Verbeekite, the Long-Unknown Crystal Structure of Monoclinic

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Verbeekite, the Long-Unknown Crystal Structure of Monoclinic PdSe2 Elisabeth Selb,† Martina Tribus,‡ and Gunter Heymann*,† †

Institut für Allgemeine, Anorganische und Theoretische Chemie, Leopold-Franzens-Universität Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria ‡ Institut für Mineralogie und Petrographie, Leopold-Franzens-Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria S Supporting Information *

ABSTRACT: Verbeekite, a monoclinic polymorph of PdSe2, was reported for the first time in 2002 by Roberts et al. The mineral has been discovered in the Musonoi Cu-Co-Mn-U mine, Democratic Republic of Congo, and was named after Dr. Théodore Verbeek, the first geoscientist who studied the palladium mineralization there (1955−1967). Until today, the crystal structure of this very rare mineral has been unknown. By syntheses via multianvil high-pressure/high-temperature methods at 11.5 GPa and 1300 °C, synthetic verbeekite could be obtained in a high degree of purity and comparatively good crystal quality, which made it possible to determine the full crystal structure for PdSe2 verbeekite from single-crystal X-ray diffractometer data: I2/a, a = 671.0(2) pm, b = 415.42(8) pm, c = 891.4(2) pm, β = 92.42(3)°, V = 248.24(4) Å3, R1 = 0.0368, wR2 = 0.0907 (all data). In contrast to layered PdS2-type PdSe2, verbeekite exhibits a novel crystal structure type of dichalcogenides of the platinum-group metals with (Se2)2− dimer anions connecting the layers. The possibility of different arrangements of the characteristic (Se2)2− dumbbells is the reason for the various polymorphs of the dichalcogenides, with now five known PdSe2 representatives. 1992, on the basis of the data of Olsen et al.5 and Takabatake et al.6 In the following, we will focus on the binary palladium diselenides. Altogether, there are now five polymorphs of PdSe2 known. The most common polymorph is the ambient-pressure PdS2-type PdSe2, which crystallizes orthorhombically in the space group Pbca.7 The crystal structure is built up from corrugated layers consisting of square-planar (PdSe4)2−-units connected via common corners. The cohesion of the layers takes place via van der Waals forces. Under pressure the layered structure of PdS2-type PdSe2 transforms into pyrite-type PdSe2 with a cubic structure (Pa3̅).8 Through compressing and converging the layers against each other, an octahedral coordination geometry at the palladium atoms is formed. Soulard et al. studied this pressure-induced phase transition of PdSe2 at room temperature in 2004, by using the diamond anvil cell technique.9 Despite the fact that the transition from layered-type PdSe2 to pyrite-type PdSe2 was determined to be reversible, for PdS 2 and PdSe 2 another orthorhombic modification was recently reported, where the length of the c axis lies between the ideal cubic configuration and the orthorhombic ambient pressure PdS2-type modification. These compounds were obtained by high-pressure/hightemperature methods and were verified via single-crystal Xray diffraction methods.10 A full publication concerning these second orthorhombic palladium dichalcogenides is in preparation. Already in 1978 Larchev et al. even reported about a

