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A naturally-occurring new lead-based halocuprate(I)
Journal of Solid State Chemistry 238 (2016) 9–14
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A naturally-occurring new lead-based halocuprate(I) Mark D. Welch a,n, Michael S. Rumsey a, Annette K. Kleppe b a b
Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 29 February 2016 Accepted 2 March 2016 Available online 2 March 2016
Pb2Cu(OH)2I3 is a new type of halocuprate(I) that is a framework of alternating [Pb4(OH)4]4+ and [Cu2I6]4 units. The structure has been determined in orthorhombic space group Fddd to R1 ¼0.037, wR2 ¼ 0.057, GoF¼1.016. Unit cell parameters are a ¼16.7082(9) Å, b ¼20.8465(15) Å, c ¼ 21.0159(14) Å, V¼ 7320.0(8) Å3 (Z¼32). There is no synthetic counterpart. The structure is based upon a cubane-like Pb4(OH)4 nucleus that is coordinated to sixteen iodide ions. Cu þ ions are inserted into pairs of adjacent edge-sharing tetrahedral sites in the iodide motif to form [Cu2I6]4- groups. The Raman spectrum of Pb2Cu(OH)2I3 has two O-H stretching modes and as such is consistent with space group Fddd, with two non-equivalent OH groups, rather than the related space group I41/acd which has only one nonequivalent OH group. Consideration of the 18-electron rule implies that there is a Cu ¼ Cu double bond, which may be consistent with the short Cu…Cu distance of 2.78 Å, although the dearth of published data on the interpretation of Cu…Cu distances in halocuprate(I) compounds does not allow a clear-cut interpretation of this interatomic distance. The orthorhombic structure is compared with that of the synthetic halocuprate(I) compound Pb2Cu(OH)2BrI2 with space group I41/acd and having chains of corner-linked CuI4 tetrahedra rather than isolated Cu2I6 pairs. The paired motif found in Pb2Cu(OH)2I3 cannot be achieved in space group I41/acd and, conversely, the chain motif cannot be achieved in space group Fddd. As such, the space group defines either a chain or an isolated-pair motif. The existence of Pb2Cu(OH)2I3 suggests a new class of inorganic halocuprate(I)s based upon the Pb4(OH)4 group. & 2016 Elsevier Inc. All rights reserved.
Keywords: New lead-based halocuprate(I) Mineral
1. Introduction Monovalent copper has a rich chemistry on account of its ability to adopt a wide range of coordination compounds, including organometallics. Halocuprates are of considerable interest to the solid-state community on account of their magnetic, semiconducting and photo-luminescent properties [1]. Halocuprates with monovalent copper, designated “halocuprates(I)”, have a diverse and complex structural chemistry defined by different halocuprate groups, including single “molecular” units [CuX2] , [CuX3]2 , [Cu2X4]2 and [Cu2X6]4 [2]. These units can polymerise as chains, e.g. [(Cu2X4)2 ]n. Although numerous organometallic halocuprates have been synthesized, there remain significant challenges to understanding the fundamental crystallisation pathways involved and how these can be modified to control structure topology [3]. The diversity of synthetic halocuprates(I), almost all of which are organometallics, contrasts with the rarity of Cu(I) minerals in Nature. The latter occur in geologically unusual settings that allow a closed-system, highly-evolved localised geochemistry to operate that can produce novel structures and chemical variants n
Corresponding author.
http://dx.doi.org/10.1016/j.jssc.2016.03.005 0022-4596/& 2016 Elsevier Inc. All rights reserved.
which often have no synthetic counterparts (e.g. [4]) In this paper we report the structure and comment on the possible significance of a naturally-occurring novel Pb-halocuprate(I) of composition Pb2Cu(OH)2I3 that is composed of [Pb4(OH)4]4 þ and [Cu2X6]4 groups.
2. Materials and methods 2.1. Sample The rock specimen (Natural History Museum catalogue #BM84642) from which the crystal was extracted for structure determination comes from the world-renowned Broken Hill CuZn-Pb ore deposit in NSW, Australia [5]. The new halocuprate occurs as small (o 1 mm) yellow crystalline aggregates in association with cuprite Cu2O, marshite CuI and galena PbS. It formed at the same time as marshite, rather than originating from it. 2.2. Chemical analysis Energy-dispersive X-ray analysis for elements with Z 48 was carried out on two different uncoated and unpolished fragments of
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M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
BM84642 using a Zeiss EVO 15LS scanning electron microscope operated at 20 kV and 3 mA, coupled with an Oxford Instruments XMax 80 at the Natural History Museum in London. Qualitative analysis using the Oxford Instruments INCA software package indicated that Cu, Pb and I were the only elements present, except for a possible trace of Br in some of thirteen analyses. 2.3. Single-crystal XRD data collection A small yellow transparent crystal was detached from specimen BM84642 and split into smaller pieces to separate off a tenacious adhering fine-grained crust of an impurity phase. One Table 1 Summary of the data collection and structure refinement of Pb2Cu(OH)2I3. Crystal data Chemical formula Crystal system Space group a (Å) b (Å) c (Å) V (Å3) Z Dx (Mg m 3) calc μ (mm 1)
Pb2Cu(OH)2I3 Orthorhombic Fddd 16.7082(9) 20.8465(15) 21.0159(14) 7320.0(8) 32 6.47 49.07
Data collection Diffractometer Radiation, wavelength (Å) Crystal Max. Med. Min. dimensions (mm) Temperature (K) Scan type, frame-width (deg.), frame-time (s) Absorption correction Tmin, Tmax Reflections used for cell, I47s(I) Reflections measured Rs Independent reflections Independent reflections with I4 2s (I) Rint (mmm) θmin, θmax (deg.) Index range Data completeness to 26°θ (%)
Refinement Reflections, restraints, parameters R1[I 42s(I)], R1(all) wR2[I4 2s(I)], wR2 (all) GoF (F2) Weighting scheme coefficients a, b (Δ/s)max Δρmax, Δρmin (e Å 3)
piece of this crystal (0.07 0.05 0.04 mm) was attached to a non-diffracting amorphous-carbon fibre (0.01 mm diameter), itself glued to a glass fibre support. X-ray data were collected using an XcaliburE four-circle diffractometer equipped with an Eos CCD detector (Agilent Industries). MoKα radiation (λ ¼0.71073 Å) was used at 45 kV and 40 mA. A frame-width of 1° in ω and a frametime of 240 s were used. A full sphere of data was collected to θ ¼30°. However, the small size of the crystal resulted in poorer reflection merging for the weaker higher-angle data, e.g. Rint ¼0.112 for θ ¼30°. After testing several θ thresholds, a satisfactory Rint of 0.074 was obtained for data to 26°θ, and these reflections were used for structure determination. The ratio of unique reflections to refined parameters is 24. Information relevant to the data collection is summarised in Table 1. Reflection intensities were corrected for Lorentz, polarisation and absorption effects and converted to structure factors using CrysalisPros. A multi-scan absorption correction (ABSPACK) based upon averaging of symmetry-equivalent reflections was used. 2.4. Raman spectroscopy Unpolarised Raman spectra were recorded from a second crystal fragment (0.01 0.02 0.035 mm) from the same original crystal. Spectra were collected in 180° back-scattering geometry over the range 90–3800 cm 1 using a LabRAM HR800 Raman spectrometer (Horiba Jobin Yvon) at beamline I15, Diamond Light Source (UK). The instrument was equipped with 2400 g grating and a CCD detector. The spectra were excited by the 532-nm line of a 300 mW Torus laser focused down to a 0.01 mm spot on the crystal and collected through a 0.075 mm confocal aperture. Laser power at the sample position was 1.8 mW and exposure times varied from 2 60 s for the low wavenumber region (o1000 cm 1) to 2 300 s for the region above 43000 cm 1. The intrinsic resolution of the spectrometer is o 1 cm 1 and calibrations are accurate to ± cm 1. The frequency of each Raman band was obtained by fitting Voigtian line profiles using a leastsquares algorithm.
