduction

To date, in the system Pb/As/O/(H), two approved and crystallographically fully characterized mineral phases are known to exist, viz. paulmooreite, PbII2AsV2O5, and schultenite, PbIIHAsVO4. Paulmooreite represents a lead(II) oxoarsenate(III) with a pyroarsenite anion, As2O54– [1], whereas schultenite is a lead(II) oxoarsenate(V) with a hydrogenarsenate anion, HAsO42– [2]. Other synthetic phases for which their crystal structures have been determined include lead(II) arsenite, PbIIAsIII2O4 [3], the lead(II) oxoarsenates(V) PbII3(AsVO4)2 [4], Pb8IIO5(AsVO4)2 [5], and PbIIAsV2O6 [6], as well as the lead(IV) oxoarsenate(V) PbIV(HAsVO4)2(H2O) [7]. Although other synthetic lead(II) oxoarsenates(V) have been reported to exist and compiled in one of the older handbooks on lead and its compounds [8], structural details of these phases still are missing. Since these compounds might also be present in nature as yet-unidentified mineral phases, detailed structure analyses were desirable.
In this article, single-crystal growth procedures and crystal structure refinements of PbII2AsV2O7, PbII(H2AsVO4)2, PbII5(AsVO4)3OH, and NaPbII4(AsVO4)3 are reported and the results comparatively discussed with related phases, expanding our knowledge about the crystal chemistry of lead(II) oxoarsenates(V).

2. Materials and Methods

2.1. Syntheses and Single-Crystal Growth Procedures

All used chemicals were of pro analysis quality.
Pb2As2O7. Single-crystals of lead(II) diarsenate(V) were obtained from the melt, starting with polycrystalline PbHAsO4. The latter was prepared by slow addition of aqueous solutions of Na2HAsO4 (1.9 g in 50 mL water) to Pb(NO3)2 (3.3 g in 50 mL water) in a molar ratio of 1:1. The precipitated PbHAsO4 was filtered off, washed with water and ethanol, and dried overnight. X-ray powder diffraction revealed a single-phase material. An amount of 0.5 g of PbHAsO4 was then placed in a platinum crucible covered with a lid. The crucible was heated within 24 h to 850 °C and then cooled within 99 h to room temperature. Crystals of Pb2As2O7 were present as a minor constituent in the recrystallized melt with Pb3(AsO4)2 as the main constituent because Pb2As2O7 melts incongruently [9]. Polycrystalline single-phase Pb2As2O7 was prepared by heating PbHAsO4 at 600 °C for one day.
Pb(H2AsO4)2. A concentrated Pb(NO3)2 solution (10 g in 50 mL water) was refluxed for one day with 8 mL concentrated arsenic acid (80%wt). After cooling to room temperature, plate-like crystals could be isolated next to recrystallized Pb(NO3)2 crystals. The latter were easily distinguishable from the bis(dihydrogen)arsenate due to their cubic symmetry.
Pb5(AsO4)3OH. Hydrothermal treatment of PbHAsO4 in 20%wt ammonia solution at 210 °C for five days (300 mg loading, Teflon container with 6 mL capacity, filling degree ca. 2/3) resulted in single-phase material of synthetic hydroxymimetite, Pb5(AsO4)3OH, but with crystals too small for standard laboratory single-crystal X-ray diffraction. Larger single-crystals were grown by following a slight modification of the procedure reported by McDonnell and Smith [10]. PbHAsO4 (see above) was added to 20 mL of a warm KOH (10%wt) solution until saturation. The solution was separated from the remaining material by filtration. The clear filtrate was then poured into 150 mL of cold water, resulting in a colorless flocculent precipitate. After standing and sedimentation for about 30 min, the solid was filtered off, and the filtrate was allowed to slowly evaporate at room temperature. After about five days, the solution became cloudy and after three additional days, an off-white solid started to crystallize. Rod-shaped single-crystals of Pb5(AsO4)3OH were manually isolated from this material under a polarizing microscope. Phase analysis of the bulk using the program Highscore [11] revealed Pb5(AsO4)3OH as the main phase (80%wt), and KPb2(CO3)2(OH) [12] and Pb3(CO3)2(OH)2 (hydrocerrusite) [13] as minor side products (10%wt each).
NaPb(AsO4)3. The sodium lead arsenate phase was obtained as a minor by-product during single-crystal growth studies of Pb5(AsO4)3OH under hydrothermal conditions. For that purpose, 0.3 g PbHAsO4 were suspended in 6 mL water to which 1 g NaOH were added. The mixture was placed in a Teflon container (filling degree 70%) that was heated under autogenous pressure in a steel autoclave at 210 °C for five days. Crystals of NaPb(AsO4)3 appeared as block-like crystals next to yet-unidentified polycrystalline material.
Pb4As2O9. Attempts to prepare Pb4As2O9 were made by heating a mixture of PbO (0.795 g) and As2O5 (0.205 g) in the molar ratio 4:1 and a mixture of PbO (0.398 g) and PbHAsO4 (0.618 g) in the molar ratio 1:1, respectively. For both batches, the educts were thoroughly milled and heated in a platinum crucible for three days at 750 °C for the first and at 680 °C for the second batch, followed by intermediate grindings after each day. Phase analysis of the products by PXRD revealed Pb3(AsO4)2 and Pb8(AsO4)2O5 as the only reaction products in both cases, with an approximate ratio of 2:1 (program Highscore [11]).

