Lead Oxychloride Borates Obtained under Extreme Conditions

Aug 25, 2016 - The 6s2 lone electron pair is stereochemically active on Pb(1)–Pb(9) atoms, whereas it is inert on Pb(10). All of the Pb coordination...
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Lead Oxychloride Borates Obtained under Extreme Conditions Oleg I. Siidra,*,† Houria Kabbour,‡ Olivier Mentre,‡ Evgeny V. Nazarchuk,† Philip Kegler,§ Diana O. Zinyakhina,† Marie Colmont,‡ and Wulf Depmeier¶ †

Department of Crystallography, Saint-Petersburg State University, University emb. 7/9, 199034 St. Petersburg, Russia UCCS, UMR 8181, Université Lille Nord de France, USTL, 59655 Villeneuve d’Ascq, France § Institute for Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany ¶ Institute of Geosciences, Kiel University, 24118 Kiel, Germany ‡

S Supporting Information *

ABSTRACT: [Pb10O4]Pb2(B2O5)Cl12 (1) and [Pb18O12]Pb(BO2OH)2Cl10 (2) were obtained via high-temperature highpressure experiments. [O12Pb18]12+ and [O4Pb10]12+ oxocentered structural units of different dimensionality are excised from the ideal [OPb] layer in tetragonal α-PbO. 2 is formed with an excess of lead oxide component, and 1 is formed with an excess of borate and halide reagents. The structure of 2 can be visualized as the incorporation of {Pb(10)Cl4(BO2OH)2} clusters into alternating PbO and chloride layers, with the existence of square vacancies in both. However, the structure of 1 is described as the intrusion of [O4Pb10]12+ tetramers linked by disordered Pb(B2O5) groups into a halogen three-dimensional matrix. The structure of 2 contains 10 symmetrically independent Pb positions. The 6s2 lone electron pair is stereochemically active on Pb(1)−Pb(9) atoms, whereas it is inert on Pb(10). All of the Pb coordinations in the structure of 2, in accordance with ECCv (volume eccentricity) parameters and the density of states (DOS), can be subdivided into three groups. The current study is the first attempt to analyze this unusual behavior in structurally complex oxyhalide material with the rare case of Pb2+ cations, demonstrating both stereochemically active and inactive behavior of the lone pair via charge and first-principle calculations.

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with Cl− anions and additional Pb2+ cations between them. Also, one Pb2+ chloride-borate Pb2(B5O9)Cl with hilgarditetype structure is known.9 Lead oxychloride materials with triangular TO3 (where T = B, C, N) anions are scarce. Boron is a unique element for triangularly coordinated cations (B, C, N), BO3 groups may protonate and polymerize into BnOm units of different dimensionality. Pb borates are of special interest, since they often demonstrate remarkable nonlinear optical properties.10 Note that one complex lead oxychloride mineral containing well-defined borate and carbonate anions was previously reported.6g Mereheadite, with a chemical formula of Pb47O24(OH)13Cl25(BO3)2(CO3), is assumed to form at relatively high temperatures and pressures in natural environments. While we have made numerous attempts to synthesize lead oxyhalide compounds with borate anions using both a “rapid quenching route” and high-temperature hydrothermal conditions,11 these efforts were unsuccessful. Therefore, it was decided to shift to high-pressure and high-temperature conditions, which would mimic natural conditions of similar in composition natural mineral systems. Successful syntheses resulted in obtaining of two novel lead oxychloride borates

INTRODUCTION Lead oxyhalides with various additional anions and cations represent an important class of inorganic materials with various potential and existing applications. They were recently the focus of intensive investigation, because of the presence of promising optical,1 luminiscent,2 photocatalytic,3 electrochemical,4 and ionic conductive properties.5 Lead oxychlorides are also known as a class of minerals that form in the oxidation zones of mineral deposits.6 Both synthetic and natural lead oxyhalides demonstrate impressive structural richness induced by the formation of units of different dimensionality with OPb4oxocentered tetrahedra.7 Structural architectures of these materials emerge as a result of a synergy between electronic and bonding requirements of the Pb2+ cations with stereochemically active 6s2 lone electron pairs and highly polarizable and soft halogen X− (X = Cl, Br, I) anions. Most lead oxyhalides contain additional O atoms, which act as very strong Lewis bases, with a strength of ∼0.5 v.u. (calculated as a formal charge divided by four) and explain stereochemically active behavior of lone pair on Pb2+ cations.7 PbO−PbCl2−H3BO3 is one of the least-explored lead oxyhalide systems. There is only one “pure” (i.e., without additional cations and anions) lead oxychloride borate material known to date.8 The crystal structure of [Pb4O]Pb2(BO3)3Cl is based on isolated OPb4 tetrahedra and isolated BO3 triangles © 2016 American Chemical Society