Xcalibur E (1 K Eos detector) MoKα, 0.71073 Yellow transparent plate 0.07 0.05 0.04 293(2) ω, 1.0, 240 Multi-scan 0.506, 1 3089 13,405 0.046 1809 1368 0.074 3.12, 25.98 20 rh r 20, 25r k r 25, 25r l r 25 99.8
3. Results 3.1. Structure determination The unit cell is metrically pseudotetragonal (Table 1). Systematic absences are consistent with space group Fddd, although I41/acd had only a few minor space-group violators. However, reflection merging is very poor for 4/mmm with Rint ¼ 0.213, whereas it is far better for mmm (Rint ¼0.074). Consequently, Fddd was chosen for structure solution and refinement. However, as we shall show, it is instructive to consider both space groups as possible candidates for the structure determination, as there is a similar
1809, 0, 75 0.037, 0.059 0.052, 0.057 1.016 0.0125, 0 o 0.001 1.2, 1.1
Table 2 Atom coordinates and atom displacement parameters Uij (Å2) of Pb2Cu(OH)2I3.
Pb(1) Pb(2) Cu(1) Cu(2) I(1) I(2) I(3) O(1) O(2)
x
y
z
U11
U22
U33
U23
U13
U12
Ueq
0.29378(3) 0.44897(3) ⅛ ⅛ 0.12740(8) 0.12311(7) 0.00732(6) 0.4374(5) 0.3095(5)
0.03061(1) 0.12199(3) ⅛ ⅛ 0.02848(5) 0.02217(5) 0.38712(5) 0.0586(5) 0.1231(4)
0.12678(4) 0.03257(3) 0.4311(1) 0.2987(1) 0.51550(5) 0.02507(4) 0.13529(5) 0.1273(5) 0.0598(4)
0.0329(3) 0.0339(3) 0.032(2) 0.034(2) 0.0316(5) 0.0285(5) 0.0252(5) 0.025(5) 0.030(5)
0.0211(3) 0.0348(3) 0.042(2) 0.039(2) 0.0224(5) 0.0266(5) 0.0281(5) 0.034(5) 0.012(4)
0.0351(4) 0.0259(3) 0.034(2) 0.036(2) 0.0295(6) 0.0257(6) 0.0248(6) 0.015(5) 0.020(5)
0.0011(3) 0.0005(3) 0 0 0.0021(4) 0.0019(4) 0.0006(4) 0.001(5) 0.000(4)
0.0019(3) 0.0071(3) 0 0 0.0012(5) 0.0010(5) 0.0001(4) 0.002(5) 0.004(4)
0.0037(3) 0.0006(3) 0.001(2) 0.003(2) 0.0002(5) 0.0003(5) 0.0022(4) 0.002(5) 0.004(5)
0.0297(2) 0.0315(2) 0.0361(7) 0.0366(7) 0.0278(2) 0.0269(2) 0.0260(2) 0.025(2) 0.020(2)
M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
synthetic halocuprate(I) that has space group I41/acd [6]. Structure determination in space group Fddd was carried out using SHELX [7] within the WinGX program suite [8]. Separate refinements were carried out using X-ray scattering factors and anomalous dispersion coefficients for ions and neutral atoms taken from International Tables for Crystallography Volume C [9]. Structure solution by direct methods for space group Fddd found two Pb atoms, three I atoms and two Cu atoms. The two nonequivalent Pb atoms form a tetrahedron of two pairs that is characteristic of the well-known “cubane-type” Pb4(OH)4 group, and this identification was confirmed by finding two nonequivalent O atoms in subsequent least-squares refinement. The very minor difference between refinements using ionised and neutral scattering factors relates to site occupancies. For the ionised model site occupancies are 0.97(1) for both Pb2 þ sites, 0.96(2) for both Cu þ sites, and 0.98(1) for all three I sites. Thus, the refined occupancies using the ionised model indicate essentially full occupancy of Pb, Cu and I sites, which is consistent with the simple chemistry indicated by EDS analysis. By comparison, the neutral-atom model gave slightly lower site occupancies for all Pb, Cu and I sites: 0.93(2) for both Pb sites, 0.91(1) for Cu(1), 0.93 (1) for Cu(2) and 0.94(1) for all three I sites. The slightly better Table 3 Bond distances (Å), polyhedral volumes (Å3), Quadratic Elongationsa and Bondangle Variances (Degree2)a for Pb2Cu(OH)2I3. Pb(1) O(1) O(2)
I(3)
2.472(9) 2.402(9) 2.424(9) 3.492(1) 3.473(1) 3.561(1) 3.568(1) 3.604(1)
Vol
54.09
I(1) I(2)
Pb(2) O(1) O(1) O(2) I(1)
I(2) I(3)
2.39(1) 2.400(9) 2.401(9) 3.534(1) 3.585(1) 3.580(1) 3.588(1) 3.608(1)
Vol
54.00
Cu(1) I(1) x2 I(3) x2 o Cu(1)-I 4 Vol QE BAV
2.683(3) 2.626(2) 2.655 9.42 1.013 50.1
Cu(2) I(2) x2 I(3) x2 o Cu(2)-I 4 Vol QE BAV
2.646(2) 2.622(2) 2.634 9.30 1.006 24.0
Cu(1)…Cu(2)
2.782(4)
a
Polyhedral distortion parameters BAV (bond-angle variance) and QE (quadratic elongation) are a measure of the distortion of the intra-polyhedral bond angles from the ideal polyhedron and a measure of the distortion of bond lengths from the ideal polyhedron, respectively, as defined by [18].