2.2. Single-Crystal X-ray Diffraction and Structure Analysis

Single-crystals were optically preselected under a polarizing microscope, embedded in perfluorinated polyether for protection from air and humidity, and mounted on MiTeGen MicroLoopsTM. The X-ray diffraction studies were conducted at room temperature on a Bruker APEX-II CCD diffractometer (Bruker-AXS, Madison, WI, USA) using MoKα radiation. Data collection was handled and optimized with Apex-2 [14], data reduction was performed with Saint [14] and absorption effects were corrected with the semi-empirical multi-scan procedure of Sadabs [15]. Experimental details of the data collections and refinements are collated in Table 1.
All crystal structures were initially solved with Shelxs (using direct methods) [16] and refined with Shelxl [17] (version 2018/3). For the purpose of better comparison between the isotypic pairs Pb2As2O7/Pb2P2O7 and Pb(H2AsO4)2/Pb(H2PO4)2, respectively, atom labelling, atomic coordinates, and the unit cell settings were adapted from the corresponding phosphate structure [18,19]. This explains why the chosen unit cell of Pb(H2AsO4)2 is not reduced [20] (reduced unit cell parameters are: a = 5.9984(8), b = 7.9497(10), c = 8.6137(10) Å, α = 108.888(5), β = 108.449(6), γ = 96.128(5)°). The hydrogen atom positions in the crystal structure of Pb(H2AsO4)2 were discernible from difference-Fourier maps and were refined freely for H7 and with a distance constraint of 0.85 Å for all other H atoms; all H atoms in this structure were refined with a common Uiso parameter. In the crystal structure of Pb5(AsO4)3OH, the O atom of the hydroxyl group was refined as being disordered over the center of symmetry at (0, 0, 0), leading to two occupied sites with a site occupation factor of 0.5 each. Its H atom could not be located and thus was not included in the model but is considered in the formula and other numerical parameters in the CIF. In the crystal structure of NaPb(AsO4)3, two of the Pb sites were found to be statistically occupied with Na. Each of the mixed-occupied sites, M1a and M1b, were refined with the same coordinates and displacement parameters of the two elements under consideration of full occupancies and charge neutrality for the compound. The NaPb(AsO4)3 crystal under investigation was found to be twinned in a 1:1 ratio by mirroring perpendicular to the c axis.
Further details of the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre (CCDC) on quoting the depository numbers listed at the end of Table 1. The data can be obtained free of charge via www.ccdc.cam.ac.uk/structures.

2.3. Powder X-ray Powder Diffraction (PXRD)

Powder diffraction data were measured on a recently calibrated (NIST LaB6 standard) PANalytical X’Pert PRO diffractometer with Cu-K α ¯ radiation in Bragg-Brentano geometry (X’Celerator multi-channel detector, silicon zero background sample holder, 2.546° scan length, 25 s exposure time per scan length, 2θ range 5°–70°; the scans were finally converted into 0.02° step-size bins). Temperature-dependent PXRD data were collected under atmospheric conditions with a HTK1200 Anton-Paar high-temperature oven chamber mounted on the diffractometer. The samples were finely ground and placed on a glass ceramic (MarcorTM) sample holder with 0.5 mm depth. The zero point was automatically adjusted during the measurements with a PC-controllable alignment stage. The samples were heated with 10 °C/h to the respective temperature and kept for 15 min before measurement of each step to ensure temperature-stability. Refinement of unit-cell parameters were performed with the program Topas [21].

3. Results and Discussion

Table 2 lists selected interatomic distances for all crystal structures.

3.1. PbII2AsV2O7

Pb2As2O7 belongs to the family of diarsenates. In a previous study on this phase [23], isotypism with the diphosphate analogue Pb2P2O7 was suggested, and indexed power diffraction data as well as unit cell parameters for the non-reduced unit cell were given, with a = 6.86(2), b = 7.13(2), c = 12.93(3) Å, α = 99.01(10), β = 91.10(12), γ = 89.48(12)°, V = 625 Å3 for room temperature data. However, the reduced unit cell [20] parameters (a = 6.86(2), b = 7.13(2), c = 12.93(3) Å, α = 80.99(10), β = 88.90(12), γ = 89.48(12)°), the unit cell volume, and the calculated density (7.18 g·cm–3) based on the powder study differ considerably from those of the current single-crystal study (Table 1).
Pb2As2O7 is in fact isotypic with Pb2P2O7 [18] and crystallizes in the triclinic K2Cr2O7 structure type that is also referred to as the “dichromate” structure type [24,25] and adopted by many M2X2O7 phases where M is a large divalent cation and X is As or P. The asymmetric unit of Pb2As2O7 comprises four Pb, four As, and 14 O sites. The coordination numbers of the lead(II) cations are nine for Pb1, Pb2 and Pb3, and eight for Pb4, with minimum and maximum bond lengths of 2.416(5) and 3.464(5) Å (Table 2). The two As2O74– pyroarsenate (or diarsenate) anions are made up from two AsO4 tetrahedra fused by a bridging O atom. As a characteristic structural feature of inorganic pyrogroups X2O7 [25], the X―O bond lengths to the bridging O atom are significantly longer than those to the terminal atoms. The corresponding averaged (av) values of d(As–O)av = 1.769 Å for bridging and d(As–O)av = 1.667 Å for terminal O atoms are in good agreement with the compiled values for pyroarsenate groups [26,27,28]. The conformation of the two pyroarsenate anions is ecliptic, with virtually identical As–O–As bridging angles (∠(As2–O4–As1) = 126.0(3)°, ∠(As3–O11–As4) = 126.3(2)°), and As···As separations (As1···As2 = 3.1518(8) Å; As3···As4 = 3.1575(8) Å). In the crystal structure of Pb2As2O7, the pyroarsenate anions are arranged in layers at z ≈ 0, ½, extending parallel to (001) (Figure 1).
The metrics of the triclinic unit cell of Pb2As2O7 (Table 1) suggest a possible phase transition to a tetragonal phase crystallizing in the β-Ca2P2O7 structure type [29], with atriclinic, btriclinic ≈ atetragonal ≈ 7 Å and ctriclinic ≈ ½ctetragonal ≈ 13 Å. For that purpose, temperature-dependent XRPD measurements were conducted (Figure 2).
As a result, Pb2As2O7 shows no phase transition into a higher-symmetric phase upon heating until melting of the compound slightly below 800 °C, in good agreement with the reported melting temperature of 800 °C [9]. All unit cell parameters (Figure 3) increase more or less in a linear way with temperature, except the γ angle that shows a slight decrease. The evolution of unit cell parameters with temperature resembles that of isotypic Ba2As2O7 [30].

