Scientific paper Crystal Structure of Picotpaulite, TlFe2S3, from Allchar, F^YIl Macedonia Ton~i Bali}-@uni},1'* Ljiljana Karanovi}2 and Dejan Poleti3 1 Department of Geography and Geology, University of Copenhagen, 0ster Voldgade 10, DK-1350 Copenhagen K, Denmark. 2 Laboratory of Crystallography, Faculty of Mining and Geology, \u{ina 7, 11000 Belgrade, Serbia 3 Department of General and Inorganic Chemistry, Faculty of Technology and Metallurgy, Karnegijeva • 11000 Belgrade, Serbia * Corresponding author: E-mail: tonci@geo.ku.dk Received: 10-03-2008 Dedicated to the memory of Professor Ljubo Golic Abstract The crystal structure of the mineral picotpaulite, TlFcjSj, was solved and refined using single-crystal X-ray diffraction data collected at room temperature. The symmetry is orthorhombic, space group Cmcm, with unit-cell parameters: a = 9.083(6), b = 10.754(6), c = 5.412(4) À, V = 528.6(6) À3, Z = 4. The structure was refined to the conventional R factor 0.0532 for 226 independent reflections with I > 2o(/) and 21 variables. Picotpaulite is isostructural with minerals rasvu-mite (KFe2S3) and pautovite (CsFe2S3), as well as with a number of synthetic compounds belonging to the CsCu2Cl3 structure type. The structure consists of double chains of FeS4-tetrahedra running along [001] interconnected by TlS10 coordination polyhedra, which form zig-zag chains along the same direction. The Fe-Fe distances between neighbours along the chain direction and perpendicular to it are 2.706(2) and 2.693(6) À, respectively, indicating strong Fe-Fe interactions. The average oxidation state of Fe is +2.5, achieved by electron transfer over the close Fe sites. Thallium coordination polyhedron can be described as a combination of a square antiprism adjacent to a trigonal prism. The 6s2 electrons of Tl behave as an inert pair and the atom is situated in the centroid of its coordination. Keywords: Picotpaulite, crystal structure, mixed valence, Fe-Fe interactions, electron delocalization, s2 inert pair. 1. Introduction Picotpaulite was originally described some forty years ago from the world-known locality of rare Tl minerals, Allchar, FYR Macedonia.1 The mineral was found in realgar and in association with pyrite, lorandite and ragui-nite. However, its crystal structure was not determined. This paper presents a single-crystal X-ray diffraction study and structure solution on a sample of picotpaulite from the type deposit (Allchar) and its relation to minerals rasvumite and pautovite, as well as to some synthetic iso-structural compounds. Both isostructural minerals, rasvumite (KFe2S3) and recently described pautovite (CsFe2S3), are from the Kola Peninsula, Russia. Rasvumite is from the Khibina massif2 and pautovite is from the Palitra peralkaline pegmatite, Kedyk-verpakhk Mountain, Lovozero alkaline complex.3 Numerous isostructural synthetic compounds are known as well: - mixed-valence thioferrates AFe2S3 with A = Ba2+, K+, Rb+ and Cs+;4-6 - rubidium and caesium iron chalcogenides AFe2Y3 with A = Rb+, Cs+ and Y = Se2-, Te2-;7 - rubidium and caesium copper halides ACu2X3 with A = Rb+, Cs+ and X = Cl-, Br-, 1 -.8-13 2. Experimental Due to a perfect cleavage and plasticity of the mineral, it was very difficult to separate a suitable fragment for the X-ray study from the sample, which consisted of bundles of elongated black crystals up to 2.5 mm long (Fig. 1). Finally, by covering a larger irregular needle by nail-polish it was possible to cut off an usable fragment while the glue was still slightly viscous. The obtained fragment was a lath 0.03 x 0.07 x 0.44 mm in size with faces