Received: July 10, 2016 Published: August 25, 2016 9077

DOI: 10.1021/acs.inorgchem.6b01587 Inorg. Chem. 2016, 55, 9077−9084

Article

Inorganic Chemistry structurally related to Aurivillius phases and α-PbO: [Pb10O4]Pb2(B2O5)Cl12 (1) and [Pb18O12]Pb(BO2OH)2Cl10 (2).

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EXPERIMENTAL SECTION

Materials and Instrumentation. α-PbO (powder, ≥99.999%, Aldrich), PbCl2 (powder, 98%, Aldrich) and H3BO3 (powder, ≥99.999%, Aldrich) reagents in 0.5:0.3:0.01 ratio were used to obtain single crystals of 1, and the same reagents, used in a ratio of 0.25:0.5:0.3 were used to obtain 2. The mixture was ground in an agate mortar under acetone and, after drying, loaded into a platinum capsule. Electron microprobe analysis (Hitachi Model TM 3000) was performed for 1 and 2. Qualitative electron microprobe analysis revealed no other elements, except Pb and Cl, with the atomic number greater than 11 (Na). For powder X-ray diffraction of 1 and 2, 10 crystals of each phase were preliminary checked by single-crystal X-ray analysis and later crushed and glued with epoxy into balls. Powder X-ray diffraction (XRD) data were collected with a Rigaku R-AXIS Rapid II singlecrystal diffractometer that was equipped with a cylindrical image plate detector, using Debye−Scherrer geometry (with d = 127.4 mm). Experimental and calculated powder XRD data (Co Kα) are in good agreement and are given in Figures 1S and 2S in the Supporting Information. Synthesis. The high-temperature/high-pressure experiments were performed using the piston cylinder module of a Voggenreiter Model LP 1000-540/50 system. Therefore, the starting materials were weighed, combined in the desired ratio, mixed, and finely grounded. The mixture was then filled into a platinum capsule (outer diameter = 4 mm, wall thickness = 0.2 mm, length = 7 mm). The capsule was sealed on both sides with an impulse microwelding device (Lampert, Model PUK U3) and placed into the center of a 1/2-in. piston cylinder talc−Pyrex assembly. The calibration procedure of the piston cylinder module was previously described.13 The run pressure of 3.5 GPa was applied within 10 min and kept constant for the complete experimental run. After the desired pressure was reached, the temperature program was started. First, the temperature was increased to 750 °C within 70 min. After a dwell time of 120 min, the temperature was slowly decreased to 200 °C. The cooling time for that step was 110 h (cooling rate = 5 °C/h). At the desired temperature, the experiment was quenched to room temperature automatically. The quenching time of the samples is ∼2−3 s. After quenching, the experiment was decompressed in a period of 30 min. The capsule then was extracted out of the high-pressure assembly and broken for further investigations. The product consisted of white transparent block crystals of 1 in the mass of crystalline PbCl2 and unidentified white nontransparent amorphous phase. Single crystals of 2 were obtained using the same procedure. (See Figure 1.) Crystallographic Studies. Single crystals of 1 and 2 were mounted on a thin glass fibers for XRD analysis, using a Bruker Model APEX II DUO X-ray diffractometer with a microfocus X-ray tube operated using Mo Kα radiation at 50 kV and 40 mA. The data were integrated and corrected for absorption using a multiscan-type model using the Bruker programs APEX and SADABS. More than a hemisphere of XRD data were collected. Crystallographic information is summarized in Table 1. Atomic coordinates and additional structural information are provided in the Supporting Information. Partial Charges and Lone Pair Localization. An ordered model (Table 1S in the Supporting Information) was built from the experimental structure to be able to perform all calculations for 2. One of the two statistically occupied OH(9) ligands was chosen. The most plausible BO2OH group was preserved, and a hydrogen atom from the structural relaxation was incorporated.am was placed ∼1 Å from OH(9). We used the Henry’s model for determination of partial charges using nonempirical scales of atomic electronegativity and hardness implemented in the program PACHA.14 Results are given in Table 1S and show very similar partial charges for Pb(1)−Pb(9) sites in the range from ca. +0.51 to +0.55 (and to a smaller value of ca. +0.47 for Pb(10)). The partial charge for the single B and H sites were calculated as +0.55 and −0.01, respectively, which confirms the O−H