3.2. Pb(H2AsO4)2

Pb(H2AsO4)2 adopts the Ca(H2PO4)2 structure type [31] and crystallizes isotypically with Pb(H2PO4)2 [19,32,33] and Ba(H2AsO4)2 [30]. The triclinic crystal structure of Pb(H2AsO4)2 comprises one Pb, two As, eight O, and five H atom positions. Except one H atom (H7) being located on an inversion center (Wyckoff position 1 d, site symmetry 1 ¯ ) and one H atom (H8) showing half-occupancy, all other atoms are located on general sites of space group P 1 ¯ with full occupation. The crystal structure of Pb(H2AsO4)2 can be described as being made up from [PbO8] polyhedra (range of Pb–O distances 2.442(2)—3.117(3) Å) that share common edges O1—O1′ and O5—O5′ to form chains extending along [001]. These chains are flanked by dihydrogen arsenate anions parallel to the chain direction at shorter Pb–O distances (<2.7 Å) and additionally bonded to dihydrogen arsenate anions that are part of an adjacent chain at longer distances (3.11 Å). In this way, a layered arrangement along (010) is accomplished. An excessive array of hydrogen bonds links two adjacent layers and consolidates the crystal packing.
The As1–O bond lengths appear to be normal for the dihydrogen arsenate group H2As(1)O4, with two significantly longer As–OH bonds (≈1.72 Å; O4, O3) and two shorter As―O bonds (≈1.65 Å; O2, O1), whereas the bond lengths of the H2As(2)O4 tetrahedron are atypical for a dihydrogen arsenate group: one shorter As–O bond of 1.665 Å to O5, two somewhat longer As–OH bonds (≈1.67 Å; O7, O8), and one long As–OH bond of 1.72 Å (O8) are observed. The As(2)O4 group shows very short interpolyhedral O···O distances of ≈2.45 Å between the two pairs of inversion-related OH groups involving atoms O7 and O8. For such very strong hydrogen bonds, an asymmetric O–H···O hydrogen bond with a disordered H atom (50% occupation each), or a symmetric O···H···O hydrogen bond with an ordered H atom located on the inversion center are possible in the present case. On basis of difference–Fourier maps, the first possibility was chosen for the hydrogen bond O8–H···O8′ that is located between two layers, and the second possibility for the hydrogen bond O7–H···O7′ that is located within a layer (Figure 4). Table 3 gives numerical details of the hydrogen bonding scheme for Pb(H2AsO4)2.
The hydrogen bonding situation in Pb(H2AsO4)2 resembles that in the related alkaline earth arsenate Ba(H2AsO4)2 [30] or in the two polymorphs of Na5H3(SeO4)4(H2O)2 [34,35,36,37] where interpolyhedral O···O distances in the range 2.44–2.48 Å are observed, and for which either asymmetrical or symmetrical hydrogen bonds were modelled. A special case in this context is Pb(H2PO4)2. The first two structure refinements of Pb(H2PO4)2 did not consider H atom positions in the model [18,32], whereas in the latest refinement of Pb(H2PO4)2 all H atom positions were automatically placed by the refinement program [33]. The resulting hydrogen bonding scheme, however, is highly questionable, with two O–H···O angles less than 130° and an O···O distance of 3.36 Å for one of the reported hydrogen bonds.
It has to be stressed that modelling of H atom positions based on conventional X-ray diffraction measurements is ambiguous, in particular under consideration of the presence of heavy atoms in the structure (here Pb, As). Although in many cases, high-quality X-ray data have allowed the determination of H atoms for such or similar structures, only much more reliable neutron diffraction data will prove satisfactory whether the chosen model is correct or not.

3.3. Structural Comparison of Pb2As2O7 and Pb(H2AsO4)2 with Their Isotypic Ba and as Analogues

For a structural comparison of the isotypic structures of M2X2O7 and M(H2XO4)2 (M = Pb, Ba; X = P, As), respectively, the program compstru [38], available at the Bilbao Crystallographic Server [39], was used. The comparison allows for the quantification of the influence caused by the exchange of the anion (phosphate versus arsenate) and the cation (barium versus lead(II)). Atomic displacements for the atom pairs in the two sets of isotypic structures as well as numerical values for the degree of lattice distortion (S), the maximum distance between the atomic positions of paired atoms, the arithmetic mean of all distances (dav), and the measure of similarity (Δ) are compiled in Table 4, with the respective lead arsenate as the reference structure.
M2X2O7. The unit cell parameters are similar for the three structures. The highest absolute displacement in the pair Pb2As2O7/Pb2P2O7 is observed for one of the terminal O atoms of the XO4 tetrahedra (O14, 0.2918 Å); in the pair Pb2As2O7/Ba2As2O7, the maximum displacement applies to the atom pair O12 (lead compound) and O14 (barium compound) of the same XO4 tetrahedron. Overall, numerical values show a high similarity of the three structures, revealing that the stereochemical influence of the 6s2 electron lone pair at the PbII cations is not pronounced in this case.
M(H2XO4)2. The unit cell parameters were very similar for the pair Pb(H2AsO4)2/Pb(H2PO4)2. The highest absolute displacement relates to O5, which is not bonded to a hydrogen atom and is associated with the shortest Pb–O bond in the structure. The low degree of lattice distortion (S = 0.0151) and the low value for the measure of similarity (Δ = 0.047) indicate a high similarity of the two lead structures, in contrast to the pair Pb(H2AsO4)2/Ba(H2AsO4)2. In this case, the unit cell parameters of Ba(H2AsO4)2 differ considerably from those of the lead structures, in particular for the b and c axes, which lead to a high degree of lattice distortion (S = 0.1107). Although the definition of isotypism according to the Commission on Crystallographic Nomenclature is formally fulfilled (§1.2 and §1.3 in [40]) for the pair Pb(H2AsO4)2/Ba(H2AsO4)2 (neglecting H atoms), the great difference in the metrics alone has an impact on the structural similarity that appears to be low (Δ = 0.703). The highest absolute atomic displacement was nearly 2 Å for O7 that is involved in formation of the symmetrical hydrogen bond in Pb(H2AsO4)2 and in an ordered asymmetric hydrogen bond in Ba(H2AsO4)2, respectively. This is an indication that the dissimilarity in the crystal structures of Pb(H2AsO4)2 and Ba(H2AsO4)2 is not primarily associated with the substitution Pb → As or As → P but with a change in the hydrogen-bonding system.

