Oleg I. Siidra

Anion-centered tetrahedra in inorganic compounds.

Sergey V. Krivovichev,*,†,‡ Olivier Mentre,́ Oleg I. Siidra,† Marie Colmont, and Stanislav K. Filatov† †St. Petersburg State University, Department of Crystallography, University Emb. 7/9, 199034 St. Petersburg, Russia ‡Institute of Silicate Chemistry, Russian Academy of Sciences, Makarova Emb. 6, 199034 St. Petersburg, Russia UCCS, Equipe de Chimie du Solide, UMR CNRS 8181, ENSC LilleUST Lille, BP 90108, 59652 Villeneuve d’Ascq Cedex, France

Minerals and synthetic Pb(II) compounds with oxocentered tetrahedra: review and classification

The crystal structures of minerals and inorganic compounds with OPb4 oxocentered tetrahedra are reviewed. It is shown that the OPb4 tetrahedral units may link by sharing common Pb atoms to form structural units of various shape and dimensionality. These units determine basic topology of the structures and influence their stability and properties. The high strength of the OPb4 te trahedral units involves interplay between high basicity of additional O2– anions and stereochemical activity of the 6s2 lone electron pairs on Pb2+ cations. The structural chemistry of polycations based upon OPb4 tetrahedra, in general, follows major trends previously observed for cation-centered tetrahedral units (silicates, phosphates, metal sulphides with MS4 tetrahedra, etc.). One may conclude that the basic structural correlations depend upon size and charge parameters of the ions only, irrespective of their positive or negative sign.

Crystal chemistry of the mendipite-type system Pb(3)O(2)Cl(2)-Pb(3)O(2)Br(2)

Abstract The crystal structures of the mendipite series Pb3O2Cl2—Pb3O2Br2 have been solved by direct methods. The structures are based upon [O2Pb3]2+ double chains of edge-sharing OPb4 tetrahedra. There are three symmetrically independent Pb2+ cations. The number of nonequivalent halogen sites is two (X1, X2). Short Pb—O bonds are located on one side of the Pb2+ cations and weak Pb-X bonds are located on the other side of the Pb2+ coordination sphere. The evident strong distortion of the Pb2+ coordination polyhedra is due to the stereoactivity of the 6s2 lone electron pairs of the Pb2+ cations. Pb1-X2 and Pb2-X2 bonds are the most sensitive to the X site occupancy, which is in agreement with the non-linear behavior of the a and c parameters. Determination of unit-cell parameters by single crystal studies showed strong deviation from Vegards rule. Nonlinearity of the lattice parameters is caused by selective ordering of the halide anions over X1 and X2 sites. Br atoms prefer the X2 position, whereas Cl prefers the X1 site. The angle between two adjacent OPb4 tetrahedra was determined to analyze the influence of halogen atoms on the structure of the [O2Pb3]2+ chain. Different occupancy of the X1 site by Cl and Br atoms leads to most pronounced angular changes. These observations may be interpreted as adaptation of the [O2Pb3]2+ double chains to the large halide ions in the crystal structures of the mendipite series compounds.

Crystal chemistry of the mendipite-type system Pb3O2Cl2-Pb3O2Br2

Abstract The crystal structures of the mendipite series Pb3O2Cl2—Pb3O2Br2 have been solved by direct methods. The structures are based upon [O2Pb3]2+ double chains of edge-sharing OPb4 tetrahedra. There are three symmetrically independent Pb2+ cations. The number of nonequivalent halogen sites is two (X1, X2). Short Pb—O bonds are located on one side of the Pb2+ cations and weak Pb-X bonds are located on the other side of the Pb2+ coordination sphere. The evident strong distortion of the Pb2+ coordination polyhedra is due to the stereoactivity of the 6s2 lone electron pairs of the Pb2+ cations. Pb1-X2 and Pb2-X2 bonds are the most sensitive to the X site occupancy, which is in agreement with the non-linear behavior of the a and c parameters. Determination of unit-cell parameters by single crystal studies showed strong deviation from Vegards rule. Nonlinearity of the lattice parameters is caused by selective ordering of the halide anions over X1 and X2 sites. Br atoms prefer the X2 position, whereas Cl prefers the X1 site. The angle between two adjacent OPb4 tetrahedra was determined to analyze the influence of halogen atoms on the structure of the [O2Pb3]2+ chain. Different occupancy of the X1 site by Cl and Br atoms leads to most pronounced angular changes. These observations may be interpreted as adaptation of the [O2Pb3]2+ double chains to the large halide ions in the crystal structures of the mendipite series compounds.