1. INTRODUCTION The name of the very rare mineral verbeekite honors the first geoscientist Dr. Théodore Verbeek, who studied the palladium mineralization in the Musonoi Cu-Co-Mn-U mine in the Democratic Republic of Congo (1955−1967), where verbeekite occurs. For the natural form of verbeekite, the formula (Pd0.99Cu0.02)Se1.99 can be found. Roberts et al. performed Xray structure analysis on verbeekite for the first time in 2002 and indexed a C-centered monoclinic cell with the lattice parameters a = 665.9(7) pm, b = 412.4(5) pm, c = 443.8(6) pm, β = 92.76°, and V = 121.7(4) Å3 and therefore deduced C2/m, C2, and Cm as possible space groups (SGs). Due to the fact that only 33 reflections could be measured, a determination of the crystal structure was not possible and, to date, the crystal structure of verbeekite is unknown.1 By means of multianvil high-pressure/high-temperature methods we were able to produce synthetic verbeekite-type PdSe2 in comparatively good crystal quality, which enabled us to determine the crystal structure by single-crystal diffraction methods with the monoclinic cell parameters a = 671.0(2) pm, b = 415.42(8) pm, c = 891.4(2) pm, β = 92.42(3)°, and V = 248.24(4) Å3 in the space group I2/a. In addition to verbeekite, only palladseite (Pd17Se15)2 and oosterboschite ((Pd,Cu)7Se3)3 are still known as naturally occurring binary Pd−Se minerals, found in the same Musonoi Cu-Co-Mn-U mine. All other known Pd-Se compounds are of synthetic nature. Currently, various different compositions with palladium-rich or selenium-rich stoichiometries of binary compounds are known. A revised and evaluated description of the binary Pd-Se system was presented by Okamoto4 in © XXXX American Chemical Society

Received: February 28, 2017

A

DOI: 10.1021/acs.inorgchem.7b00544 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry fourth polymorph with marcasite-type structure (SG Pnnm) synthesized at 7.5 GPa and 600−900 °C.8 Here, palladium exhibits an octahedral Se coordination and, in contrast to the pyrite-type structure, where the octahedra are interconnected via common corners, the linkage now takes place via edges and corners. With an improvement in the crystal quality by means of high-pressure/high-temperature crystallization the longknown monoclinic modification of PdSe2, verbeekite, has now been fully characterized by single-crystal diffraction methods.

Table 1. Crystal Data and Structure Refinement of Verbeekite-Type PdSe2a empirical formula molar mass, g mol−1 cryst syst space group formula units per cell, Z powder diffractometer radiation powder data a, pm b, pm c, pm β, deg V, Å3 single-crystal diffractometer radiation single-crystal data a, pm b, pm c, pm β, deg V, Å3 calcd density, g cm3 cryst size, mm3 temp, K abs coeff, mm−1 F(000), e detector dist, mm θ range, deg range in hkl no. of total rflns no. of data/ref params no. of rflns with I ≥ 2σ(I) Rint, Rσ goodness of fit on F2 abs cor R1/wR2 for I ≥ 2σ(I) R1/wR2 (all data) largest diff peak/hole, e Å−3 transmissn min/max

2. EXPERIMENTAL SECTION 2.1. High-Pressure/High-Temperature Syntheses. Verbeekitetype PdSe2 can be obtained in a high degree of purity and reasonable crystallinity via high-pressure/high-temperature multianvil synthesis according to eq 1, using palladium powder (purity >99.95%, Strem Chemicals, Inc.) and selenium powder (purity >99%, Fluka) in a ratio of 1:2. For enhanced crystal growth, an excess of selenium was added to act as a flux. Our experiments also showed that verbeekite is formed even at lower pressures (above 5.5 GPa), but then it is in combination with other binary palladium chalcogenides as side phases.2b,11 11.5 GPa, 1300 ° C

Pd + 2Se ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ PdSe2

(1)