parallel to {010}, {100}, and {001}, respectively, and only slightly bent on its ends. The diffraction spots were visibly elongated and distorted, but it was still possible by a careful refinement of the crystal lattice orientation and choice of the integration volume in reciprocal space to obtain usable intensities of reflections for a structure solution and refinement. Room-temperature data were collected on a Bruker-AXS four-circle diffractometer with a Smart 1000CCD detector and a flat graphite monochromator, using Mo^a radiation (X = 0.71073 À) from a fine-focus sealed X-ray tube. The sample-to-detector distance and collimator size were 38.7 and 0.5 mm, respectively. The software SMART was used for data collection, SAINT for integration of intensities and Lp correction. The absorption effects were corrected by a combination of face-indexed (program XPREP from the SHELXTL package) and multi-scan met- Fig. 1. A group of picotpaulite crystals, from which a fragment was isolated for the crystal structure analysis. The elongated habit, perfect cleavage and pronounced plastic deformation can be seen. The approximate dimensions of the field of view are 3 x 3.5 mm. hod (program SADABS). This approach significantly decreased Rint value from 0.30 to 0.08. All software is produced by Bruker-AXS. The structure was solved by direct methods14 and refined using full-matrix least-squares on F2 and the SHELXL97 software15 implemented in the WinGX system Table 1. Crystal data and some details of the structure refinement for picotpaulite. Formula Crystal size (mm) Absorption correction factors Data measurement temperature (K) Crystal description Formula weight, Mr, Crystal system, space group (No.) a (À) b (À) c (À) y (À3) Z Dx (g cm-3) ^ (mm-1) F(000) Range for data collection, 0 (°) Range of Miller indices Reflections collected / unique Reflections / restraints / parameters Observed reflections [I > 2o(I)] Extinction coefficient, k + Goodness-of-fit, S R indices [I > 2o(I)] R indices (all data) (^/^)max „ 3 Largest difference peak and hole (e À 3) TlFe2S3 0.03 x 0.07 x 0.44 T . = 0.022, T = 0.405 min ' max 298(1) black lath-shaped fragment 412.25 orthorhombic, Cmcm (63) 9.083(6) 10.754(6) 5.412(4) 528.6(6) 4 5.180 36.84 724 4.49 - 25.02 -10 < h <10 -12 < k < 12 -6 < l < 6 1934 / 274 (Rint = 0.083) 274 / 0 / 21 int 226 0.0004(8) 1.125 R1 = 0.0532, wr2 = 0.1310 ++ R1 = 0.0663, wr2 = 0.1429 ++ < 0.0001 ^Pmax = 4.60, ^Pmi„= -2.21 + F' = Fck[1+0.001Fc2X3/sin(20)]- w = 1/[c2(Fo^) + (0.0848 P)2 + 16.9765P] where P = (Fo2 + 2Fc2)/3 ++ Table 2. Atomic coordinates, equivalent isotropic+ and anisotropic displacement parameters (À2) for picotpaulite. Atom x .y Z U11 U22 U33 U23 Ul3 U12 Tl 0.5 0.1609(2) 0.25 0.0448(8) 0.045(1) 0.060(1) 0.029(1) 0 0 0 Fe 0.3518(3) 0.5 0 0.0203(9) 0.020(2) 0.037(2) 0.004(1) -0.001(1) 0 0 S1 0.5 0.6212(7) 0.25 0.019(2) 0.022(4) 0.028(4) 0.008(3) 0 0 0 S2 0.2191(7) 0.3827(6) 0.25 0.028(1) 0.025(3) 0.046(3) 0.014(2) 0 0 -0.013(2) + is defined as one third of the trace of the orthogonalized U^ tensor. ++ The anisotropic displacement factor exponent takes the form: -2'K2[h^a*^Ujj + ... + 2hka*b*Ul2 ]. of programs.16 The highest and the lowest residuals in the final AF map were centred on Tl. Further crystal and refinement details can be found in Table 1. The final atomic parameters are listed in Table 2. a) 3. Results and Discussion 3. 