3.4. Pb5(AsO4)3OH

Pb5(AsO4)3OH is a member of the vast family of the apatite [M(1)2][M(2)3](XO4)3Y supergroup [41,42] and belongs to the apatite group, subgroup mimetites-H (mimetite is the Cl-endmember Pb5(AsO4)3Cl [43,44]). Although a possible mineral phase “mimetite-OH” with composition Pb5(AsO4)3OH has not yet been approved, SEM-EDS analyses of specimen from Styria (Austria) and the Blackforest (Germany) support the existence of such a mineral species [45,46]. A synthetic hydrous phase “Pb4(PbOH)(AsO4)3·H2O” (=Pb5(AsO4)3(OH)·H2O) [47]] has previously been reported, but its composition was later questioned and revised to Pb4(PbOH)(AsO4)3 (=Pb5(AsO4)3(OH)) [48]. Subsequent structural studies of this phase were limited to a qualitative assessment to the apatite structure type and unit cell parameters from powder X-ray data [49] until the crystal structure was fully refined from synchrotron powder data under consideration of small amounts of incorporated carbonate, leading to a refined composition of Pb5(AsO4)3(OH)0.87(CO3)0.04 [22]. The current refinement of Pb5(AsO4)3(OH) is based on the first single-crystal study and reports all atoms with anisotropic displacement parameters. The previously given unit cell parameters from room-temperature powder data are slightly larger than those of the current single-crystal study (Table 1): a = 10.154, c = 7.515 Å, V = 671.02 Å3 [49]; a = 10.187, c = 7.523 Å, V = 676.303(13) Å3 [22]; a = 10.14(8), c = 7.50(1) Å, V = 669 Å3 [50]. Unit cell parameters predicted from elemental radii using pattern recognition and artificial intelligence methods, a = 9.8939, c = 7.6039 Å, V = 644.617 Å3 [51], are much smaller than all experimentally determined values. As emphasized by White and ZhiLi [42], it seems likely that most of the reported apatites are in fact somewhat non-stoichiometric, particularly with respect to X anions; furthermore, it is difficult to determine such compositional variations directly. For the final crystal structure model of Pb5(AsO4)3(OH), full occupation of the Y site with OH was considered, and without any incorporation of carbonate.
The apatite structure is well known and has been reviewed some time ago in a crystal-chemical context [42]. Therefore, only the most important structural features are discussed here. Pb1 is located on Wyckoff position 4 f (site symmetry 3) and has a trigonal-prismatic environment if Pb–O distances less than 2.8 Å are considered (three more O atoms cap the lateral faces of the trigonal prism at distances of 3.0 Å). As a quantitative measure for the reliability of the structure model of apatites, the metaprism twist angle (φ), i.e., the O(1)–M(1)–O(2) twist angle projected on (001) of the [M(1)O6] metaprism was introduced with a usual variation of 5° ≤ φ ≤ 25° for inconspicuous structure refinements for apatites crystallizing in space group type P63/m [52]. From the current refinement of Pb5(AsO4)3OH, φ amounts to 21.9°. This value matches with the metaprism angle of 21.6° in the analogous crystal structure of the mineral hydroxylpyromorphite, Pb5(PO4)3OH, from single-crystal data [53]. The trigonal [Pb1O6] metaprism in Pb5(AsO4)3OH has a polyhedral volume of 20.46 Å3 (calculated with the Volcal option in Platon [54] and shares faces to build up chains running parallel to [001]. Pb2 (Wyckoff position 6 h, m..) is surrounded by one arsenate O atom (O1) at a short distance of 2.36 Å, and four arsenate O atoms (O3) at longer distances of ≈2.62 Å. The next nearest O atoms are the hydroxyl O atom O4 at 2.88 Å, followed by O3 at 3.06 Å. The hydroxyl group is disordered around the Wyckoff 2 b position (0, 0, 0), with a value for the z coordinate of 0.568 and located in channels (diameter 5.071 Å) formed by the surrounding Pb2 atoms. The As atom is situated on Wyckoff position 6 h and exhibits three equal bond lengths of 1.655 Å to O3 (2x) and O2, and a slightly longer bond length to O1 (1.681 Å). The average As–O bond length of 1.662 Å is in very good agreement with literature data of 1.667 (18) for As–O bonds to nonprotonated O atoms [55] (the calculated overall mean As–O bond length including also As–O(H) groups is 1.689 Å [56]). The crystal structure of Pb5(AsO4)3OH is depicted in Figure 5.
In comparison with the previous refinement of Pb5(AsO4)3OH from powder synchrotron X-ray data [22], the Pb–O and As–O bond lengths are the same within the 3σ range, with the exception of the Pb–O4 bond involving the hydroxyl group (Table 2). The latter bond is by 0.2 Å shorter in the model refined from powder data with an isotropic refinement of all atoms and the hydroxyl O atom at z = 0.3743 showing the same type of disorder than modelled for the current single-crystal X-ray data (anisotropic refinement of all atoms with the hydroxyl O atom). In the model from single-crystal X-ray data, O4 shows a strong motion along the channel direction as indicated by its unilateral anisotropic displacement parameter (Figure 5, inset). On the other hand, the hydroxyl group of analogous Pb5(PO4)3OH (space group P63/m) was modelled in different ways. Whereas for natural hydroxylpyromorphite (containing small amounts of fluorine; single-crystal data; [53]) and synthetic Pb5(PO4)3OH (single-crystal data; [57]) the hydroxyl O atom was not disordered and found to be situated at the 2 b Wyckoff position (z = 0); it was treated as disordered in another refinement of synthetic Pb5(PO4)3OH (neutron data from polycrystalline material) with z = 0.3565 [58]. All other structurally determined M5(XO4)3OH apatite phases reported up to date were modelled with a disordered hydroxyl group, viz. Ca5(PO4)3OH (P63/m; z = 0.195) [59], Ca5(AsO4)3OH (P63/m; z = 0.1919) [60], Sr5(PO4)3OH (P63/m; z = 0.1856) [61], Sr5(AsO4)3OH (P63/m; z = 0.1919) [62], Ba5(PO4)3OH (P63; z = 0.148) [63], and Cd5(PO4)3OH (z = 0.1880) [64].