Figure 1. General scheme of synthesis and structural transformations in 1 and 2. [O12Pb18]12+ and [O4Pb10]12+ structural units of different dimensionality are excised from the ideal [OPb] layer in tetragonal αPbO (left top). [O12Pb18]12+ layer in 2 contains square-shaped vacancies corresponding to the removal of PbO4 group. H3BO3 and PbCl2 are used a source of borate groups and chloride anions, respectively. Under the high-pressure and high-temperature conditions, 1 and 2 are formed. 2 is formed with an excess of lead oxide component and 1 is formed with an excess of borate and halide reagents. The structure of 2 can be imagined as the incorporation of {Pb(10)Cl4(BO2OH)2} clusters into alternating PbO and chloride layers, with the formation of square vacancies in both. Whereas the structure of 1 can be described as the intrusion of [O4Pb10]12+ tetramers linked by disordered Pb(B2O5) groups into a threedimensional halogen matrix. covalence. Partial charges for O range from ca. −0.44 to −0.40 and those for Cl range from ca. −0.38 to −0.36. This charge model was considered for further rough LP localization favoring the morecovalent Pb−Cl bonds, but the effective overlap reinforces the Pb−O overlap. The latter was verified by the calculation of the electronic structure. The lone-pair (LP) localization for each Pb site was performed using the Verbaere method, implemented in the program Hybride.15 The electrostatic E field on the Pb atoms is calculated as the induced polarization P = αE roughly equal to the Pb−LP dipolar momentum (−2d), which enables location of self-consistent LP coordinates. We have used the Pb2+ Shannon polarizability of α = 6.58 A3.16 Computational Methods. Density functional theory (DFT) electronic structure calculations were carried out using the Vienna Ab-initio Package (VASP).14 The spin-polarized calculations were performed using the Projected Augmented Wave Method (PAW),15a applying the generalized gradient approximation (GGA). The GGA potential was developed using the Perdew−Burke−Ernzerhof (PBE) functional.15b A geometry optimization was performed using the ordered model of 2 described above to get a more-accurate structure for the following calculations. It was performed using restrictions on the unit-cell parameters and atomic positions. All of these were fixed except OH(9) and H atoms, which were allowed to relax in all directions (x, y, z). The relaxation was carried out using a plane-wave energy cutoff of 550 eV and 19 k-points in the irreducible Brillouin zone. The convergence was reached with residual Hellman−Feynman forces of 2σ(I)] wR2 R1 (all data)

1

2

[Pb10O4]Pb2(B2O5) Cl12 120 Mo Kα 3077.45

[Pb18O12] Pb(BO2OH)2Cl10 120 Mo Kα 4602.96

monoclinic P21/c

triclinic P1̅

10.665(7) 17.405(12) 8.406(6) 90 95.79(1) 90 1552.3(18) 6.584 65.869 17607 0.044 1.030 0.027 0.061 0.034

8.8923(7) 10.7545(8) 11.9094(9) 102.789(2) 102.584(2) 105.149(2) 1025.05(14) 7.453 78.384 14655 0.060 1.029 0.038 0.099 0.045

distances involving OH(9), namely, B−OH(9) and Pb10-OH(9). Moderate rearrangements occurred locally with the two latter distances demonstrating reasonable discrepancies with experiment (i.e., 6% and 1.5%, respectively). For accurate electronic structure calculations using this convergence model, a plane-wave energy cutoff of 550 eV, 148 k-points in the irreducible Brillouin zone, and an energy convergence criterion of 10−6 eV were used.