1. Description of the Structure The core of the complex Fe polysulfide anions in pi-cotpaulite are planar [Fe2S2] clusters, which contain two Fe atoms bridged by two S1 atoms. These cores are terminated by four S2 atoms. Two S1 and two S2 atoms are a) b) Fig. 2. (a) The pairs of tetrahedra with the atomic numbering scheme (displacement ellipsoids at 50 % probability level), and (b) the double chains of FeS4-tetrahedra in picotpaulite as viewed approximately along [010] (a-axis horisontal, c-axis vertical). Fig. 3. Part of the chains of FeS4-tetrahedra in picotpaulite (a), and corresponding part of sheets in mackinawite (b). bonded to one Fe atom forming a slightly distorted tetrahedron (Fig. 2a). The tetrahedra pairs sharing S1-S1 edges are placed in the planes parallel to (100) and are further interconnected to infinite linear double chains by sharing S1-S2 edges positioned close to planes parallel to (001), as shown in Fig. 2b. The double chains running parallel to [001] are responsible for the pronounced columnar to needle-like habit of the mineral. 1dentical double chains were found in rasvumite and other isostructural compounds. 1n related mackinawite (FeS) structure,1718 edge-sharing FeS4-tetra-hedra are further polymerized in infinite sheets and each iron is bonded to four equidistant sulphur atoms (Fig. 3). The bond distances and angles of picotpaulite are given in Table 3. Similar to rasvumite, in picotpaulite three tetrahedral S-Fe-S angles [108.8(2), 106.3(2), 110.8(2) °] are close to the ideal value (109.5 °), but the fourth one (S2-Fe-S2) is larger and has the value of 113.8(3)°. The four sulphur atoms form an almost perfect tetrahedron, as can be seen from a negligible volume distortion calculated according to Makovicky and Balic-Žunic19, which amounts to only 0.2% (Table 4). The same holds true for all other isostructural compounds. The difference in bonding angles is mostly due to the displacement of Fe atom from the centre of the tetrahedron towards the unshared S2-S2 edge, reflected in eccentricity, which is the largest distortion parameter of the coordination polyhedron (Table 4). The symmetry of the Fe site and its coordination polyhedron is 2 (C2), which puts the two Fe atoms from a [Fe2S2] cluster on a common two-fold axis. As can be seen from Table 3, the differences in the lengths of the tetrahe-dral edges arising from the difference to the ideal tetrahe-dral symmetry result in a variation from the shortest S1-S2 edges of 3.62 À to the longest S1-S1 edge of 3.76 À. 1t is interesting that the shortest edge is between the Fe-Fe neighbours along the c-axis, whereas the longest one lies between the Fe-Fe pairs of [Fe2S2] cluster, which runs across the chain (Fig. 3). As in other isostructural compounds, the large cation (A = Tl) is bonded to ten S atoms (Fig. 4a). The site symmetry of the Tl atom and of its coordination polyhedron is m2m (C2v). The coordination polyhedron can be described as containing two adjacent parts: a square anti-prism (housing the eight shortest bonds) and a trigonal prism sharing one of the lateral faces with one square base of the antiprism (Fig. 4a). Tl atom is situated almost perfectly in the ideal centre of the coordination. The calculated displacement is less than 0.001 À. Tl-polyhedra are interconnected in zigzag chains extending along [001], where every Tl-polyhedron shares two rectangular faces of the trigonal-prism part with neighbouring polyhedra. (Fig. 4b). The lath-like morphology of picotpaulite crystals and the perfect cleavage along the two pinacoids: {010} and {100}, where the former one predominates, can be explained by the distribution of Tl-S bonds. The four shortest Tl-S bonds [3.392(4) À] connect the Fe-S chains to form slabs parallel to {010}. The remaining Tl-S bonds are longer (>3.49 À) and more easily disrupted. A