Prewittite, KPb1.5Cu6Zn(SeO3)2O2Cl10, a new mineral from Tolbachik fumaroles, Kamchatka peninsula, Russia: Description and crystal structure

Abstract Prewittite, ideally KPb1.5Cu6Zn(SeO3)2O2Cl10, was found in the fumarole field of the second cinder cone of the North Breach of the Great fissure Tolbachik eruption (1975-1976, Kamchatka peninsula, Russia). It occurs as separate olive-green tabular crystals up to 0.2 mm in maximum dimension. It has vitreous luster and brownish-green streak. Prewittite is orthorhombic, space group Pnnm, a = 9.132(2), b = 19.415(4), c = 13.213(3) Å, V = 2342.6(9) Å3, Z = 4, Dcalc = 3.89 g/cm3, Dmeas = 3.90(2) g/cm3. The eight strongest lines of the powder X-ray diffraction pattern are {I [d(Å)] hkl}: 70 (8.26) 110; 60 (7.53) 101; 90 (4.111) 220, 132, 141; 100 (3.660) 212, 123; 40 (2.996) 223; 50 (2.887) 062; 40 (2.642) 322, 214; 40 (2.336) 073, 180, 244. Prewittite is biaxial (-). The optical orientation is X = a, Y = c, Z = b. The mineral has clear pleochroism: X, Y - olive green, Z - red-brown. The mineral is very brittle with the perfect cleavage on (010) and (101). The most developed crystal forms are {010}, {001}, and {101}. The chemical composition determined by the electron-microprobe is (wt%): K2O 1.76, PbO 21.18, CuO 33.24, ZnO 8.00, SeO2 15.74, Cl 26.06, O=Cl -5.88, total 100.10. The empirical formula derived on the basis of O+Cl = 18 and sum of positive charges of cations equal to 26 is K0.53Pb1.33Cu5.87Zn1.38Se1.99O7.67Cl10.33. The crystal structure was solved by direct methods and refined to an agreement index R1 = 0.034 on the basis of 1522 independent reflections with I ≥ 2σI. It is based upon metal oxide selenite chloride layers parallel to (010) and linked through K-Cl and Pb-Cl bonds to the K and Pb atoms located in the interlayer. The mineral name honors Charles T. Prewitt (b. 1933) in recognition of his important contributions to crystal chemistry of minerals and planetary materials.

The crystal structure and chemistry of mereheadite

Abstract The crystal structure of mereheadite (monoclinic, Cm, a = 17.372(1), b = 27.9419(19), c = 10.6661(6) Å, β = 93.152(5)°, V = 5169.6(5) Å3) has been solved by direct methods and refined to R1 = 0.058 for 6279 unique observed reflections. The structure consists of alternating Pb-O/OH blocks and Pb-Cl sheets oriented parallel to the (201) plane and belongs to the 1:1 type of lead oxide halides with PbO blocks. It contains 30 symmetrically independent Pb positions, 28 of which belong to the PbO blocks, whilst two positions (Pb12 and Pb16) are located within the tetragonal sheets of the Cl− anions. Mereheadite is thus the first naturally occurring lead oxychloride mineral with inter-layer Pb ions. The coordination configurations of the Pb atoms of the PbO blocks are distorted versions of the square antiprism. In one half of the coordination hemisphere, they are coordinated by hard O2−and OH− anions whose number varies from three to four, whereas the other coordination hemisphere invariably consists of four soft Cl− anions located at the vertices of a distorted square. The Pb12 and Pb16 atoms in between the PbO blocks have an almost planar square coordination of four Cl− anions. These PbCl4 squares are complemented by triangular TO3 groups (T = B, C) so that a sevenfold coordination is achieved. The Pb-O/OH block in mereheadite can be obtained from the ideal PbO block by the following list of procedures: (1) removal of some PbO4 groups that results in the formation of square-shaped vacancies; (2) insertion of TO3 groups into these vacancies; (3) removal of some Pb atoms (that correspond to the Pb1A and Pb2A sites), thus transforming coordination of associated O sites from tetrahedral OPb4 to triangular OHPb3; and (4) replacement of two O2−anions by one OH− anion with twofold coordination; this results in formation of the 1 × 2 elongated rectangular vacancy. The structural formula that can be derived on the basis of the results of single-crystal structure determination is Pb47O24(OH)13Cl25(BO3)2(CO3). Welch et al. (1998) proposed the formula Pb2O(OH)Cl for mereheadite, which assumes that neither borate nor carbonate is an essential constituent of mereheadite and their presence in the mineral is due to disordered replacements of Cl− anions. However, our study demonstrates that this is not the case, as BO3 and CO3 groups have well-defined structural positions confined in the vacancies of the Pb-O/OH blocks and are therefore essential constituents. Our results also show that mereheadite is not a polymorph of blixite, but is in fact related to symesite. Symesite thus becomes the baseline member of a group of structurally-related minerals.