A 14/8-assembly crucible made of hexagonal boron nitride (HeBoSint P100, Henze BNP GmbH, Kempten, Germany) was filled with a homogenized stoichiometric mixture of palladium and selenium. Within 5 h, a pressure of 11.5 GPa was reached through compression of the Walker module. Afterward, the sample was heated to 1300 °C over the next 15 min and the temperature kept constant for the following 30 min. To improve the crystallinity of verbeekite-type PdSe2, the sample was gently cooled to 450 °C over 90 min. In a subsequent decompression step, the press was released and the sample was returned to normal-pressure conditions in 14.5 h. The sample could be well-separated from the surrounding crucible material, and the polycrystalline sample appears silvery to black with metallic luster. The powdered sample is dark gray and is stable in air. Further information about the multianvil technique and constructions of the various assemblies can be found in numerous references.12 2.2. EDX Data. The PdSe2, verbeekite crystals were semiquantitatively investigated by the use of a JEOL JSM-6010LV scanning electron microscope with a Quantax (Bruker Nano, Germany) energy dispersive system (EDX) for element identification. Small particles of the samples were placed on adhesive carbon platelets, and suitable regions of the crystals were selected for measurement points. The experimentally observed compositions (35 ± 3 atom % Pd, 65 ± 3 atom % Se) were close to the ideal ones. No impurity elements with an atomic number larger than that of sodium (detection limit of the instrument) were observed. 2.3. X-ray Diffraction and Data Collection. Powder diffractometry on a STOE Stadi P diffractometer with (111) curved Ge monochromated Mo Kα1 radiation (λ = 70.93 pm) was used to characterize the polycrystalline sample, which was obtained by multianvil high-temperature/high-pressure methods. The powdered sample was mounted between two acetate films with high-vacuum grease and fixed in a sample holder to attach it on the instrument. A Dectris MYTHEN2 1K microstrip detector with 1280 strips was used to collect the diffraction intensities. Using the parameters derived from the single-crystal structure model, a Rietveld refinement of monoclinic PdSe2 was performed with the DIFFRACplus-TOPAS 4.2 software package (Bruker AXS, Karlsruhe, Germany). Peak shapes were modeled by Thompson− Cox−Hastings pseudo-Voigt profiles,13 and the background was fitted through Chebychev polynomials up to the 12th order. A measured instrument function for reflection profiles derived from the refinement of a LaB6 standard14 took into account instrument contributions. The refined lattice parameters agree well with those obtained from the single-crystal data (see Table 1). Figure 1 displays the results of the Rietveld refinement of verbeekite-type PdSe2. As a side phase, rhombohedral selenium15 was identified in a proportion of 5.5%.

a

PdSe2 264.32 monoclinic I2/a (No. 15) 4 STOE Stadi P Mo Kα1 (λ = 70.93 pm) 667.48(6) 414.23(5) 901.4(2) 91.98(1) 249.08(6) Bruker D8 Quest Mo Kα (λ = 71.07 pm) 671.0(2) 415.4(1) 891.4(2) 92.42(3) 248.24(4) 7.07 0.025 × 0.025 × 0.020 292(2) 36.4 456 40 4.6−30.0 ±9, ±5, ±12 2810 363/16 333 0.0519, 0.0276 1.166 multiscan16 0.0329/0.0893 0.0368/0.0907 2.78/−1.07 0.448/0.747

Standard deviations are given in parentheses.

Some of the crushed sample, treated at high pressures and high temperatures, was embedded in perfluoropolyalkylether (viscosity 1800 cSt) to isolate single crystals under the microscope. The irregularly shaped black metallic crystals were fixed on the tip of MicroMounts (MiTeGen, LLC, Ithaca, NY, USA) with a diameter of 20 μm. A Bruker D8 Quest diffractometer with Photon 100 detector system and Incoatec Microfocus source generator (multilayered optic, monochromatized Mo Kα radiation, λ = 71.073 pm) was used for collection of the reflection data. The APEX 2 program package16 improved collection strategies, concerning ω and φ scans. As a result, data sets of complete reciprocal spheres up to high angles with high completeness could be obtained. The data collection occurred in uncorrelated mode, and images with overexposed reflections were retaken with a Ni attenuator. In this way, it was possible to accumulate the diffraction intensity of weak reflections in addition to the intense (311) reflection at 14.95° 2θ. This great difference in intensities (see Figure 2) made previous structural investigations of verbeekite-type PdSe2 impossible.1a For the integration of the reflection intensities, the program SAINT16 used a narrow-frame algorithm and a correction of the reflection intensities with regard to absorption effects was performed with the program SADABS,16 on the basis of the B


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Inorganic Chemistry

Figure 1. XRD pattern (Mo Kα1 radiation) of monoclinic PdSe2, simultaneously refined with rhombohedral Se15 (Rexp = 1.33, Rwp = 1.75, Rp = 1.24, and GOF = 1.31).