3.5. NaPb4(AsO4)3

NaPb4(AsO4)3 is also a member of the apatite supergroup [41], but represents a lacunar apatite where the Y position, i.e., the anion located in the channels formed by M2 cations, remains empty (Figure 6). It has been suggested that the stereochemically active lone pair of PbII that occupies a volume close to that of oxygen allows lacunar apatite structures to be stable [65]. In contrast to a previous refinement of NaPb4(AsO4)3 (polycrystalline material prepared by a ceramic route) based on laboratory powder X-ray data and performed in space group P63/m [66], the current single-crystal X-ray data for hydrothermally grown crystals clearly revealed a lower symmetry in space group P 3 ¯ due to the violation of reflection condition l = 2n for 00l reflections. For example, reflection 003 has an average Fobs2 value of 1040(33). A check for a possibly missed higher symmetry using the Addsym feature in Platon [54] gave no indication for a higher spacer group symmetry on basis of the current X-ray data. However, the corresponding phosphate phase NaPb4(PO4)3 (single-crystal X-ray data) has also been reported to crystallize in space group P63/m in the apatite-type of structure [67,68]. The question of whether the lower space group symmetry of NaPb4(AsO4)3 is a systematic feature or is related with different preparation conditions (hydrothermal versus ceramic route) and associated polymorphism must remain unanswered for the time being. Future studies on basis of single-phase material from different preparation routes, high-resolution diffraction data, and complementary techniques are definitely needed to achieve a deeper insight.
According to space group symmetry P 3 ¯ , NaPb4(AsO4)3 belongs to the belovite group within the apatite supergroup [41]. To the best of the author’s knowledge, NaPb4(AsO4)3 is the first representative of a lacunar apatite comprising solely of XO4 groups as anions, adopting the belovite structure type. In comparison with Pb5(AsO4)3(OH), the unit cell volume is reduced by about 4.5% due to incorporation of smaller NaI (ionic radii: NaI [IX] = 1.24 Å versus PbII [IX] = 1.35 Å [69]). In comparison with the apatite structure type in space group P63/m, the M1 position related with the metaprism is split into two positions in the belovite structure type (M1a, M1b) that each are located on Wyckoff position 2 d (site symmetry 3). Both positions show occupational disorder of Na/Pb, however, with different absolute ratios. Whereas M1a shows a minor fraction of lead(II) (Pb1a: site occupancy 0.15), the situation for M1b is reversed (Pb1b: site occupancy 0.85). The polyhedral volumes of the metaprisms reflect this behavior, with a smaller volume of 18.07 Å3 for Na-rich [M1aO6] and a greater volume of 19.04 Å3 for Pb-rich [M1bO6]. The angles φ for the corresponding metaprisms amount to 24.5° for O1–(M1a)–O2 and to 45.6° for O2–(M1b)–O3. The latter value is the highest observed for any apatite structure [52] and reflects the high distortion of the [M1bO6] polyhedron due to a considerable twist of the opposite trigonal faces. Like in the higher-symmetric apatite structure, the two different trigonal prisms are capped by three additional O atoms at the lateral faces, with distances of 3.202(8) Å for [M1aO6] and 3.070 for Å for [M1bO6]. The M2 site of NaPb4(AsO4)3 is located on a general position (6 g) of space group P 3 ¯ . According to refinements of the s.o.f. for this site, significant amounts of NaI are not incorporated, and consequently only PbII is situated on this position. The channel diameter for the empty channel defined by Pb2 atoms is 5.150 Å. In comparison with Pb5(AsO4)3(OH), the As–O bond lengths of the AsO4 tetrahedron in the crystal structure of NaPb4(AsO4)3 are slightly enlarged (d(As–O)av = 1.684 Å).

3.6. Pb4As2O9

From an investigation of the PbO/As2O5 phase diagram, the phase Pb4As2O9 was reported to exist up to a temperature of 740 °C when decomposition through a peritectoid is observed [9]. In the present study, annealing temperatures above (750 °C) and below (680 °C) the peritectoid were chosen for phase mixtures with a molar ratio of PbO:As2O5 = 4:1. However, in both cases, only the phases Pb3(AsO4)2 and Pb8(AsO4)2O5 were observed as reaction products. Therefore, different molar ratios and/or annealing temperatures might be necessary to reproduce the reported formation of Pb4As2O9 [9].