Crystal chemistry of layered Pb oxychloride minerals with PbO-related structures: Part I. Crystal structure of hereroite, [Pb32O20(O,□)](AsO4)2[(Si,As,V,Mo)O4]2Cl10

Abstract The crystal structure of hereroite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R1 = 0.054 for 6931 unique observed reflections. The mineral is monoclinic C2/c, a = 23.139(4), b = 22.684(4), c = 12.389(2) Å, β = 102.090(3)°, and V = 6358.8(18) Å3. The structure contains 16 independent Pb sites in strongly asymmetric coordination by O and Cl atoms. There are two tetrahedral sites, from which one (As) is occupied solely by As, whereas the second (T) has the mixed occupancy of [Si0.48As0.29V0.15Mo0.09]. There are in total 21 O sites. The O1-O8 sites belong to the AsO4 and TO4 tetrahedral oxyanions. The other O atoms (O9-O20) are tetrahedrally coordinated by Pb atoms, thus being central for the OPb4 oxocentered tetrahedra. The OPb4 tetrahedra share edges to form the [O21Pb32]22+ layers that can be described as derivatives of the [OPb] layer from the structure of tetragonal PbO (litharge). The [O21Pb32]22+ layer in hereroite can be obtained from the [OPb] layer by removal of blocks of oxocentered tetrahedra, which results in formation of double-square sevenfold and square fourfold cavities. The cavities are occupied by the AsO4 and TO4 tetrahedra, respectively. The topology of the [O21Pb32]22+ layer is complex and can be described as a combination of modules extracted from the layers of OPb4 tetrahedra present in the structures of kombatite and symesite. The topological functions of tetrahedra within the layer are analyzed using the square lattice method, which shows that each symmetry-independent tetrahedron has its own topological function in the layer construction. The structure of hereroite belongs to the 2:1 type of layered Pb oxyhalides and consists of alternating PbO-type layers and Cl sheets oriented parallel to the (010) plane.

Chloroxiphite Pb3CuO2(OH)2Cl2: structure refinement and description in terms of oxocentred OPb4 tetrahedra

Abstract The crystal structure of chloroxiphite, Pb3CuO2(OH)2Cl2, from Merehead Quarry (monoclinic, P21/m, a = 6.6972(8), b = 5.7538(5), c = 10.4686(14) Å, β = 97.747(10)º, V = 399.72(8) Å3) has been refined to R1 = 0.041. The structure contains three symmetrically unique Pb sites and one Cu site. The strong distortion of the Pb2+ coordination polyhedra is due to the stereoactivity ofthe s2 lone electron pairs on the Pb2+ cations. The Cu-site is coordinated by four OH- groups to form an almost planar Cu(OH)4 square that is complemented by two apical Cl- anions, forming an elongated [Cu(OH)4Cl2] octahedron. Because of the large size and variability of coordination polyhedra around Pb2+ cations and the strength of the Me-O bonds in comparison to the Me-Cl bonds (Me = metal), it is convenient to describe the structure of chloroxiphite in terms of oxocentred OPb4 tetrahedra. The O1 atom is tetrahedrally coordinated by four Pb2+ cations forming relatively short and strong O-Pb bonds. The OPb4 tetrahedra link together via common edges to form [O2Pb3]2+ double chains. The difference between chloroxiphite and other natural oxyhalides is the presence of Cu2+ cations which form an independent structural unit that links to units formed by OPb4 tetrahedra. In this sense, chloroxiphite can be considered as a modular structure consisting of both strong cation- and anion-centred units.