Figure 2. Experimental powder pattern of PdSe2 (top) and labeled theoretical reflection positions of PdSe2 (verbeekite) from Roberts et al.1a and theoretical powder pattern (bottom) simulated from the single-crystal data of the monoclinic verbeekite-type PdSe2 presented here. semiempirical “multiscan” approach. The experimental details are given in Table 1.

found. The general systematic extinctions of hkl h + k + l ≠ 2n, h0l h, l ≠ 2n, 0kl k + l ≠ 2n, hk0 with h + k ≠ 2n, 0k0 with k ≠ 2n, h00 with h ≠ 2n, and 00l with l ≠ 2n as well as the special extinction conditions hkl with l ≠ 2n and h ≠ 2n led to the space group I2/a (No. 15). The nonstandard setting I2/a instead of C2/c was chosen because of a better comparison of our and Roberts’ unit cells. Furthermore, the C2/c setting exhibited a β angle larger than 120°, which should be avoided according to the IUCr. The unit cell of Roberts et al. was checked with CELL_NOW16 and found to be unsuitable for our diffraction data. By the use of the “intrinsic phasing” method,17 implemented in the APEX 2 program package,16 the initial positional parameters were deduced. This solution was followed by full-matrix least-squares refinements based on F2 executed with the program SHELXL-2013.18 The correct composition of the compound was verified by refining the occupation parameters in separate series of least-squares cycles.

3. RESULTS AND DISCUSSION 3.1. Structure Refinements. In 2002 Roberts et al. reported a monoclinic polymorph of PdSe 2 , denoted verbeekite.1a A determination of the unit cell was carried out from the indexing of 33 reflections, resulting in the following cell parameters: a = 665.9(7) pm, b = 412.4(5) pm, c = 443.8(6) pm, β = 92.76°, and V = 121.7(4) Å3. As possible space groups (SGs) C2/m, C2, and Cm were given. Figure 2 gives a comparison between the indexed unit cell of Roberts et al. (red lines) and the experimental and simulated powder patterns of our structure proposition. It can be seen that, in the case of the cell of Roberts et al., several reflections remain unindexed. Under consideration of these additional reflections, a new monoclinic cell with approximately doubled c axis was C


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Inorganic Chemistry

Table 4. Selected Interatomic Distances (pm) of Verbeekitea

Additionally, all sites were refined with anisotropic displacement parameters. Slightly increased residual electron density values are due to the crystal quality and the lamellar habitus. A twin refinement or refinement of disorder was not meaningful. The correctness of the space group was checked with the ADDSYM19 routine of the PLATON program package.20 Detailed information about the single-crystal structure determination can be found in Table 1. The positional parameters, anisotropic displacement parameters, interatomic distances, and angles are given in Tables 2−5, respectively.

Pd−SeA Pd−SeB Se−Se SeA−SeB Pd−Pd a

248.5(1) (2×) 249.64(9) (2×) 300.89(9) (2×) 244.2(2) 394.59(3)


Table 5. Selected Interatomic Angles (deg) in Verbeekitea SeA−Pd−SeA SeB−Pd−SeB SeA−Pd−SeB SeB−Pd−SeA

Table 2. Atomic Coordinates and Isotropic Equivalent Displacement Parameters Ueq (Å2) for Verbeekite-Type PdSe2a a

atom

Wyckoff position

x

y

z

Ueq

Pd Se

4d 8f

1/4 0.5551(2)

1/4 0.1683(2)

1/4 0.10613(9)

0.0148(3) 0.0158(3)

180.0 180.0 93.72(3) (2×) 86.28(3) (2×)


a

Space group I2/a. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. Standard deviations are given in parentheses.