11
result for the ionised model is consistent with well-defined ionic character for Pb, Cu and I in Pb2Cu(OH)2I3. In both models there were, as expected, significant correlations between site occupancies and the overall scale-factor (OSF). Consequently, as the refined occupancies were close to unity (particularly for the ionised model), in the final stages of refinement occupancies of all sites were fixed at unity. Structure solution and refinement procedures were identical for both models, differing only in the X-ray scattering factors used (ionised/neutral). In terms of final agreement indices (R1, wR2), Goodness-of-Fit, atom coordinates, displacement parameters and bond distances, the two models are almost identical. As the ionised model gave a result that is more consistent with the compositional data (full occupancies by single elements) and the expected primarily ionised character of Pb, Cu and I in this compound, we only describe the result for this model below. CIFs and structure factors for refinements using ionised and neutral scattering factors are deposited with the journal. Refinement of Pb2 þ , Cu þ and I occupancies indicated that all sites are filled by their respective ions; there was no suggestion of minor bromine at the iodine site. Although a reasonable result was obtained, examination of the list of most disagreeable reflections suggested likely twinning and this was confirmed by ROTAX which indicated pseudomerohedral twinning on [0–11]. Incorporating this twinning into the refinement reduced indices from R1 ¼0.055, wR2 ¼0.102 and GoF ¼1.241 to final values for full anisotropic refinement of R1 ¼ 0.037, wR2 ¼0.059 and GoF¼ 1.016. The minor twin constitutes 10% of the crystal [BASF¼0.097(2)]. The unmodelled residual electron density maxima and minima are 1.2 and 1.1 e/Å3. The structure of Pb2Cu(OH)2I3 is described in Section 3.2 below. For comparison, we tested refinement of the structure in tetragonal space group I41/acd for a unit cell a ¼14.8178(16) Å, c¼ 16.705(2) Å, V¼3667.9(7) Å3, related to the Fddd cell by [0 ½ ½/0 ½ ½/1 0 0]. The axis of merohedral twinning corresponding to [0–11] of the orthorhombic structure is [ 110]. However, this is a diad axis of the tetragonal space group and so twinning cannot be refined. ROTAX did not indicate any plausible additional twinning for the tetragonal structure. We discuss this structure in relation to the correct orthorhombic structure in Section 3.1.2. 3.1.1. Crystal structure Atom coordinates and atom displacement parameters are given in Table 2. Key interatomic distances and polyhedral parameters are given in Table 3, and bond-valence calculations are summarised in Table 4. A CIF file containing a list of calculated and observed structure factors is deposited with the journal. Both oxygen atoms are bonded to three Pb atoms and their bond-valence sums clearly show that they are OH groups (Table 4). The asymmetric unit contains two Pb atoms, two Cu atoms, three I atoms and two OH groups. The formula is Pb2Cu(OH)2I3 (Z ¼32). The structure of Pb2Cu(OH)2I3 is shown in Fig. 1. It is a framework
Table 4 Bond-valence values and sums (valence units, vu) for Pb2Cu(OH)2I3 calculated using the parameters of Brese and O’Keeffe [15]. O(1)
O(2)
I(1)
I(2)
I(3)
BVS cations (vu)
Pb(1)
0.379
0.457 0.430
0.166 0.130
0.154 0.121
0.108
1.934
Pb(2)
0.472 0.459
0.458
0.114 0.115
0.113 0.119
0.107
1.968
0.284 x1↓x20.287 x1↓x20.786
1.054 1.112
Cu(1) Cu(2) BVS anions (vu)
0.243 x1↓x21.310
1.350
0.768
0.269 x1↓x20.776
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M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
Pb
O
I
(b) c I(2) I(2) Cu(2) I(3) Cu(1)
b (a)
2.783
I(3)
I(1)
I(1)
(c)
Fig. 1. (a) Crystal structure of Pb2Cu(OH)2I3 projected on to (100). The structure is a framework composed of alternating [Pb4(OH)4]4 þ and [Cu2I6]4 groups, shown as balland-spoke and paired blue tetrahedra, respectively. Lead atoms are black, oxygen atoms are red, iodine atoms are mauve. (b) A single cubane-type [Pb4(OH)4]4 þ group shown with anisotropic displacement ellipsoids at the 68% level. (c) A [Cu2I6]4 group comprising a pair of edge-sharing tetrahedra; Cu-I and Cu…Cu distances are shown (Å). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Pb4(OH)4I16 cluster
Pb2Cu(OH)2I3
Pb4(OH0.5F0.5)4Cl18 cluster
bideauxite
Fig. 2. Polyhedral clusters of Pb2Cu(OH)2I3 and the topologically-related mineral bideauxite Pb2Ag(OH)2F2Cl3. Arrows indicate the two additional Cl atoms of the bideauxite polyhedral cluster that distinguish it from that of Pb2Cu(OH)2I3. Both clusters are centred on a tetrahedron of oxygen atoms of the cubane-type Pb4(OH)4 group. See text for discussion.
comprising an alternation of two structural elements: (i) a cubanelike [Pb4(OH)4]4 þ group (Fig. 1b), and (ii) a [Cu2I6]4 halocuprate group composed of two edge-sharing CuI4 tetrahedra (Fig. 1c). The [Pb4(OH)4]4 þ group is bonded to sixteen iodide ions (Fig. 1b). Pairs of Cu þ ions are located at pairs of edge-sharing tetrahedral sites within the iodide motif to form non-polymerised [Cu2I6]4 groups. Six halocuprate groups surround each [Pb4(OH)4]4 þ nucleus, and each
halocuprate group is shared between six adjacent [Pb4(OH)4]4 þ groups. Five long Pb–I bonds (3.48–3.61 Å) are needed to complete the coordination of each Pb atom, contributing totals of 0.68 and 0.57 valence units to Pb(1) and Pb(2) bond-valence sums, respectively. Four Pb(OH)3I5 polyhedra are centred on a tetrahedron of O atoms to form a Pb4(OH)4I16 cluster (Fig. 2). The volumes of the Pb(1) and Pb (2) polyhedra are 54.0 Å3 and 54.1 Å3, respectively.
M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
1
3
2
1
2
1
2
= 3.801 Å
2
3.775
2
1
3
13
2
1
1
2
1
= 3.770 Å
Fig. 3. Environments of the hydroxyl O(1) and O(2) atoms of the Pb4(OH)4 group of Pb2Cu(OH)2I3. Potential O(H)…I donor-acceptor distances (Å) are indicated. See text for discussion. Iodine and lead atoms are identified by numbers. Oxygen atoms are shown as red spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The two non-equivalent Cu tetrahedra are similar (Table 3), but the Cu2I6 group is not centrosymmetric as it is in, for example, Tl4Cu2I6 [10] and some organometallics. Cu-I bond distances of Pb2Cu(OH)2I3 (2.622–2.683 Å) are comparable to that of the CuI4 tetrahedra of CuI marshite (2.625 Å) and synthetic Tl4Cu2I6 (2.652– 2.678 Å). In this respect, we note that the Cu-I bond of the CuI2Br2 tetrahedron of the chain halocuprate(I) Pb2Cu(OH)2BrI2 [6] at 2.527 Å is much shorter than in these other structures and is almost the same as the Cu-Br bond (2.538 Å). Is it possible that Pb2Cu(OH)2BrI2 is actually Pb2Cu(OH)2Br3? Bond-valence sums for I(1–3) are quite low at around 0.8 vu (Table 4). It is possible that minor additional bond-valence is supplied by O-H…I hydrogen bonds (Fig. 3). However, we find that low (0.75–0.9 vu) bond-valence sums for halogens involved in forming long Pb-X bonds in Pb-oxyhalides are common (e.g. [11–14]), perhaps reflecting a structural bias or limitation of the dataset compiled by Brese and O’Keeffe [15]. Bond-valence sums of 1.31 vu for O(1) and 1.35 vu for O(2) clearly indicate that these oxygen atoms form OH groups. The Cu(1,2) bond-valence sums confirm the monovalent state of copper in this phase.
3.1.2. Comparison with refinement in I41/Acd Although reflection merging in 4/mmm indicated that tetragonal space group I41/acd was unlikely to be a correct structure for Pb2Cu(OH)2I3, we were aware of a synthetic halocuprate with space group I41/acd and the same stoichiometry but with one third of the I atoms replaced by Br, namely Pb2Cu(OH)2BrI2 [6]. Consequently, by way of comparison we tested refinement in I41/acd. Structure solution found one Pb atom, one Cu atom and two I atoms. After a few cycles of least-squares refinement the O atom (again an OH group) was located and completed the Pb4(OH)4 group. Corner-linked CuI4 tetrahedra form chains extending parallel to the c axis. However, agreement indices after anisotropic refinement remained high at R1 ¼ 0.17, wR2 ¼0.39, GoF¼1.346, and there was a high residual maximum of 25 e/Å3 at a distance 2.79 Å from the Cu atom. This atom was added to the refinement model as Cu and the occupancies of both Cu sites were coupled and refined together to values of 0.49(1) and 0.51. At this stage anisotropic structure refinement gave R1 ¼0.11, wR2 ¼0.25, GoF ¼1.186 and could not be improved further, as weighting of reflections was clearly problematic; weighting coefficients (SHELX, [7]) for the final tetragonal structure are a ¼0.0918, b¼1394.49, compared
Fig. 4. The unpolarised single-crystal Raman spectrum of Pb2Cu(OH)2I3 with mode frequencies indicated. The two well-resolved peaks at 3440–3460 cm 1 are due to the two non-equivalent OH groups of the orthorhombic structure.