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available in The Cambridge Crystallographic Data Centre (CCDC).

Acknowledgments

Open Access Funding by TU Wien. The X-ray center of TU Wien is acknowledged for providing access to the single-crystal and powder X-ray diffractometers. The author thanks Ton Spek (Utrecht) for implementing an extended Volcal algorithm (based on the original code by Larry Finger) into Platon.

Conflicts of Interest

The author declares no conflict of interest.

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Minerals 11 01156 g001 550
Figure 1. The crystal structure of Pb2As2O7 in a projection along [ 1 ¯ 00]. Anisotropic displacement parameters are given at the 74% probability level with Pb in blue, O in white, and As in red. AsO4 tetrahedra (red) are given in polyhedral representation, and Pb–O bonds shorter than 2.7 Å as solid bonds, and between 2.7 and 3.5 Å as open bonds.
Figure 1. The crystal structure of Pb2As2O7 in a projection along [ 1 ¯ 00]. Anisotropic displacement parameters are given at the 74% probability level with Pb in blue, O in white, and As in red. AsO4 tetrahedra (red) are given in polyhedral representation, and Pb–O bonds shorter than 2.7 Å as solid bonds, and between 2.7 and 3.5 Å as open bonds.
Minerals 11 01156 g001
Minerals 11 01156 g002 550
Figure 2. Temperature-dependent XRPD of Pb2As2O7.
Figure 2. Temperature-dependent XRPD of Pb2As2O7.
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Minerals 11 01156 g003 550
Figure 3. Pb2As2O7. Evolution of unit cell parameters with temperature.
Figure 3. Pb2As2O7. Evolution of unit cell parameters with temperature.
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Minerals 11 01156 g004 550
Figure 4. The crystal structure of Pb(H2AsO4)2 in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. Ordered H atoms are shown as light-gray spheres, and disordered H atoms as dark-gray spheres; hydrogen bonds are shown as green lines.
Figure 4. The crystal structure of Pb(H2AsO4)2 in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. Ordered H atoms are shown as light-gray spheres, and disordered H atoms as dark-gray spheres; hydrogen bonds are shown as green lines.
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Minerals 11 01156 g005 550
Figure 5. The crystal structure of Pb5(AsO4)3OH in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. The O atom of the hydroxyl group is given in yellow, and the [Pb1O6] metaprism as a blue polyhedron. In the inset, the center of symmetry is indicated by a black dot, and the disordered positions of the hydroxyl O atom (half occupation) by yellow and white ellipsoids.
Figure 5. The crystal structure of Pb5(AsO4)3OH in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. The O atom of the hydroxyl group is given in yellow, and the [Pb1O6] metaprism as a blue polyhedron. In the inset, the center of symmetry is indicated by a black dot, and the disordered positions of the hydroxyl O atom (half occupation) by yellow and white ellipsoids.
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Minerals 11 01156 g006 550
Figure 6. The crystal structure of NaPb4(AsO4)3 in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. The [M1aO6] metaprism is displayed as a turquoise polyhedron and the [M1bO6] metaprism as a blue polyhedron.
Figure 6. The crystal structure of NaPb4(AsO4)3 in a projection along [00 1 ¯ ]. Color codes and displacement ellipsoids are as in Figure 1. The [M1aO6] metaprism is displayed as a turquoise polyhedron and the [M1bO6] metaprism as a blue polyhedron.
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Table
Table 1. Details of single-crystal X-ray data collections and structure refinements.
Table 1. Details of single-crystal X-ray data collections and structure refinements.
CompoundPb2(As2O7)Pb(H2AsO4)2Pb5(AsO4)3OHNaPb4(AsO4)3
Temperature/°C— 23(1) —
Diffractometer— Bruker-AXS APEX-II CCD detector —
Radiation; λ/Å— Mo K α ¯ ; 0.71073 —