Structural complexity of lead silicates: Crystal structure of Pb21[Si7O22]2[Si4O13] and its comparison to hyttsjöite

Abstract The crystal structure of Pb21[Si7O22]2[Si4O13] has been solved on crystals grown by crystallization from melt. The compound is hexagonal, P63/m, a = 9.9244(5), c = 34.2357(16) Å, V = 795.28(6) Å3, R1 = 0.042 for 3361 unique observed reflections. The structure contains five symmetrically independent Si sites tetrahedrally coordinated by O atoms. The Si1O4, Si3O4, and Si4O4 tetrahedra share corners to form branched heptameric [Si7O22]16- units, whereas the Si2O4 and Si5O4 tetrahedra form the tetrameric [Si4O13]10- anions. The structure contains six symmetrically independent Pb sites with the PbOn coordination polyhedra distorted due to the stereochemical activity of the lone electron pairs. The structure can be described as a stacking of layers of the two types, A and B. The A-type layer contains [Si7O22]16- units, Pb1, Pb2, Pb3, and Pb4 sites, whereas the B-type layer contains [Si4O13]10- anions, together with Pb5, Pb6, and Pb6A sites. Stacking of the layers can be described as a sequence ...AA′BAA′B..., where A and A′ denote A layers with opposite orientations of the tripod-shaped silicate heptamers. The crystal structure of Pb21[Si7O22]2[Si4O13] has many similarities to that of hyttsjöite, which contains the same layers consisting of tripod-shaped [Si7O22]16- anions. In both title compound and hyttsjöite, the anions are stacked together in such a way that ellipsoidal cavities with dimensions of ca. 10 × 6 × 6 Å3 are created. The cavities are occupied by the ClPb6 octahedra in hyttsjöite and by “empty” Pb6 octahedra in Pb21[Si7O22]2[Si4O13]. Analysis of structural and chemical complexity in the PbO-SiO2 system indicates that the most chemically complex phases (in terms of complexity of relations between chemical components) appear to be the most complex from the structural point of view as well. The title phase is the most structurally and chemically complex phase in the system. Structural organization of crystalline phases in the PbO-SiO2 system can be described as controlled by the Pb:Si ratio. For the phases with Pb:Si <2, their structures contain Pb2+ ions and silicate anions. For the phases with Pb:Si <2, the structures contain “additional” O atoms, i.e., atoms that are not bonded to Si. These atoms form OPb4 tetrahedra, which are the next strongest structural subunits in the structure after silicate anions. The structures of the phases with Pb:Si <2 can therefore be described as based upon silicate anions and polynuclear cationic units consisting of edge- and corner-sharing OPb4 tetrahedra.

Cr(VI) Trioxide as a Starting Material for the Synthesis of Novel Zero-, One-, and Two-Dimensional Uranyl Dichromates and Chromate-Dichromates

Six different dichromate-based uranyl compounds were obtained. Their structures belong to four principally different but related structure types with different dimensionality of basic structural units. The units in Cs2(UO2)(Cr2O7)(NO3)2 (1) and (C6H11N2)2(UO2)(Cr2O7)2(H2O) (2) are unique, and these are the first “pure” uranyl-dichromates known to date. The compounds Rb2(UO2)(CrO4)(Cr2O7) (3), (C2NH8)2(UO2)(CrO4)(Cr2O7) (4), (C2NH8)2(UO2)(CrO4)2(Cr2O7)(H2O)2 (5), and (C3NH10)2(UO2)(CrO4)2(Cr2O7)(H2O)2 (6) are novel representatives of a rather small group of inorganic compounds containing both isolated CrO4 tetrahedra and dichromate Cr2O7 groups. The structures of 5 and 6 contain compositionally identical but topologically different ∞(2)[(UO2)(CrO4)2(Cr2O7)](2–) sheets (thus corresponding to different geometrical isomers), which have not been reported previously in inorganic compounds. All novel phases have been prepared with an excess of CrO3. “Pure” dichromates are formed at pH < 1.5 and with prior hydrothermal treatment of uranyl-chromate solution, whereas mixed chromate-dichromates are formed at higher pH > 2 values.