Figure 2. General projection of the crystal structure of (a) 1 and (b) {[Pb10O4]Pb2(B2O5)}12+ bands. The disordered interstitial Pb(B2O5) is highlighted.

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1. B2O5 groups provide the interconnection of these tetramers into one-dimensional (1D) chains. Split Pb(6A) and Pb(6B) sites not bonded to O(1) and O(2) are attached under and above B2O5 groups. Thus, resulted bands are composed of ordered rigid [O4Pb10]12+ groups and disordered Pb(B2O5) interstitial regions. The space between them is filled by Cl− anions (see Figure 2a). The structure of 2 contains 10 symmetrically independent Pb positions (see Figure 3). All of Pb sites are fully occupied in the structure of 2. In order to evaluate the coordination environment of Pb2+ cations, volume eccentricity (ECCv) eccentricity parameters17 were calculated using the computer program IVTON v.2.18 The ECCv parameter is equal to zero for Pb(10) site, which manifests the location of this atom on an inversion center and, as a consequence, its highly symmetrical coordination. In contrast, Pb(4) and Pb(6) coordination environments are highly asymmetrical, with the absence of any even weak Pb−O or Pb−Cl bonds in one of the coordination hemispheres. The rest of Pb sites (Pb(1), Pb(2), Pb(3), Pb(5), Pb(7), Pb(8), Pb(9)) demonstrate rather usual ECCv values (Figure 3). In one-half of the coordination hemisphere, they are coordinated by hard O2− and OH− anions, the number of which varies from three to four. Another coordination hemisphere consists of three or four soft Cl− anions located at the vertices of a distorted square or triangle. The Pb(10) atoms between the PbO related blocks have almost planar square coordination of four Cl− anions. These Pb(10)-

RESULTS AND DISCUSSION The structure of 1 contains 6 symmetrically independent Pb sites. All Pb sites have strongly asymmetric coordination with 2−5 short Pb−O bonds in one coordination hemisphere. The coordination is complemented by several additional longer bonds that vary from site to site. The Pb(3) and Pb(5) atoms form 3 Pb−Cl bonds, and Pb(2) forms 4 Pb−Cl bonds, whereas the Pb(1) and Pb(4) are bonded to five Cl atoms each. The Pb(6) site is split into Pb(6A) and Pb(6B) atomic positions with the total occupancy of 1. The disorder observed for the Pb(6A) and Pb(6B) sites correlates with disordered B2O5 group. Boron site is split into B(1A) and B(1B). All of the Cl atoms in the structure of 1 are fully ordered and have variable coordination environments. The structure of 1 contains five symmetrically independent O sites. O(1) and O(2) atoms are tetrahedrally coordinated by four Pb atoms, forming OPb4 oxocentered tetrahedra7. The ⟨O−Pb⟩ bond lengths in the tetrahedra are in the range of 2.27−2.42 Å, in agreement with the average value of 2.33 Å obtained previously. The O(3), O(4), and O(5) sites belong to the BO3 triangles. The O(4) oxygen site is split into O(4A) and O(4B). O(5) site has siteoccupation factor (sof) of 50%. BO3 triangles share a common corner-forming B2O5 group. Observed pronounced disorder of the diborate group is due to the high flexibility of borate units and high-pressure synthesis conditions. Disorder of borate units 9079

DOI: 10.1021/acs.inorgchem.6b01587 Inorg. Chem. 2016, 55, 9077−9084

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Figure 3. Coordination polyhedra and volume eccentricity (ECCv) parameters of Pb2+ in the structure of 2. Localized Pb2+ lone-pair positions are marked by green. All of the Pb−O, Pb−Cl bond lengths ≤3.55 Å are taken into consideration.