Space group (# no.)P 1 ¯ (# 2)P 1 ¯ (# 2)P63/m (# 176)P 3 ¯ (# 147)
Formula units Z4222
Formula weight676.22489.061469.721268.51
Crystal dimensions/mm0.10 × 0.03 × 0.030.10 × 0.06 × 0.010.10 × 0.01 × 0.010.04 × 0.04 × 0.02
Crystal descriptionlight-yellow fragmentcolorless platelight-yellow needlecolorless block
a/Å7.13790(10)7.9497(10)10.1266(3)10.0230(14)
b/Å7.14000(10)8.6137(10)10.1266(3)10.0230(14)
c/Å13.0681(3)5.9984(8)7.5010(2)7.3117(15)
α/°83.3602(11)108.888(5)9090
β/°86.6710(10)96.128(5)9090
γ/°89.9469(10)108.449(6)120120
V/Å3660.42(2)358.51(8)666.16(4)636.1(2)
μ/mm − 160.85132.69370.42960.606
X-ray density/g·cm–36.8014.5317.3276.623
Range θmin–θmax1.57–40.182.69–36.732.32–30.002.35–30.28
Rangeh − 12 → 12 − 13 → 13 − 14 → 14 − 14 → 14
k − 12 → 12 − 14 → 14 − 14 → 14 − 14 → 14
l−23 → 23 − 10 → 10 − 10 → 9 − 10 → 10
Measured reflections70,28324,488957632,342
Independent reflections824035796991279
Obs. reflections [I >2σ(I)]695130174821207
Ri0.0740.0600.0770.063
Transmis. coeff. Tmin; Tmax0.1644; 0.56970.3792; 0.74720.1107; 0.54110.2245; 0.6244
Structure solution and refinement— Shelxs and Shelxl —
Absorption correction— Sadabs —
Number of parameters2001144064
Ext. coef. (Shelxl)0.00118(8)0.00095(16)--
Diff. elec. dens. max; min/e– Å–3 (dist./Å, atom)6.23 (0.61, Pb2);
−5.53 (0.59, Pb1)		1.47 (0.68, Pb1);
− 1.24 (0.71, Pb1)		5.60 (0.67, Pb2);
−3.05 (0.76, Pb2)		2.61 (0.43, (Pb/Na)3);
− 1.69 (0.57, Pb1)
R[F2 > 2σ(F2)]0.03460.02370.03800.0211
wR2(F2 all)0.08480.04030.08710.0452
Goof1.0981.0141.0801.096
CSD number2099610209961120996122099609
Table
Table 2. Selected interatomic distances.
Table 2. Selected interatomic distances.
Pb2As2O7
Pb1O62.416(5)Pb4O7 v2.463(4)
Pb1O2 i2.485(5)Pb4O3 i2.479(5)
Pb1O102.521(5)Pb4O72.514(5)
Pb1O32.563(5)Pb4O5 ix2.523(5)
Pb1O1 ii2.673(5)Pb4O142.673(5)
Pb1O7 iii2.878(5)Pb4O2v2.695(5)
Pb1O4 ii2.965(5)Pb4O63.140(5)
Pb1O14 iii3.319(5)Pb4O123.226(5)
Pb1O5 iii3.464(5)As1O11.657(4)
Pb2O13 iv2.417(4)As1O31.669(4)
Pb2O12 i2.484(5)As1O21.679(5)
Pb2O8 ii2.624(5)As1O41.777(4)
Pb2O10 i2.631(5)As2O51.649(5)
Pb2O14 v2.817(5)As2O61.671(5)
Pb2O22.883(6)As2O71.679(4)
Pb2O11 v2.903(4)As2O41.760(4)
Pb2O12.987(5)As3O91.649(4)
Pb2O9 ii3.369(5)As3O101.674(4)
Pb3O9 vi2.441(5)As3O81.683(4)
Pb3O132.546(5)As3O11x1.760(4)
Pb3O8 vii2.555(5)As4O141.651(4)
Pb3O12 viii2.642(5)As4O131.669(4)
Pb3O82.753(5)As4O121.672(4)
Pb3O52.871(6)As4O111.778(4)
Pb3O1ii2.912(4)
Pb3O103.021(5)
Pb3O63.149(6)
Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) −x, −y + 1, −z + 1; (iii) x, y + 1, z; (iv) x, y, z + 1; (v) −x + 1, −y, −z + 1; (vi) x, y − 1, z; (vii) −x, −y + 1, −z; (viii) x − 1, y, z; (ix) x + 1, y, z; (x) −x + 1, −y + 1, −z.
Pb(H2AsO4)2
Pb1O5 i2.442(2)As1O21.652(2)
Pb1O1 ii2.529(2)As1O11.657(2)
Pb1O22.559(2)As1O4 vi1.719(2)
Pb1O5 iii2.575(2)As1O3 vii1.727(2)
Pb1O1 iv2.584(2)As2O51.665(2)
Pb1O7 v2.701(2)As2O71.673(2)
Pb1O42.944(2)As2O81.676(2)
Pb1O63.117(3)As2O61.722(2)
Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) −x, −y + 1, −z + 1; (iii) x, y + 1, z; (iv) x, y, z + 1; (v) −x + 1, −y, −z + 1; (vi) x, y − 1, z; (vii) −x, −y + 1, −z;
Pb5(AsO4)3(OH)
Pb1O2 i2.534(8) 2.534Pb2O1 vi2.355(14) 2.398
Pb1O2 ii2.534(8)Pb2O3 vii2.616(9) 2.618
Pb1O2 iii2.534(8)Pb2O3 viii2.616(9)
Pb1O1 iv2.774(10) 2.754Pb2O3 iv2.664(11) 2.691
Pb1O1 v2.774(10)Pb2O3 v2.664(11)
Pb1O12.774(10)Pb2O4 xi2.88(3) 2.693
Pb1O3 v3.009(13) 2.944Pb2O4 xii2.88(3)
Pb1O3 iv3.009(13)Pb2O2 vi2.937(12) 3.058
Pb1O33.009(13)
As1O2 vi1.655(11) 1.674
As1O3 xiii1.655(10) 1.691
As1O31.655(10)
As1O11.681(13) 1.711
Symmetry codes: (i) x − y, x, −z; (ii) −x + 1, −y + 1, −z; (iii) y, −x + y + 1, −z; (iv) −y + 1, x − y + 1, z; (v) −x + y, −x + 1, z; (vi) −y + 1, x − y, z; (vii) y, −x + y, z + 1/2; (viii) y, −x + y, −z; (xi) −x, −y, z − 1/2; (xii) −x, −y, −z + 1; (xiii) x, y, −z + 1/2.
Values in italics are from the powder synchrotron study [22].
NaPb4(AsO4)3
(Na/Pb)1aO2i2.428(7)Pb2O1vi2.248(7)
(Na/Pb)1aO2ii2.428(7)Pb2O4ii2.433(7)
(Na/Pb)1aO2iii2.428(7)Pb2O4viii2.509(7)