Additional details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, D76344 Eggenstein-Leopoldshafen, Germany (fax, (+49)7247808-666; e-mail, crysdata@fiz-karlsruhe.de), on quoting the deposition number CSD 432630 (PdSe2, verbeekite). 3.2. Crystal Chemistry. Verbeekite, PdSe2, which could be synthesized at pressures between 5.5 and 11.5 GPa and temperatures of 1300 °C via high-pressure/high-temperature multianvil synthesis, is built up from one palladium site and one selenium site. Comparable to the case for all other PdSe2 polymorphs, the structure of verbeekite consists of Pd2+ cations and (Se2)2− dumbbell-shaped anions. These (Se2)2− anions are found in several compounds: e.g., in pyrite- and marcasite-type structures8 as well as in the layered structures of the PdS2 type7,9 and the recently described HP-PdSe210b structure. In pyrite- and marcasite-type PdSe2, the Pd atoms are coordinated by six (Se2)2− anions in an octahedral geometry. In contrast, PdS2-type PdSe2 has a layered structure type and the layers consist of square-planar (PdSe4)2−-units, which arise from the coordination of four (Se2)2− anions around one Pd2+ cation. For HP-PdSe2 the coordination geometry is comparable to the PdS2-type coordination, because the two structures differ mainly in the lengths of the orthorhombic c axis and consequently in the layer distances. In Figure 3 a comparison of the layered PdS2-type structure and the verbeekite structure with highlighted (Se2) 2− anions is presented. A view perpendicular to the layers is given in Figure 4. The (Se2)2− dimer anions in the layered structure types are oriented within each corrugated layer (see Figure 4 (bottom)). This is the essential difference from the structure of verbeekite showing (Se2)2− dimer anions connecting the corrugated layers. Below, a

Figure 3. Crystal structure of verbeekite (top) with a view along the a axis and layered crystal structure of PdS2-type PdSe2 (bottom) with a view along the b axis.

closer look at the verbeekite structure is given in comparison to the other PdSe2 polymorphs. As in the PdS2-type structure, there are square-planar (PdSe4)2− units with slightly differing Pd−Se distances extending from 248 (SeA) to 250 pm (SeB) (see Figure 5). Within the planar (PdSe4)2− units SeA−Pd−SeB and SeB−Pd− SeA angles of 93.7 and 86.3° are found, respectively. These planar (PdSe4)2− units are connected via common corners forming corrugated layers. At first glance, Figure 3

Table 3. Anisotropic Displacement Parameters (Å2) for Verbeekitea

a

atom

U11

U22

U33

U23

U13

U12

Pd Se

0.0113(4) 0.0143(4)

0.0198(5) 0.0184(5)

0.0134(4) 0.0147(4)

−0.0007(3) −0.0009(3)

0.0006(3) 0.0013(3)

0.0010(3) 0.0003(3)

Space group I2/a. Standard deviations are given in parentheses. D


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Inorganic Chemistry

glide plane is responsible for the displacement of the layers instead of a c-glide plane in PdS2-type structures. Subsequently, this means selenium atoms from adjacent layers are much closer in verbeekite than in PdS2-type structures and form (Se2)2− dimers, as shown in Figure 3 (top). Consequently, a three-dimensional network structure is built up instead of the layered structure type of PdSe2. The Se−Se distances of the (Se2)2− dumbbells from PdS2-type PdSe2 (only within the layers) and verbeekite-PdSe2 extends from 2389 to 244 pm. Thus, the distances are still significantly shorter than the 2-fold van der Waals radius (190 pm)21 and slightly longer than the 2fold covalent radius (117 pm)21 of selenium. The range of distances in (Se2)2− dimers documented in the literature considering the former known polymorphs vary from 233 to 244 pm,7−9,22 with the longest Se−Se distances occurring at the octahedrally coordinated Pd2+ atoms of pyrite and marcasite. Since the (PdSe4)2− units of verbeekite are more tilted than in PdS2-type structures, the distances between adjacent palladium atoms within the ab planes decrease from 411 pm in PdS2-type PdSe2 to 395 pm in verbeekite.9 Hence, an even more closedmeshed network within the ab planes of verbeekite can be observed. Table 6 gives an overview of the crystallographic densities of the PdSe2 polymorphs, where verbeekite (7.07 g cm−3) is located in the range of PdS2-type PdSe2 (6.77 g cm−3) and high-pressure PdSe2 (6.98 g cm−3).10b In 2002, Roberts et al.1a proposed AuTe2 (calaverite)23 as a possible prototype structure. This has now turned out to be incorrect, because AuTe2 (SG C2/m) shows a different linking pattern of square-planar (AuTe4)2−-units. To our knowledge, the crystal structure of monoclinic PdP2 and its isostructural representative NiP2 are the most similar crystal structures in comparison to verbeekite-type PdSe2.24 Zachariasen reported the crystal structure of PdP2 in the same monoclinic bodycentered cell choice (SG I2/a) as presented for verbeekite. Table 7 gives a comparison of the unit cells of the isopointal compounds PdP2 and verbeekite-type PdSe2. In contrast to PdP2, the square-planar (PdX4)2−-units (X = P, Se) of the verbeekite structure are more tilted against each other than in PdP2. This situation is comparable to the differences we observed before by comparing PdS2-type PdSe2 with verbeekite (see Figures 3 and 6). As a result, the Pd−Pd distances of 415 pm in PdP2 within the layers are larger than comparable Pd−Pd distances of 395 pm in verbeekite.24a The different bonding numbers of phosphorus and selenium causes another difference concerning the connectivity of the anions. In comparison to the selenium atoms in verbeekite, which form separated (Se2)2− dumbbell-shaped anions connecting the layers, the phosphorus atoms in PdP2 form endless chains running along the a axis (see Figure 6). The distances of the phosphorus atoms inside the chains are almost equal at 222 and 220 pm, whereas the distances of the Se atoms of 365 pm