very easy plastic deformation and bending of needles obtained by cleavage is also understandable from the character of Tl-S bonding. Gliding of Fe-S chains relative to each other should be realively easily accomplished by recombination of Tl-S bonds, which must be very weak because of their length and the high Tl coordination number. An interesting aspect of the crystal structure is the packing of the large atoms (S and Tl). Sulphur and thallium atoms are arranged in a cubic eutaxy (Fig. 5). The eutactic planes, parallel to (221), are composed of double rows of sulphur atoms running parallel to [-110] and in- Table 3. The coordination tables of cations for picotpaulite. The bold numbers in the diagonals are the bond distances (À), the numbers above the diagonal the bond angles (°), and the numbers below the diagonal the ligand-ligand distances (À). Tl S2 S2 S2 S2 S2 3.392 ( 4) 164.1 ( 2) 105.9 ( 2) 71.9 ( 2) S2 6.718 ( 6) 3.392 ( 4) 71.9 ( 2) 105.9 ( 2) S2 5.412 ( 4) 3.980 ( 9) 3.392 ( 4) 164.1 ( 2) S2 3.980 ( 9) 5.412 ( 4) 6.718 ( 6) 3.392 ( 4) S2 6.008 ( 8) 3.973 ( 7) 6.008 ( 8) 3.973 ( 7) S2 3.973 ( 7) 6.008 ( 8) 3.973 ( 7) 6.008 ( 8) S1 6.415 ( 6) 3.445 ( 9) 3.445 ( 9) 6.415 ( 6) S1 3.445 ( 9) 6.415 ( 6) 6.415 ( 6) 3.445 ( 9) S2 3.700 ( 6) 5.434 ( 8) 3.700 ( 6) 5.434 ( 8) S2 5.434 ( 8) 3.700 ( 6) 5.434 ( 8) 3.700 ( 6) Fe S2 S2 S1 S1 S2 2.208( 5) 113.8( 3) 106.3( 2) 110.8( 2) S2 3.700( 6) 2.208( 5) 110.8( 2) 106.3( 2) S1 3.618( 8) 3.719( 5) 2.311( 5) 108.8( 2) S1 3.719( 5) 3.618( 8) 3.757( 8) 2.311( 5) S2 S2 S1 S1 S2 S2 121.5 ( 2) 70.5 ( 2) 121.5 ( 2) 70.5 ( 2) 3.493 ( 7) 5.103(10) 3.719 ( 5) 3.719 ( 5) 7.038(10) 5.406(10) 70.5 ( 2) 121.5 ( 2) 70.5 ( 2) 121.5 ( 2) 93.8 ( 2) 3.493 ( 7) 3.719 ( 5) 3.719 ( 5) 5.406(10) 7.038(10) 133.9 ( 1) 59.2 ( 1) 59.2 ( 1) 133.9 ( 1) 63.4 ( 1) 63.4 ( 1) 3.580 ( 5) 5.412 ( 4) 6.304 ( 9) 6.304 ( 9) 59.2 ( 1) 133.9 ( 1) 133.9 ( 1) 59.2 ( 1) 63.4 ( 1) 63.4 ( 1) 98.2 ( 1) 3.580 ( 5) 6.304 ( 9) 6.304 ( 9) 63.9 ( 2) 102.1 ( 2) 63.9 ( 2) 102.1 ( 2) 166.7 ( 2) 99.4 ( 2) 123.0 ( 1) 123.0 ( 1) 3.593 ( 7) 3.980 ( 9) 102.1 ( 2) 63.9 ( 2) 102.1 ( 2) 63.9 ( 2) 99.4 ( 2) 166.7 ( 2) 123.0 ( 1) 123.0 ( 1) 67.3 ( 2) 3.593 ( 7) e o o ffl" M TT Z e g n Z e o SS e Sä v T3(S o ss e « <3 I B9 T3 .0 C3 O ss » T3 k n o H Q k k n o H Q k 12 s ^ B s o o cn CO ■it 00 \D c^ c^ ,—1 o \D IN c^ \D IN CO P^ C^ CO CO CO CO CO CO 00 c^ IN c^ CO c^ c^ CO O 00 00 v^ P^ c^ v^ c^ IN c^ IN c^ IN = average bond distance, VP = volume of the coordination polyhedron, VD = volume distortion, EC = eccentricity, AS = asphericity (for CN 4 it is 0 by definition), BV = valence sum from bond valence calculations. All parameters are calculated by program IVTON. 32 For the calculation of bond valences the parameters of Brese and O'Keeffe 33 were used. Fig. 5. The crystal structure of picotpaulite represented as a cubic eutaxy of large (S light and Tl dark) atoms with layers parallel to (221). Fe atoms represented as small black spheres. One unit cell with the orientation of crystal axes is indicated. In the lower right corner the bases of the four adjacent occupied tetrahedral holes associated with the uppermost layer are indicated. have ten S and two Tl neighbours, whereas S2 has eight S and four Tl neighbours. As typical for the cubic eutaxy, the surrounding eutactic atoms form a cuboctahedron. As a measure of discrepancy of the arrangement from an ideal eutaxy, volume discrepancies of the surrounding atoms from an ideal cuboctahedron can be used. Using the centroid of coordination20 the volumes of circumscribed spheres around the coordination cuboctahedra of Tl, S1 and S2 can