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M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
with a ¼0.0124 and b¼0 for the orthorhombic structure. The reason for this failure was obvious: the second Cu site implies a second CuI4 chain intertwined with the first, resulting in doubling of I coordination-an impossible structure. The fact that occupancies of both Cu sites are ½ is easily explained as due to a “ghost” of the correct Fddd structure superimposed upon the incorrect space group I41/acd. Thus, a convincing case can be made for the orthorhombic model being the correct choice. Further confirmation of the correctness of the Fddd structure is provided by Raman spectroscopy, as described in Section 3.2. 3.2. Raman spectrum The Raman spectrum of Pb2Cu(OH)2I3 is shown in Fig. 4. It has three main groups of modes: (i) a group consists of at least four intense and sharp bands between 90 and 150 cm 1; (ii) a cluster of six or more weaker and broader bands occurs in the range 200–400 cm 1, and (iii) a well-resolved doublet between 3400 and 3500 cm 1. The vibrational modes of Pb2Cu(OH)2I3 can be described mainly in terms of the internal vibrations of its two structural motifs, the cubane-like Pb4(OH)4 cluster and the Cu2I6 group, assuming that these units are only weakly coupled. By comparison with mode assignments made for Cu2Cl6 [16] and Pb4(OH)4 [17], Raman bands observed in the range 90–150 cm 1 can be assigned to Pb–O–Pb bending motions as well as external modes of Cu2I6 including Cu2I6 librations, while bands in the range 200–350 cm 1 can be associated with stretching motions, Pb–O stretching and stretching vibrations of the Cu2I6 group. Below 450 cm 1, assignment of individual Raman modes of Pb2Cu(OH)2I3 to one or the other of its structural motifs is ambiguous because the two dominant frequency ranges for mode activity of the Cu2Cl6 and Pb4(OH)4 units are identical. Nevertheless, the total number of bands observed in the range 50–140 cm 1 and 200–450 cm 1 is consistent with the number of modes previously observed for Cu2Cl6 and Pb4(OH)4. The two modes of Pb2Cu(OH)2I3 observed at 3443 and 3455 cm 1 can be assigned to the OH stretching vibrations of the Pb4(OH)44 þ unit; the two OH stretching modes reflecting the two nonequivalent OH sites of the orthorhombic structure. These are the first experimentally reported OH stretching frequencies for [Pb4(OH)4]4 þ ; previous experiments concentrated on the lower wavenumber range up to a maximum of 1600 cm 1.
4. Discussion The inorganic halocuprate reported here has a completely novel structure and composition. The only similar phase is synthetic Pb2Cu(OH)2BrI2, which also has the cubane-type [Pb4(OH)4]4 þ group, but has chains of corner-sharing CuBr2I2 tetrahedra rather than isolated edge-sharing Cu2I6 pairs. An important point is that space group Fddd leads to a framework structure in which Pb4(OH)4 and Cu2I6 groups alternate, whereas this is not possible for space group I41/acd, which leads to the corner-linked tetrahedral chain structure exemplified by Pb2Cu(OH)2BrI2. Conversely, the corner-linked chain structure cannot be achieved in space group Fddd. The Fddd structure has two non-equivalent OH groups, whereas the tetragonal chain-based structure has one I41/acd. The unpolarised single-crystal Raman spectrum of Pb2Cu(OH)2I3 has two well-resolved peaks in the OH-stretching region with shifts of 3443 cm 1 and 3455 cm 1. As such the Raman spectrum is consistent with space group Fddd and not I41/acd. The [CuI4]3 tetrahedral group, such as occurs in CuI marshite, satisfies the 18-electron rule (11 þ4 þ 3 electrons). However, the
[Cu2I6]4 group can satisfy the 18-electron rule only if there is a Cu¼ Cu double bond. The presence of this intermetallic bond may be consistent with the short Cu…Cu distance of 2.78 Å. This distance is similar to the corresponding distance in synthetic Tl4Cu2I6 (2.61 Å) which is also a halocuprate(I) with isolated [Cu2I6]4 groups [10]; the distance is also similar to those in a number of synthetic organometallic iodocuprates(I) having polymerised halocuprate(I) units with chains of edge-sharing CuI4 tetrahedra, e.g. 2.83 Å in {C10H10N2(CuI2Br4)}n [2]. As there is a dearth of information in the literature concerning the identification of genuine Cu–Cu bonds from interatomic distances, we cannot be sure that Pb2Cu(OH)2I3 complies with or violates the 18-electron rule. Pb2Cu(OH)2I3 bears some similarity to bideauxite Pb2Ag(OH)2F2Cl3[12], space group Fd3¯ m, including a related stoichiometry. The structure of bideauxite is composed of a chequerboard framework of alternating AgCl6 octahedra and Pb4(OH0.5F0.5)4 cubane-type groups. The Cl motif is different from the I motif of Pb2Cu(OH)2I3 and results in octahedral (rather than tetrahedral) sites being occupied by the monovalent cation (Ag). The cubane group in bideauxite is bonded to eighteen Cl atoms to give a Pb4(OH0.5F0.5)4Cl18 cluster (Fig. 2). Two extra halogen atoms enter the coordination sphere of the cubane group (arrowed in Fig. 2) compared with the Pb4(OH)4I16 cluster of Pb2Cu(OH)2I3. Thus, there appear to be topological variants on the Pb2M þ (OH)2X3 stoichiometry that depend upon the halogen motif, which itself may be dependent upon halogen type (size). The interesting possibility arises of a structural dependency upon halogen type or combination. While it is possible that a tetragonal polymorph of Pb2Cu(OH)2I3 with a chain structure may occur, the presence of bromine could stabilise a Cu(I,Br)4 chain structure, as in Pb2Cu(OH)2BrI2, rather than one composed of pairs of edge-sharing Cu(I,Br)4 tetrahedra. The novel structure described here hints at the possibility of synthesizing a new class of inorganic halocuprates based upon Pb4(OH)4 and halocuprate(I) groups, with varying degrees of tetrahedral polymerization of the latter that could be engineered by modifying halogen composition.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2016.03.005.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
R. Peng, M. Li, D. Li, Coord. Chem. Rev. 254 (2010) 1–18. E. Redel, M. Fiederle, C. Janiak, Z. Anorg. Allg. Chem. 635 (2009) 1139–1147. C.H. Arnby, S. Jagner, I. Dance, Cryst. Eng. Commun. 6 (2004) 257–275. R. Turner, Mineral. Mag. 70 (2006) 629–653. W.D. Birch (Ed.), Minerals of Broken Hill, Broken Hill City Council and Museum of Victoria, Australia, 1999, p. 289. Y.-C. Jiang, M.-C. Hu, Cryst. Eng. Commun. 13 (2011) 414–417. G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112–122. L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837–838. A.J.C. Wilson (Ed.), International Tables for crystallography, vol. C, Kluwer Academic Publishers, Dordrecht, Netherlands, 1992. M. Hoyer, H. Hartl, Z. Anorg. Allg. Chem. 587 (1990) 23–28. M.A. Cooper, F.C. Hawthorne, Am. Mineral. 79 (1994) 550–554. M.A. Cooper, F.C. Hawthorne, S. Merlino, M. Pasero, N. Perchiazzi, Can. Mineral. 37 (1999) 915–919. E. Bonaccorsi, M. Pasero, Mineral. Mag. 67 (2003) 15–21. O.I. Siidra, S.V. Krivovichev, R.W. Turner, M.S. Rumsey, J. Spratt, Am. Mineral. 98 (2013) 248–255. N.E. Brese, M. O’Keeffe, Acta Crystallogr. B47 (1991) 192–197. A. Oueslati, F. Bulou, K. Calvayrac, M. Adil, Gargouri, F. Hell, Vib. Spectrosc. 64 (2013) 10–20. J.O. Jensen, J. Mol. Struct. Theochem 587 (2002) 111–121. K. Robinson, G.V. Gibbs, P.H. Ribbe, Science 172 (1971) 567–570.