(Na/Pb)1aO1iv2.564(7)Pb2O3xv2.568(7)
(Na/Pb)1aO1v2.564(7)Pb2O2ix2.805(6)
(Na/Pb)1aO1vi2.564(7)Pb2O32.912(8)
(Na/Pb)1aO4vii3.202(8)As1O2ix1.673(5)
(Na/Pb)1aO4viii 3.202(8)As1O3xvii1.675(7)
(Na/Pb)1aO4ix3.202(8)As1O1xviii1.689(6)
(Na/Pb)1bO2x2.435(7)As1O41.699(7)
(Na/Pb)1bO2xi2.435(7)
(Na/Pb)1bO2xii2.435(7)
(Na/Pb)1bO3xiii2.773(8)
(Na/Pb)1bO32.773(8)
(Na/Pb)1bO3xiv2.773(8)
(Na/Pb)1bO1i3.070(8)
(Na/Pb)1bO1v3.070(8)
(Na/Pb)1bO1vi3.070(8)
Symmetry codes: (i) −x + 1, −y + 1, −z; (ii) x − y, x, −z; (iii) y, −x + y + 1, −z; (iv) y, −x + y, −z; (v) −x, −y + 1, −z; (vi) x − y + 1, x + 1, −z; (vii) x, y + 1, z; (viii) −y, x − y, z; (ix) −x + y + 1, −x + 1, z; (x) −x + 1, −y + 1, −z + 1; (xi) x − y, x, −z + 1; (xii) y, −x + y + 1, −z + 1; (xiii) −y + 1, x − y + 1, z; (xiv) −x + y, −x + 1, z; (xv) y, −x + y, −z + 1; (xvii) −x + y, −x, z; (xviii) −x, −y, −z.
Table
Table 3. Pb(H2AsO4)2. Hydrogen-bond geometry/Å.
Table 3. Pb(H2AsO4)2. Hydrogen-bond geometry/Å.
D–H···AD–HH···AD···AD–H···A
O3–H3···O8vi0.85 (1)1.81 (1)2.660 (3)176 (3)
O4–H4···O2ix0.85 (1)1.87 (2)2.685 (3)160 (4)
O6–H6···O30.85 (1)1.96 (1)2.805 (3)170 (5)
O7–H7···O7v1.23 (1)1.23 (1)2.450 (4)180 (1)
O8–H8···O8x0.85 (1)1.63 (2)2.458 (5)165 (7)
Symmetry codes: (v) −x + 1, −y, −z; (vi) x, y, z + 1; (ix) −x + 1, −y + 1, −z + 1; (x) −x + 2, −y + 1, −z + 1.
Table
Table 4. Atom pairs and their absolute distances |u|/Å in the isotypic structures of M2X2O7 and M(H2XO4)2 (M = Pb, Ba; X = P, As), as well as degree of lattice distortion (S), arithmetic mean of the distances (dav/Å), and measure of similarity (Δ). H atoms were not considered for the comparison.
Table 4. Atom pairs and their absolute distances |u|/Å in the isotypic structures of M2X2O7 and M(H2XO4)2 (M = Pb, Ba; X = P, As), as well as degree of lattice distortion (S), arithmetic mean of the distances (dav/Å), and measure of similarity (Δ). H atoms were not considered for the comparison.
Pb2As2O7Pb2P2O7 [18]|u|Ba2As2O7 [30] (1)|u|
a = 7.13790, b = 7.14000, c = 13.06810 Å,
α = 83.3602, β = 86.6710, γ = 89.9469° 		a = 6.9140, b = 6.9660, c = 12.7510 Å,
α = 83.180, β = 88.860, γ = 89.640° 		a = 7.3996, b = 7.3812, c = 13.3261 Å,
α = 83.116, β = 86.446, γ = 89.792°
Pb1Pb10.0609Ba20.2396
O10O100.0699O70.1127
Pb2Pb20.0709Ba30.0571
As2P20.0751As20.0562
O8O80.0792O50.0653
As3P30.0845As40.0447
O12O120.1220O140.2650
O2O20.1266O30.0404
O4O40.1332O40.0907
Pb4Pb40.1367Ba10.1397
As1P10.1390As10.0735
O5O50.1459O20.0537
O7O70.1561O110.1227
As4P40.1721As30.1624
O11O110.1756O130.1020
Pb3Pb30.1780Ba40.1873
O3O30.1803O90.1343
O13O130.1873O60.2288
O6O60.1977O80.1125
O9O90.2333O120.1897
O1O10.2390O100.0968
O14O140.2918O10.1930
S 0.0189 0.0170
dmax. (Å) 0.2918 0.2650
dav. (Å) 0.1480 0.1258
Δ 0.041 0.043
Pb(H2AsO4)2Pb(H2PO4)2 [19]|u|Ba(H2AsO4)2 [30] (1)|u|
a = 7.9497, b = 8.6137, c = 5.9984 Å,
α = 108.888, β = 96.128, γ = 108.449° 		a = 7.823, b = 8.315, c = 5.856 Å,
α = 108.24, β = 96.90, γ = 108.61° 		a = 7.2453, b = 7.4341, c = 8.1890 Å,
α = 104.685, β = 96.210, γ = 110.276°
Pb1Pb10.0082Ba10.9896
As2P20.0125As20.5838
As1P10.0226As10.4551
O3O30.0767O30.7901
O8O80.0795O80.9794
O7O70.1049O71.9663
O4O40.1160O20.7304
O1O10.1284O40.8412
O6O60.1309O51.7444
O2O20.1328O10.4070
O5O50.1340O60.6014
S 0.0151 0.1107
dmax. (Å) 0.1340 1.9663
dav. (Å) 0.0860 0.9172
Δ 0.047 0.703
(1) Original unit cell of Ba2As2O7 was transformed by -b,-a,-c to have the same setting as Pb2As2O7.
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Weil, M. Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data. Minerals 2021, 11, 1156. https://doi.org/10.3390/min11111156

AMA Style

Weil M. Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data. Minerals. 2021; 11(11):1156. https://doi.org/10.3390/min11111156

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Weil, Matthias. 2021. "Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data" Minerals 11, no. 11: 1156. https://doi.org/10.3390/min11111156

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