be calculated as 198.5, 202.4, and 201.0 À3, respectively, whereas the polyhedral volumes are 109.6, 114.7, and 111.5 À3, respectively. VJVp ratio for a regular cuboctaheron is [4/5(2)1'2]n = 1.7772. The values for Tl, S1, and S2 are 1.8111, 1.7646, and 1.8027, respectively, which gives +2%, -1%, and +1% volume discrepancy from an ideal cuboctahedron as defined by Makovicky and Bali}-@uni}.19 Furthermore, the atoms in a perfect eutaxy lie in the centroids of their surroundings, and the surrounding atoms lie on the surface of the common sphere. Calculating the discrepancies from this ideal situation, one obtains 9%, 3% and 4%, respectively, as the values of eccentricities, whereas the asphericities amount to 9%, 2% and 5%, respectively. As can be seen, all distortion parameters show that the arrangement deviates by less than 10% from an ideal cubic eutaxy. 3. 2. Comparison With Similar Structures Among the family of thioferrates isostructural with picotpaulite different valence states of iron are expected for BaFe2S3 and the rest of the group. According to stoic-hiometry, the former should contain Fe(II) and the rest a mixture of Fe(II) and Fe(III). The Mössbauer-spectros-copy study of synthetic rasvumite, KFe2S321 showed a presence of only one type of Fe, in accordance with the crystal structure study, with characteristics intermediate to Fe(II) and Fe(III). At the same time, the spectroscopical -6.81 (1) characteristics are very similar to those observed for Fe in BaFe2S3.22 The geometrical characteristics of Fe are indeed very similar in all structures (Table 4) and do not show variations expected from the differences in Fe valence. The bond-valence calculations according to the formula suggested by Brown and Altermatt23 predict for all cases Fe(III), contrary both to stoichiometry and the Mössb-auer measurements. Closer to reality are the results of calculation according to the empirical bonding equation 1 where R^ is the bond distance, suggested by Hoggins and Steinfink24 for metal-iron-sulphide compounds. The formula gives 2.77 for the valence of Fe in picotpaulite and 2.63 for Fe in BaFe2S3, emphasizing again the discrepancy between the geometrical parameters and the expected stoichiometry of the latter compound. The expected Fe3+-S, (Fe2+/Fe3+)-S and Fe2+-S bond distances are 2.26, 2.31 and 2.36 À, respectively.25 In K3Fe2S426 and mineral sternbergite, AgFe2S3,27 there are single tetrahedral FeS2 chains with +2.5 as a formal oxidation state of Fe ions. The observed average Fe-S and Fe-Fe distances are 2.31 and 2.84 À, respectively, for Fe2K3S4 and 2.28 and 2.76 À, respectively, for sternbergite. In picotpaulite the corresponding average distances are 2.26 and 2.70 À, respectively, and they are very close to the average distances for the whole AFe2S3 series containing double chains (Table 4). Finally, in mackinawite, with FeS4-tetrahedra interconnected in infinite sheets (Fig. 3) the equivalent values are 2.26 À for Fe-S and 2.60 À for Fe-Fe distance. Therefore, an increase in the polymerization of tetrahedra is accompanied by a decrease of both Fe-S and Fe-Fe distances, and this increases a possibility for electron delocalization. It can be concluded that the double chains in picot-paulite and isostructural thioferrates represent relatively rigid units stabilized by Fe-Fe interactions. The electronic structure of the units enables combination with large cations of various oxidation states, with a maintenance of the same character of bonding. In the case of structures with monovalent large cations, the electron transfer is necessary just for the achievement