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
A naturally-occurring new lead-based halocuprate(I) Mark D. Welch a,n, Michael S. Rumsey a, Annette K. Kleppe b a b
Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 29 February 2016 Accepted 2 March 2016 Available online 2 March 2016
Pb2Cu(OH)2I3 is a new type of halocuprate(I) that is a framework of alternating [Pb4(OH)4]4+ and [Cu2I6]4 units. The structure has been determined in orthorhombic space group Fddd to R1 ¼0.037, wR2 ¼ 0.057, GoF¼1.016. Unit cell parameters are a ¼16.7082(9) Å, b ¼20.8465(15) Å, c ¼ 21.0159(14) Å, V¼ 7320.0(8) Å3 (Z¼32). There is no synthetic counterpart. The structure is based upon a cubane-like Pb4(OH)4 nucleus that is coordinated to sixteen iodide ions. Cu þ ions are inserted into pairs of adjacent edge-sharing tetrahedral sites in the iodide motif to form [Cu2I6]4- groups. The Raman spectrum of Pb2Cu(OH)2I3 has two O-H stretching modes and as such is consistent with space group Fddd, with two non-equivalent OH groups, rather than the related space group I41/acd which has only one nonequivalent OH group. Consideration of the 18-electron rule implies that there is a Cu ¼ Cu double bond, which may be consistent with the short Cu…Cu distance of 2.78 Å, although the dearth of published data on the interpretation of Cu…Cu distances in halocuprate(I) compounds does not allow a clear-cut interpretation of this interatomic distance. The orthorhombic structure is compared with that of the synthetic halocuprate(I) compound Pb2Cu(OH)2BrI2 with space group I41/acd and having chains of corner-linked CuI4 tetrahedra rather than isolated Cu2I6 pairs. The paired motif found in Pb2Cu(OH)2I3 cannot be achieved in space group I41/acd and, conversely, the chain motif cannot be achieved in space group Fddd. As such, the space group defines either a chain or an isolated-pair motif. The existence of Pb2Cu(OH)2I3 suggests a new class of inorganic halocuprate(I)s based upon the Pb4(OH)4 group. & 2016 Elsevier Inc. All rights reserved.
Keywords: New lead-based halocuprate(I) Mineral
1. Introduction Monovalent copper has a rich chemistry on account of its ability to adopt a wide range of coordination compounds, including organometallics. Halocuprates are of considerable interest to the solid-state community on account of their magnetic, semiconducting and photo-luminescent properties [1]. Halocuprates with monovalent copper, designated “halocuprates(I)”, have a diverse and complex structural chemistry defined by different halocuprate groups, including single “molecular” units [CuX2] , [CuX3]2 , [Cu2X4]2 and [Cu2X6]4 [2]. These units can polymerise as chains, e.g. [(Cu2X4)2 ]n. Although numerous organometallic halocuprates have been synthesized, there remain significant challenges to understanding the fundamental crystallisation pathways involved and how these can be modified to control structure topology [3]. The diversity of synthetic halocuprates(I), almost all of which are organometallics, contrasts with the rarity of Cu(I) minerals in Nature. The latter occur in geologically unusual settings that allow a closed-system, highly-evolved localised geochemistry to operate that can produce novel structures and chemical variants n
Corresponding author.
http://dx.doi.org/10.1016/j.jssc.2016.03.005 0022-4596/& 2016 Elsevier Inc. All rights reserved.
which often have no synthetic counterparts (e.g. [4]) In this paper we report the structure and comment on the possible significance of a naturally-occurring novel Pb-halocuprate(I) of composition Pb2Cu(OH)2I3 that is composed of [Pb4(OH)4]4 þ and [Cu2X6]4 groups.
2. Materials and methods 2.1. Sample The rock specimen (Natural History Museum catalogue #BM84642) from which the crystal was extracted for structure determination comes from the world-renowned Broken Hill CuZn-Pb ore deposit in NSW, Australia [5]. The new halocuprate occurs as small (o 1 mm) yellow crystalline aggregates in association with cuprite Cu2O, marshite CuI and galena PbS. It formed at the same time as marshite, rather than originating from it. 2.2. Chemical analysis Energy-dispersive X-ray analysis for elements with Z 48 was carried out on two different uncoated and unpolished fragments of
10
M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
BM84642 using a Zeiss EVO 15LS scanning electron microscope operated at 20 kV and 3 mA, coupled with an Oxford Instruments XMax 80 at the Natural History Museum in London. Qualitative analysis using the Oxford Instruments INCA software package indicated that Cu, Pb and I were the only elements present, except for a possible trace of Br in some of thirteen analyses. 2.3. Single-crystal XRD data collection A small yellow transparent crystal was detached from specimen BM84642 and split into smaller pieces to separate off a tenacious adhering fine-grained crust of an impurity phase. One Table 1 Summary of the data collection and structure refinement of Pb2Cu(OH)2I3. Crystal data Chemical formula Crystal system Space group a (Å) b (Å) c (Å) V (Å3) Z Dx (Mg m 3) calc μ (mm 1)
Pb2Cu(OH)2I3 Orthorhombic Fddd 16.7082(9) 20.8465(15) 21.0159(14) 7320.0(8) 32 6.47 49.07
Data collection Diffractometer Radiation, wavelength (Å) Crystal Max. Med. Min. dimensions (mm) Temperature (K) Scan type, frame-width (deg.), frame-time (s) Absorption correction Tmin, Tmax Reflections used for cell, I47s(I) Reflections measured Rs Independent reflections Independent reflections with I4 2s (I) Rint (mmm) θmin, θmax (deg.) Index range Data completeness to 26°θ (%)
Refinement Reflections, restraints, parameters R1[I 42s(I)], R1(all) wR2[I4 2s(I)], wR2 (all) GoF (F2) Weighting scheme coefficients a, b (Δ/s)max Δρmax, Δρmin (e Å 3)
piece of this crystal (0.07 0.05 0.04 mm) was attached to a non-diffracting amorphous-carbon fibre (0.01 mm diameter), itself glued to a glass fibre support. X-ray data were collected using an XcaliburE four-circle diffractometer equipped with an Eos CCD detector (Agilent Industries). MoKα radiation (λ ¼0.71073 Å) was used at 45 kV and 40 mA. A frame-width of 1° in ω and a frametime of 240 s were used. A full sphere of data was collected to θ ¼30°. However, the small size of the crystal resulted in poorer reflection merging for the weaker higher-angle data, e.g. Rint ¼0.112 for θ ¼30°. After testing several θ thresholds, a satisfactory Rint of 0.074 was obtained for data to 26°θ, and these reflections were used for structure determination. The ratio of unique reflections to refined parameters is 24. Information relevant to the data collection is summarised in Table 1. Reflection intensities were corrected for Lorentz, polarisation and absorption effects and converted to structure factors using CrysalisPros. A multi-scan absorption correction (ABSPACK) based upon averaging of symmetry-equivalent reflections was used. 2.4. Raman spectroscopy Unpolarised Raman spectra were recorded from a second crystal fragment (0.01 0.02 0.035 mm) from the same original crystal. Spectra were collected in 180° back-scattering geometry over the range 90–3800 cm 1 using a LabRAM HR800 Raman spectrometer (Horiba Jobin Yvon) at beamline I15, Diamond Light Source (UK). The instrument was equipped with 2400 g grating and a CCD detector. The spectra were excited by the 532-nm line of a 300 mW Torus laser focused down to a 0.01 mm spot on the crystal and collected through a 0.075 mm confocal aperture. Laser power at the sample position was 1.8 mW and exposure times varied from 2 60 s for the low wavenumber region (o1000 cm 1) to 2 300 s for the region above 43000 cm 1. The intrinsic resolution of the spectrometer is o 1 cm 1 and calibrations are accurate to ± cm 1. The frequency of each Raman band was obtained by fitting Voigtian line profiles using a leastsquares algorithm.