LP displacement. The O(1)−O(6) sites belong to “additional” O atoms that are tetrahedrally coordinated by four Pb atoms each, thus forming OPb4 oxocentered tetrahedra. As mentioned above, “additional” O atoms act as strong Lewis bases. But the Pb(10) atom is generally coordinated by 4 Cl− anions, which may explain the stereochemically passive behavior of the 6s2 lone electron pair. O(7)−OH(9) atoms coordinate B atoms. This BO3 group is rather distorted, and it most probably contains a protonated terminal OH(9) oxygen atom split into O(9A) and O(9B) sites and directed into the cavity discussed below in detail. B−OH(9A),OH(9B) distances are significantly elongated, in accordance with protonation of this oxygen site. Note that the formation of protonated BO2OH groups was described previously in several high-pressure syntheses of different borate materials.19 From the viewpoint of the bond-valence theory, the Oa−Pb bonds (Oa denote additional O atoms) are the shortest and, therefore, the strongest in the structure of 2, which makes it reasonable to consider the Pb−O substructure consisting of OPb4 tetrahedra as an independent structural unit interacting with X atoms via weak Pb−Cl bonds. Outside of these units, Pb−Cl distances (up to 3.55 Å) are necessary for the modeling of both Pb and Cl BVSs, indicating significant Pb−Cl interactions. We note also that partial charges for all additional oxygen atoms are less (Table 1S) than those for O atoms of the borate groups. The topology of the oxocentered Pb−O

Cl bonds make the most important contribution to the Pb(10) bond valence sum. Pb(10) is located within the tetragonal sheets of the Cl− anions, whereas Pb(1)−Pb(9) sites belong to the PbO-derivative blocks (see Figures 4a and 4b). The PbCl4 squares are complemented by triangular BO2OH groups with the formation of clusters shown in Figures3 and Figure 4. Complex borate [OPb4Mn2Cl2(BO3)8]16− cluster with octahedral OPb4Mn2 core is also inserted in similar fashion into both a PbO-derived layer and a chloride interlayer in the structure of the mineral vladkrivovichevite.6i Localization of two terminal hydroxyl groups OH(9A) and OH(9B) with partial occupancies reflects disorder of the borate configuration. If one neglects the disorder over several borate groups (i.e., ordered model designed for the charge and first-principle calculations and LP localizations), the elongated octahedral symmetrical coordination of Pb(10) located at an inversion center suggests that the 6s2 lone electron pair on this Pb2+ is inert. This feature is rather unusual and prompted us to carry out computational studies for the structure of 2. Results on lone-pair localization on Pb2+ cations in the structure of 2 are provided in Table 1S. Pb−LP distances are rather short (between 0.12 Å for Pb(4) and 0.3 Å for Pb(9)) and, as expected, LP are displaced opposite to the shortest Pb−O bonds. Note that the Pb−LP distances should not be taken literally but reflect the lone-pair orientation and relative stereoactivity rather well. Because of its centrosymmetric coordination, Pb(10) was found without significant Pb9080

DOI: 10.1021/acs.inorgchem.6b01587 Inorg. Chem. 2016, 55, 9077−9084

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Inorganic Chemistry

Figure 5. Topological structures of [OPb]-derived layers with 2 × 2 square vacancies in lead oxyhalides (each black square symbolizes a OPb4 tetrahedron and each white square corresponds to a vacancy; anions incorporated into the vacancies are inscribed): (a) [O12Pb18]12+ layer with square vacancies filled by BO2OH in the structure of 2; (b) [O16Pb22]12+ layer with incorporated I− anions in the structure of [Pb22O16][PbO](OH)I10(I,Br)(H2O);11 (c) [O22Pb30]16+ layer in Pb31O22X18 (where X = Br, Cl)12 (additional single vacancies are present); (d) [O24Pb44]40+ layer in the structure of mereheadite, Pb47O24(OH)13Cl25(BO3)2(CO3)6g; (e) [O7Pb10]6+ layer in symesite, Pb10(SO4)O7Cl4(H2O);6e and (f) the [O21Pb32]22+ layer in hereroite, [Pb32O20(O,□)](AsO4)2((Si,As,V,Mo)O4)2Cl106i (with two types of vacancies: S (square) and DS (double square)).