Figure 4. Crystal structure of verbeekite (top) with a view along the c axis and crystal structure of PdS2-type PdSe2 (bottom) with a view along the c axis. The inter- and intralayer (Se2)2− dimers are shown. Different layers are highlighted by different intensities of the (PdSe4)2− units.

Figure 5. Square-planar (PdSe4)2−-unit.

shows that the magnitude of corrugation in verbeekite is much more pronounced than that in PdS2-type structures. The perpendicular view of the layers of PdS2-type PdSe2 and verbeekite-type PdSe2 (see Figure 4) reveal an equal linking pattern of the (PdSe4)2− units. However, the symmetry operation, which relates the layers, differs from PdS2-type PdSe2 (Pbca) to verbeekite (I2/a). In verbeekite, a diagonal b-

Table 6. Comparison of the Lattice Parameters and Densities of the PdSe2 Polymorphsa verbeekite (I2/a) verbeekite NP-PdSe2 (Pbca) HP-PdSe2 (Pbca) PdSe2 (Pnnm) PdSe2 (Pa3̅) a

a, pm

b, pm

c, pm

β, deg

V, Å3

calcd density, g cm−3

ref

671.0(2) 665.9(7) 574.57(4) 587.93(3) 487.3(5) 610.0(6)

415.4(1) 412.4(5) 586.79(4) 593.16(4) 601.3(6) 610.0(6)

891.4(2) 443.8(6) 769.46(3) 685.99(4) 393.0(4) 610.0(6)

92.42(3) 92.76(3)

248.24(4) 121.7(4) 259.43(5) 239.23(2) 115.1(1) 226.9(1)

7.07 7.21 6.77 6.98 7.62 7.74

this work 1a 9 10b 8 8

Abbreviations: NP, normal pressure; HP, high pressure. Standard deviations are given in parentheses. E


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Inorganic Chemistry

Table 7. Comparison of the Lattice Parameters and Densitiesa the Structurally Related Compounds PdSe2 (Verbeekite) and PdP2 verbeekite (I2/a) PdP2 (I2/a) a

a, pm

b, pm

c, pm

β, deg

V, Å3

calc density, g cm−3

ref

671.0(2) 620.7(1)

415.4(1) 585.7(1)

891.4(2) 587.4(1)

92.42(3) 111.80(1)

248.24(4) 198.27

7.07 5.64

this work 24a


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00544. Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*G.H.: fax, +43(0)512-507 57099; e-mail, Gunter.Heymann@ uibk.ac.at. ORCID

Gunter Heymann: 0000-0001-8500-9159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Dr. H. Huppertz for continuous support and use of all the facilities of the Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck. This work was financially supported by the Tiroler Wissenschaf tsfond (TWF), project no. 235863. G.H. was supported by the program Nachwuchsförderung of the University of Innsbruck.