of the stabilizing mixed-valence character of Fe. However, in the case of BaFe2S3 the close Fe-Fe contacts are supposed to accommodate surplus valence electrons. Reiff et al.22 have determined a very low electrical resistivity for BaFe2S3 (0.5 Q cm in average at room temperature) and a high anisotropy of the property, with resistivity parallel to the c-axis being about ten times lower than that perpendicular to the c-axis. This effect can be explained by the delocalization of a part of electrons through the network of Fe-Fe bonds. One can observe small dissimilarities in the geometry of Fe coordinations between BaFe2S3 and the rest of the isostructural thioferrates. The small distortions of the coordination tetrahedra are reflected in different lengths of the S1-S1 and S2-S2 edges (Fig. 2). In BaFe2S3 the former is shorter than the latter, which lies on the border of the tetrahedral rows. In other thioferrates, the opposite is true. In BaFe2S3 iron atoms are situated almost perfectly in the centres of their coordination polyhedra and all bonds are almost equal in length, whereas other structures show a larger eccentricity of Fe atoms, which are displaced away from the central chain line and S1-S1 edges. Fe-Fe distances along the chain extension are shorter than those perpendicular to it in BaFe2S3, whereas the opposite situation can be observed for other thioferrates. Expressed similarity in the geometrical parameters of all Fe-S chains suggests, however, an unique bonding and electron-band structure, so the small differences are probably a consequence of a partial filling of the conduction band, which can be assumed for BaFe2S3. All of these crystal chemical characteristics also are found in the selenoferrates and teluroferrate (Table 4). Quite a different situation can be seen with Cu as the tetra-hedral cation. In accordance with the stoichiometry the valence state is Cu(I) and the bond distances are as expected, what can be seen from the results of the bond valence calculations (Table 4). The Cu-Cu distances are significantly longer than Fe-Fe and more sensitive to the type of the additional cation in the structure. The eccentricities of Cu atoms diminish with the increase of the anion size, which suggests that the eccentricity is a consequence of the cation-cation repulsion (Table 4). It is interesting that the same binuclear iron-sulphur cores containing planar Fe2S2 or more complicated clusters like those found in picotpaulite and other thioferra-tes, are also building blocks in some complex biological molecules. As the structural parts of many enzymes, ferre-doxins and other iron-sulphur proteins, the valence-delo-calized [Fe2+Fe3+] clusters are of great interest in molecular biology because of their role as active sites in electron transfer reactions.28 29 The proteins containing iron-sulphur active sites are present in all organisms and function as catalytic centres and mediators in complex redox systems, in which the iron-sulphide ions have variable formal oxidation states: [Fe2S2]0, [Fe2S2]+ and [Fe2S2]2+, which correspond to the presence of 2Fe2+, Fe2+ + Fe3+ and 2Fe3+ ions, respectively.30,31 The relationship between the size of A cation and unit cell parameters of thioferrates is shown in Fig. 6. One can see that a nearly linear relationship exists for the K, Rb and Cs compounds. In BaFe2S3 the values deviate very much from this trend, especially for the b and c parameter. The former is much longer, the latter much shorter than predicted from the K-Rb-Cs trend. The differences can a) b) c) ■ Cs ■ Rb ■ TI ■ K ■ 8a l'I' 3.45 3.50 l'I' 355 350