Figure 4. (a) General projection of the crystal structure of 2 along the a-axis. (b) Chloride interlayer with intruded PbCl4(BO2(OH))2 clusters. (c) General projection of [O12Pb18]12+ layer with square vacancies filled by BO2OH groups (OPb4 tetrahedra are shown in red).

structural unit in 2 is two-dimensional and related to the [OPb] layer typical for PbO-derivative oxyhalides.11 The OPb4 tetrahedra share common edges and corners to produce the novel [O12Pb18]12+ layer depicted in Figure 4c. The vacancies have a shape of a single square, obtained via the removal of one PbO4 group. The similar square (S) modules have been found in several natural and synthetic complex lead oxyhalides, but with different stacking sequences involved (see Figure 5). The S modules in 2 are occupied by the BO2OH groups. This is the first example of borate incorporation into [OPb] layer in lead oxyhalide synthetic materials. However, BO3 triangles were previously localized in S modules in the structure of mereheadite.6g Carbonate CO3 anions are also located in half of S vacancies in this mineral crystal structure (Figure 5d). [OPb]-derived layers containing S vacancies only were described earlier in [Pb22O16][PbO](OH)I10(I,Br)(H2O)11 (see Figure 5b) and in another mineral crystal structure symesite, Pb10(SO4)O7Cl4(H2O)6e (Figure 5e). The band structure of 2 along the high symmetry segments of the Brillouin zone is represented in Figure 6. The maximum

of the valence band (VB) is located at point U, while the minimum of the conduction band (CB) is located at point Z, which indicates an indirect band gap. The electron localization function (ELF) allows the visualization of the nodal structure of the molecular orbital, including lone pair electrons.20 Figure 7 represents three-dimensional (3D) isosurfaces at different ELF values, where the electrons associated with Pb atoms are pointing away from their close neighboring anions and toward the Cl layers. The LP stereochemistry can be visualized from the ELF on all Pb atoms as it appears lobe-shaped except for Pb(10), which shows a spherical contribution around it at ELF = 0.972 and no contribution at ELF = 0.982. 9081

DOI: 10.1021/acs.inorgchem.6b01587 Inorg. Chem. 2016, 55, 9077−9084

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Inorganic Chemistry

Figure 6. Total density of states (DOS) and projected density of states (PDOS) in the energy range from −10 eV to 7 eV for Pb(4), Pb(1), Pb(7), Pb(6), and Pb(10) with their shortest surrounding O and Cl atoms in 2. Only the closest anions, which contribute the most to the bonding with Pb (i.e., the shortest distances), are considered in this figure. The Fermi level is set to zero (0). In this figure, p-states are represented in red and s-states are shown in blue. For Pb, the s-and p-states are indicated. For O and Cl, the p-states are represented. An expanded view of the top of the valence band (VB) is shown for Pb(6) and Pb(10) and reveals strong/weak sp hybridization in the former due to the stereoactivity of the lone pair. Band structure of 2 in the energy range from −2 eV to 4 eV. The indirect band gap is highlighted in red.

Figure 7. Electron function localization (ELF) calculated for compound 2 and depicted using the program VESTA: (a) three-dimensional (3D) isosurfaces with (a) ELF = 0.972 and (b) 3D isosurfaces with ELF = 0.982. Pb, O, Cl, B, and H atoms are represented as gray, red, green, brown, and yellow balls, respectively. The ELF isosurfaces are shown in yellow.

ECCv values are geometrical parameters that allow one to qualitatively evaluate the degree of the stereochemical activity of 6s2 lone electron pair, but consideration of the DOS and PDOS is required. DOS and PDOS focused in the energy range from −10 eV to 7 eV are represented in Figure 6. The PDOS were analyzed for five representative entities of three groups of Pb polyhedra with significantly different ECCv parameters (Figure 3): Pb(1)O3Cl3 and Pb(7)O4Cl3; Pb(4)O4Cl3 and Pb(6)O4Cl3; and Pb(10)Cl4O2. For each group of Pb atoms,

very similar features are observed (Figure 6). Pb(1) and Pb(7) can be assumed to be representative of other Pb atoms with similar degrees of stereochemical activity of 6s2 lone pair (Pb(2), Pb(3), Pb(5), Pb(8), Pb(9)). The contribution to the upper VB is extended in the energy range from ca. −4 eV to the Fermi level and is composed by the mixing of O p-states with Pb p-states, also with the presence of Pb s-states just below the Fermi level. Concerning the Cl contribution, it is detailed for Pb(7)−Cl(3) (