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Figure 6. Crystal structure of PdP2 with a view along c (top) and [101] (bottom). The infinite P−P zigzag chains are highlighted.

REFERENCES

(1) (a) Roberts, A. C.; Paar, W. H.; Cooper, M. A.; Topa, D.; Criddle, A. J.; Jedwab, J. Verbeekite, monoclinic PdSe2, a new mineral from the Musonoi Cu-Co-Mn-U mine, near Kolwezi, Shaba Province, Democratic Republic of Congo. Mineral. Mag. 2002, 66, 173−179. (b) Pirard, C.; Hatert, F. The sulfides and selenides of the Musonoi ̈ mine, Kolwezi, Katanga, Democratic Republic of Congo. Can. Mineral. 2008, 46, 219−231. ̈ a New (2) (a) Davis, R. J.; Clark, A. M.; Criddle, A. J. Palladseite, Mineral from Itabira, Minas Gerais, Brazil. Mineral. Mag. 1977, 41, 123−123. (b) Geller, S. The crystal structure of Pd17Se15. Acta Crystallogr. 1962, 15, 713−721. (3) Johan, Z.; Picot, P.; Pierrot, R.; Verbeek, T. L’oosterboschite (Pd,Cu)7Se5, une nouvelle espèce minérale et la trogtalite cupropalladifère de Musonoi ̈ (Katanga). Bull. Soc. Fr. Minéral. Cristallogr. 1970, 93, 476−481. (4) Okamoto, H. The Pd-Se (palladium-selenium) system. J. Phase Equilib. 1992, 13, 69−72. (5) Olsen, T.; Røst, E.; Grønvold, F. Phase Relationships of Palladium Selenides. Acta Chem. Scand. 1979, 33a, 251−256. (6) Takabatake, T.; Ishikawa, M.; Jorda, J. L. Superconductivity and phase relations in the Pd-Se system. J. Less-Common Met. 1987, 134, 79−89. (7) Grønvold, F.; Røst, E. The crystal structure of PdSe2 and PdS2. Acta Crystallogr. 1957, 10, 329−331. (8) Larchev, V. N.; Popova, S. V. Polymorphism of palladium dichalcogenides at high pressures and temperatures. Inorg. Mater. 1978, 14, 611−612. (9) Soulard, C.; Rocquefelte, X.; Petit, P. E.; Evain, M.; Jobic, S.; Itié, J. P.; Munsch, P.; Koo, H. J.; Whangbo, M. H. Experimental and Theoretical Investigation on the Relative Stability of the PdS2- and Pyrite-Type Structures of PdSe2. Inorg. Chem. 2004, 43, 1943−1949.

within the layers are too large to describe endless selenium chains. Finally, the ternary compound Au(AsGe)25 should be mentioned with mixed As/Ge occupation, which crystallizes isopointal with the PdP2 structure type. In this compound Ge and As build up chains similar to those for P.

4. CONCLUSION Multianvil high-pressure/high-temperature experiments enabled an approach to crystallize verbeekite-type PdSe2, a fifth polymorph of palladium diselenide. The crystal structure of this rare mineral could be solved for the first time in the monoclinic space group I2/a. As in other PdSe2-polymorphs, the crystal structure of verbeekite is also built up from square-planar (PdSe4)2− units, but in contrast to the layered PdS2-type structure, the verbeekite structure exhibits (Se2)2− dumbbellshaped anions connecting the layers. To the best of our knowledge, no comparable prototype structure is known within the chalcogenides. Only the phosphides comprise compounds showing structural similarities. PdP2 has a comparable layered composition with the difference of endless P−P chains instead of (Se2)2− dimer anions. F


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Verbeekite, the Long-Unknown Crystal Structure of Monoclinic

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