Xcalibur E (1 K Eos detector) MoKα, 0.71073 Yellow transparent plate 0.07 0.05 0.04 293(2) ω, 1.0, 240 Multi-scan 0.506, 1 3089 13,405 0.046 1809 1368 0.074 3.12, 25.98 20 rh r 20, 25r k r 25, 25r l r 25 99.8
3. Results 3.1. Structure determination The unit cell is metrically pseudotetragonal (Table 1). Systematic absences are consistent with space group Fddd, although I41/acd had only a few minor space-group violators. However, reflection merging is very poor for 4/mmm with Rint ¼ 0.213, whereas it is far better for mmm (Rint ¼0.074). Consequently, Fddd was chosen for structure solution and refinement. However, as we shall show, it is instructive to consider both space groups as possible candidates for the structure determination, as there is a similar
1809, 0, 75 0.037, 0.059 0.052, 0.057 1.016 0.0125, 0 o 0.001 1.2, 1.1
Table 2 Atom coordinates and atom displacement parameters Uij (Å2) of Pb2Cu(OH)2I3.
Pb(1) Pb(2) Cu(1) Cu(2) I(1) I(2) I(3) O(1) O(2)
x
y
z
U11
U22
U33
U23
U13
U12
Ueq
0.29378(3) 0.44897(3) ⅛ ⅛ 0.12740(8) 0.12311(7) 0.00732(6) 0.4374(5) 0.3095(5)
0.03061(1) 0.12199(3) ⅛ ⅛ 0.02848(5) 0.02217(5) 0.38712(5) 0.0586(5) 0.1231(4)
0.12678(4) 0.03257(3) 0.4311(1) 0.2987(1) 0.51550(5) 0.02507(4) 0.13529(5) 0.1273(5) 0.0598(4)
0.0329(3) 0.0339(3) 0.032(2) 0.034(2) 0.0316(5) 0.0285(5) 0.0252(5) 0.025(5) 0.030(5)
0.0211(3) 0.0348(3) 0.042(2) 0.039(2) 0.0224(5) 0.0266(5) 0.0281(5) 0.034(5) 0.012(4)
0.0351(4) 0.0259(3) 0.034(2) 0.036(2) 0.0295(6) 0.0257(6) 0.0248(6) 0.015(5) 0.020(5)
0.0011(3) 0.0005(3) 0 0 0.0021(4) 0.0019(4) 0.0006(4) 0.001(5) 0.000(4)
0.0019(3) 0.0071(3) 0 0 0.0012(5) 0.0010(5) 0.0001(4) 0.002(5) 0.004(4)
0.0037(3) 0.0006(3) 0.001(2) 0.003(2) 0.0002(5) 0.0003(5) 0.0022(4) 0.002(5) 0.004(5)
0.0297(2) 0.0315(2) 0.0361(7) 0.0366(7) 0.0278(2) 0.0269(2) 0.0260(2) 0.025(2) 0.020(2)
M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
synthetic halocuprate(I) that has space group I41/acd [6]. Structure determination in space group Fddd was carried out using SHELX [7] within the WinGX program suite [8]. Separate refinements were carried out using X-ray scattering factors and anomalous dispersion coefficients for ions and neutral atoms taken from International Tables for Crystallography Volume C [9]. Structure solution by direct methods for space group Fddd found two Pb atoms, three I atoms and two Cu atoms. The two nonequivalent Pb atoms form a tetrahedron of two pairs that is characteristic of the well-known “cubane-type” Pb4(OH)4 group, and this identification was confirmed by finding two nonequivalent O atoms in subsequent least-squares refinement. The very minor difference between refinements using ionised and neutral scattering factors relates to site occupancies. For the ionised model site occupancies are 0.97(1) for both Pb2 þ sites, 0.96(2) for both Cu þ sites, and 0.98(1) for all three I sites. Thus, the refined occupancies using the ionised model indicate essentially full occupancy of Pb, Cu and I sites, which is consistent with the simple chemistry indicated by EDS analysis. By comparison, the neutral-atom model gave slightly lower site occupancies for all Pb, Cu and I sites: 0.93(2) for both Pb sites, 0.91(1) for Cu(1), 0.93 (1) for Cu(2) and 0.94(1) for all three I sites. The slightly better Table 3 Bond distances (Å), polyhedral volumes (Å3), Quadratic Elongationsa and Bondangle Variances (Degree2)a for Pb2Cu(OH)2I3. Pb(1) O(1) O(2)
I(3)
2.472(9) 2.402(9) 2.424(9) 3.492(1) 3.473(1) 3.561(1) 3.568(1) 3.604(1)
Vol
54.09
I(1) I(2)
Pb(2) O(1) O(1) O(2) I(1)
I(2) I(3)
2.39(1) 2.400(9) 2.401(9) 3.534(1) 3.585(1) 3.580(1) 3.588(1) 3.608(1)
Vol
54.00
Cu(1) I(1) x2 I(3) x2 o Cu(1)-I 4 Vol QE BAV
2.683(3) 2.626(2) 2.655 9.42 1.013 50.1
Cu(2) I(2) x2 I(3) x2 o Cu(2)-I 4 Vol QE BAV
2.646(2) 2.622(2) 2.634 9.30 1.006 24.0
Cu(1)…Cu(2)
2.782(4)
a
Polyhedral distortion parameters BAV (bond-angle variance) and QE (quadratic elongation) are a measure of the distortion of the intra-polyhedral bond angles from the ideal polyhedron and a measure of the distortion of bond lengths from the ideal polyhedron, respectively, as defined by [18].
11
result for the ionised model is consistent with well-defined ionic character for Pb, Cu and I in Pb2Cu(OH)2I3. In both models there were, as expected, significant correlations between site occupancies and the overall scale-factor (OSF). Consequently, as the refined occupancies were close to unity (particularly for the ionised model), in the final stages of refinement occupancies of all sites were fixed at unity. Structure solution and refinement procedures were identical for both models, differing only in the X-ray scattering factors used (ionised/neutral). In terms of final agreement indices (R1, wR2), Goodness-of-Fit, atom coordinates, displacement parameters and bond distances, the two models are almost identical. As the ionised model gave a result that is more consistent with the compositional data (full occupancies by single elements) and the expected primarily ionised character of Pb, Cu and I in this compound, we only describe the result for this model below. CIFs and structure factors for refinements using ionised and neutral scattering factors are deposited with the journal. Refinement of Pb2 þ , Cu þ and I occupancies indicated that all sites are filled by their respective ions; there was no suggestion of minor bromine at the iodine site. Although a reasonable result was obtained, examination of the list of most disagreeable reflections suggested likely twinning and this was confirmed by ROTAX which indicated pseudomerohedral twinning on [0–11]. Incorporating this twinning into the refinement reduced indices from R1 ¼0.055, wR2 ¼0.102 and GoF ¼1.241 to final values for full anisotropic refinement of R1 ¼ 0.037, wR2 ¼0.059 and GoF¼ 1.016. The minor twin constitutes 10% of the crystal [BASF¼0.097(2)]. The unmodelled residual electron density maxima and minima are 1.2 and 1.1 e/Å3. The structure of Pb2Cu(OH)2I3 is described in Section 3.2 below. For comparison, we tested refinement of the structure in tetragonal space group I41/acd for a unit cell a ¼14.8178(16) Å, c¼ 16.705(2) Å, V¼3667.9(7) Å3, related to the Fddd cell by [0 ½ ½/0 ½ ½/1 0 0]. The axis of merohedral twinning corresponding to [0–11] of the orthorhombic structure is [ 110]. However, this is a diad axis of the tetragonal space group and so twinning cannot be refined. ROTAX did not indicate any plausible additional twinning for the tetragonal structure. We discuss this structure in relation to the correct orthorhombic structure in Section 3.1.2. 3.1.1. Crystal structure Atom coordinates and atom displacement parameters are given in Table 2. Key interatomic distances and polyhedral parameters are given in Table 3, and bond-valence calculations are summarised in Table 4. A CIF file containing a list of calculated and observed structure factors is deposited with the journal. Both oxygen atoms are bonded to three Pb atoms and their bond-valence sums clearly show that they are OH groups (Table 4). The asymmetric unit contains two Pb atoms, two Cu atoms, three I atoms and two OH groups. The formula is Pb2Cu(OH)2I3 (Z ¼32). The structure of Pb2Cu(OH)2I3 is shown in Fig. 1. It is a framework
Table 4 Bond-valence values and sums (valence units, vu) for Pb2Cu(OH)2I3 calculated using the parameters of Brese and O’Keeffe [15]. O(1)
O(2)
I(1)
I(2)
I(3)
BVS cations (vu)
Pb(1)
0.379
0.457 0.430
0.166 0.130
0.154 0.121
0.108
1.934
Pb(2)
0.472 0.459
0.458
0.114 0.115
0.113 0.119
0.107
1.968
0.284 x1↓x20.287 x1↓x20.786
1.054 1.112
Cu(1) Cu(2) BVS anions (vu)
0.243 x1↓x21.310
1.350
0.768
0.269 x1↓x20.776
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M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
Pb
O
I
(b) c I(2) I(2) Cu(2) I(3) Cu(1)
b (a)
2.783
I(3)
I(1)
I(1)
(c)
Fig. 1. (a) Crystal structure of Pb2Cu(OH)2I3 projected on to (100). The structure is a framework composed of alternating [Pb4(OH)4]4 þ and [Cu2I6]4 groups, shown as balland-spoke and paired blue tetrahedra, respectively. Lead atoms are black, oxygen atoms are red, iodine atoms are mauve. (b) A single cubane-type [Pb4(OH)4]4 þ group shown with anisotropic displacement ellipsoids at the 68% level. (c) A [Cu2I6]4 group comprising a pair of edge-sharing tetrahedra; Cu-I and Cu…Cu distances are shown (Å). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Pb4(OH)4I16 cluster
Pb2Cu(OH)2I3
Pb4(OH0.5F0.5)4Cl18 cluster
bideauxite
Fig. 2. Polyhedral clusters of Pb2Cu(OH)2I3 and the topologically-related mineral bideauxite Pb2Ag(OH)2F2Cl3. Arrows indicate the two additional Cl atoms of the bideauxite polyhedral cluster that distinguish it from that of Pb2Cu(OH)2I3. Both clusters are centred on a tetrahedron of oxygen atoms of the cubane-type Pb4(OH)4 group. See text for discussion.
comprising an alternation of two structural elements: (i) a cubanelike [Pb4(OH)4]4 þ group (Fig. 1b), and (ii) a [Cu2I6]4 halocuprate group composed of two edge-sharing CuI4 tetrahedra (Fig. 1c). The [Pb4(OH)4]4 þ group is bonded to sixteen iodide ions (Fig. 1b). Pairs of Cu þ ions are located at pairs of edge-sharing tetrahedral sites within the iodide motif to form non-polymerised [Cu2I6]4 groups. Six halocuprate groups surround each [Pb4(OH)4]4 þ nucleus, and each
halocuprate group is shared between six adjacent [Pb4(OH)4]4 þ groups. Five long Pb–I bonds (3.48–3.61 Å) are needed to complete the coordination of each Pb atom, contributing totals of 0.68 and 0.57 valence units to Pb(1) and Pb(2) bond-valence sums, respectively. Four Pb(OH)3I5 polyhedra are centred on a tetrahedron of O atoms to form a Pb4(OH)4I16 cluster (Fig. 2). The volumes of the Pb(1) and Pb (2) polyhedra are 54.0 Å3 and 54.1 Å3, respectively.
M.D. Welch et al. / Journal of Solid State Chemistry 238 (2016) 9–14
1
3
2
1
2
1
2
2
3.775
2
1
3
13
2
1
1
2
1
Fig. 3. Environments of the hydroxyl O(1) and O(2) atoms of the Pb4(OH)4 group of Pb2Cu(OH)2I3. Potential O(H)…I donor-acceptor distances (Å) are indicated. See text for discussion. Iodine and lead atoms are identified by numbers. Oxygen atoms are shown as red spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The two non-equivalent Cu tetrahedra are similar (Table 3), but the Cu2I6 group is not centrosymmetric as it is in, for example, Tl4Cu2I6 [10] and some organometallics. Cu-I bond distances of Pb2Cu(OH)2I3 (2.622–2.683 Å) are comparable to that of the CuI4 tetrahedra of CuI marshite (2.625 Å) and synthetic Tl4Cu2I6 (2.652– 2.678 Å). In this respect, we note that the Cu-I bond of the CuI2Br2 tetrahedron of the chain halocuprate(I) Pb2Cu(OH)2BrI2 [6] at 2.527 Å is much shorter than in these other structures and is almost the same as the Cu-Br bond (2.538 Å). Is it possible that Pb2Cu(OH)2BrI2 is actually Pb2Cu(OH)2Br3? Bond-valence sums for I(1–3) are quite low at around 0.8 vu (Table 4). It is possible that minor additional bond-valence is supplied by O-H…I hydrogen bonds (Fig. 3). However, we find that low (0.75–0.9 vu) bond-valence sums for halogens involved in forming long Pb-X bonds in Pb-oxyhalides are common (e.g. [11–14]), perhaps reflecting a structural bias or limitation of the dataset compiled by Brese and O’Keeffe [15]. Bond-valence sums of 1.31 vu for O(1) and 1.35 vu for O(2) clearly indicate that these oxygen atoms form OH groups. The Cu(1,2) bond-valence sums confirm the monovalent state of copper in this phase.
3.1.2. Comparison with refinement in I41/Acd Although reflection merging in 4/mmm indicated that tetragonal space group I41/acd was unlikely to be a correct structure for Pb2Cu(OH)2I3, we were aware of a synthetic halocuprate with space group I41/acd and the same stoichiometry but with one third of the I atoms replaced by Br, namely Pb2Cu(OH)2BrI2 [6]. Consequently, by way of comparison we tested refinement in I41/acd. Structure solution found one Pb atom, one Cu atom and two I atoms. After a few cycles of least-squares refinement the O atom (again an OH group) was located and completed the Pb4(OH)4 group. Corner-linked CuI4 tetrahedra form chains extending parallel to the c axis. However, agreement indices after anisotropic refinement remained high at R1 ¼ 0.17, wR2 ¼0.39, GoF¼1.346, and there was a high residual maximum of 25 e/Å3 at a distance 2.79 Å from the Cu atom. This atom was added to the refinement model as Cu and the occupancies of both Cu sites were coupled and refined together to values of 0.49(1) and 0.51. At this stage anisotropic structure refinement gave R1 ¼0.11, wR2 ¼0.25, GoF ¼1.186 and could not be improved further, as weighting of reflections was clearly problematic; weighting coefficients (SHELX, [7]) for the final tetragonal structure are a ¼0.0918, b¼1394.49, compared
Fig. 4. The unpolarised single-crystal Raman spectrum of Pb2Cu(OH)2I3 with mode frequencies indicated. The two well-resolved peaks at 3440–3460 cm 1 are due to the two non-equivalent OH groups of the orthorhombic structure.
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with a ¼0.0124 and b¼0 for the orthorhombic structure. The reason for this failure was obvious: the second Cu site implies a second CuI4 chain intertwined with the first, resulting in doubling of I coordination-an impossible structure. The fact that occupancies of both Cu sites are ½ is easily explained as due to a “ghost” of the correct Fddd structure superimposed upon the incorrect space group I41/acd. Thus, a convincing case can be made for the orthorhombic model being the correct choice. Further confirmation of the correctness of the Fddd structure is provided by Raman spectroscopy, as described in Section 3.2. 3.2. Raman spectrum The Raman spectrum of Pb2Cu(OH)2I3 is shown in Fig. 4. It has three main groups of modes: (i) a group consists of at least four intense and sharp bands between 90 and 150 cm 1; (ii) a cluster of six or more weaker and broader bands occurs in the range 200–400 cm 1, and (iii) a well-resolved doublet between 3400 and 3500 cm 1. The vibrational modes of Pb2Cu(OH)2I3 can be described mainly in terms of the internal vibrations of its two structural motifs, the cubane-like Pb4(OH)4 cluster and the Cu2I6 group, assuming that these units are only weakly coupled. By comparison with mode assignments made for Cu2Cl6 [16] and Pb4(OH)4 [17], Raman bands observed in the range 90–150 cm 1 can be assigned to Pb–O–Pb bending motions as well as external modes of Cu2I6 including Cu2I6 librations, while bands in the range 200–350 cm 1 can be associated with stretching motions, Pb–O stretching and stretching vibrations of the Cu2I6 group. Below 450 cm 1, assignment of individual Raman modes of Pb2Cu(OH)2I3 to one or the other of its structural motifs is ambiguous because the two dominant frequency ranges for mode activity of the Cu2Cl6 and Pb4(OH)4 units are identical. Nevertheless, the total number of bands observed in the range 50–140 cm 1 and 200–450 cm 1 is consistent with the number of modes previously observed for Cu2Cl6 and Pb4(OH)4. The two modes of Pb2Cu(OH)2I3 observed at 3443 and 3455 cm 1 can be assigned to the OH stretching vibrations of the Pb4(OH)44 þ unit; the two OH stretching modes reflecting the two nonequivalent OH sites of the orthorhombic structure. These are the first experimentally reported OH stretching frequencies for [Pb4(OH)4]4 þ ; previous experiments concentrated on the lower wavenumber range up to a maximum of 1600 cm 1.
4. Discussion The inorganic halocuprate reported here has a completely novel structure and composition. The only similar phase is synthetic Pb2Cu(OH)2BrI2, which also has the cubane-type [Pb4(OH)4]4 þ group, but has chains of corner-sharing CuBr2I2 tetrahedra rather than isolated edge-sharing Cu2I6 pairs. An important point is that space group Fddd leads to a framework structure in which Pb4(OH)4 and Cu2I6 groups alternate, whereas this is not possible for space group I41/acd, which leads to the corner-linked tetrahedral chain structure exemplified by Pb2Cu(OH)2BrI2. Conversely, the corner-linked chain structure cannot be achieved in space group Fddd. The Fddd structure has two non-equivalent OH groups, whereas the tetragonal chain-based structure has one I41/acd. The unpolarised single-crystal Raman spectrum of Pb2Cu(OH)2I3 has two well-resolved peaks in the OH-stretching region with shifts of 3443 cm 1 and 3455 cm 1. As such the Raman spectrum is consistent with space group Fddd and not I41/acd. The [CuI4]3 tetrahedral group, such as occurs in CuI marshite, satisfies the 18-electron rule (11 þ4 þ 3 electrons). However, the
[Cu2I6]4 group can satisfy the 18-electron rule only if there is a Cu¼ Cu double bond. The presence of this intermetallic bond may be consistent with the short Cu…Cu distance of 2.78 Å. This distance is similar to the corresponding distance in synthetic Tl4Cu2I6 (2.61 Å) which is also a halocuprate(I) with isolated [Cu2I6]4 groups [10]; the distance is also similar to those in a number of synthetic organometallic iodocuprates(I) having polymerised halocuprate(I) units with chains of edge-sharing CuI4 tetrahedra, e.g. 2.83 Å in {C10H10N2(CuI2Br4)}n [2]. As there is a dearth of information in the literature concerning the identification of genuine Cu–Cu bonds from interatomic distances, we cannot be sure that Pb2Cu(OH)2I3 complies with or violates the 18-electron rule. Pb2Cu(OH)2I3 bears some similarity to bideauxite Pb2Ag(OH)2F2Cl3[12], space group Fd3¯ m, including a related stoichiometry. The structure of bideauxite is composed of a chequerboard framework of alternating AgCl6 octahedra and Pb4(OH0.5F0.5)4 cubane-type groups. The Cl motif is different from the I motif of Pb2Cu(OH)2I3 and results in octahedral (rather than tetrahedral) sites being occupied by the monovalent cation (Ag). The cubane group in bideauxite is bonded to eighteen Cl atoms to give a Pb4(OH0.5F0.5)4Cl18 cluster (Fig. 2). Two extra halogen atoms enter the coordination sphere of the cubane group (arrowed in Fig. 2) compared with the Pb4(OH)4I16 cluster of Pb2Cu(OH)2I3. Thus, there appear to be topological variants on the Pb2M þ (OH)2X3 stoichiometry that depend upon the halogen motif, which itself may be dependent upon halogen type (size). The interesting possibility arises of a structural dependency upon halogen type or combination. While it is possible that a tetragonal polymorph of Pb2Cu(OH)2I3 with a chain structure may occur, the presence of bromine could stabilise a Cu(I,Br)4 chain structure, as in Pb2Cu(OH)2BrI2, rather than one composed of pairs of edge-sharing Cu(I,Br)4 tetrahedra. The novel structure described here hints at the possibility of synthesizing a new class of inorganic halocuprates based upon Pb4(OH)4 and halocuprate(I) groups, with varying degrees of tetrahedral polymerization of the latter that could be engineered by modifying halogen composition.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2016.03.005.
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