Intermetallic Cerium Compounds, Crystal StructureNew intermetallic cerium compounds Ce2 7iMg (T = Ni, Cu, Pd), CqiTjCd (T -Pd, Pt, Au), and CtiTzPb (T = Pt, Au) were prepared by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. Most C e :^^ (X = Mg, Cd, Pb) compounds are stable after annealing at about 1000 K, while Ce:Pd2Mg is obtained as single phase only after melting and quenching. A thermal treatment at about 1000 K leads to decomposition in CePdMg and CePd. The eight compounds were investigated by X-ray diffraction both as powders and single crystals, and most structures were refined from single crystal data. They adopt an ordered l^Si? type structure with space group PA/mbm: a = 759.6( 1), c = 376.71 (9) pm, wR2 = 0.0562, 294 F values, 12 parameters for Ce2Ni2Mg, a = 787.41(9), c = 387.23 (7) pm, wR2 = 0.0438, 222 F2 values, 12 parameters for Ce2Cu2Mg, a = 777.14(8), c = 400.03 (7) pm, wR2 = 0.0276, 221 F2 values, 13 parameters for Ce2Pd2.03Mg0.97, a = 777.90(6), c = 393.28(6) pm, wR2 = 0.0360,317 F2 values, 12 parameters for Ce2Pd2Cd, and a = 779.90 (7), c = 389.97(7) pm, wR2 = 0.0453, 315 F2 values, 12 parameters for Ce2Pt2Cd. Refinement of the occupancy parameters revealed full occupancy for most sites. A mixed palladium/magnesium (97 % Mg / 3 % Pd) occupancy was observed only for the 2a site of Ce2Pd2.03Mg0.97, indicating a small homogeneity range for this compound. Ce2Au2Cd (a = 804.93 (7), c -393.36(6) pm) and the plumbides Ce2Pt2Pb {a = 794.63 (7), c = 381.50(6) pm) and Ce2Au2Pb (a -810.70 (7), c = 394.85(7) pm) were characterized through their Guinier powder data. The structures of the Ct2 FX compounds can be interpreted as an intergrowth of distorted AIB2 and CsCl related slabs of compositions CzTi and CeX. The course of the lattice parameters and chemical bonding within the series of C^TjX compounds are briefly discussed.

The title compounds were synthesized by reacting the elements in sealed tantalum tubes in a high-frequency furnace. They crystallize with the Mo 2 FeB 2 structure, a ternary ordered variant of the U 3 Si 2 type, space group P4/mbm. All compounds were characterized through Guinier powder patterns and the lattice parameters were obtained from least-squares fits. Four structures were refined from single crystal X-ray data: a = 740.5(1) pm, c = 372.5 (1) pm, wR2 = 0.0430, 247 F 2 o values, 13 variables for Y 2 Ni 1.90 Mg, a = 764.5(1) pm, c = 394.39(9) pm, wR2 = 0.0371, 310 F 2 o values, 12 variables for La 2 Ni 2 Mg, a = 754.4(1) pm, c = 385.20(9) pm, wR2 = 0.0460, 295 F 2 o values, 12 variables for Pr 2 Ni 2 Mg, and a = 752.53(8) pm, c = 382.33(5) pm, wR2 = 0.0183, 291 F 2 o values, and 12 variables for Nd 2 Ni 2 Mg.A refinement of the occupancy parameters indicated small de-fects on the nickel site of the yttrium compound, resulting in the composition Y 2 Ni 1.90 Mg for the investigated single crystal. The compounds with cerium, samarium, and gadolinium to thulium as rare earth component were characterized through their Guinier powder patterns. The cell colume of Ce 2 Ni 2 Mg is smaller than that of Pr 2 Ni 2 Mg, indicating intermediate-valent cerium. The structures can be considered as an intergrowth of distored AlB 2 and CsCl related slabs of compositions LnNi 2 and LnMg. Chemical bonding in La 2 Ni 2 Mg and isotypic La 2 Ni 2 In is compared on the basis of extended Hu È ckel calculations.

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Highly mineralized springs in the Scuol-Tarasp area of the Lower Engadin and in the Albula Valley near Alvaneu, Switzerland, display distinct differences with respect to the source and fate of their dissolved sulphur species. High sulphate concentrations and positive sulphur (δ(34)S) and oxygen (δ(18)O) isotopic compositions argue for the subsurface dissolution of Mesozoic evaporitic sulphate. In contrast, low sulphate concentrations and less positive or even negative δ(34)S and δ(18)O values indicate a substantial contribution of sulphate sulphur from the oxidation of sulphides in the crystalline basement rocks or the Jurassic sedimentary cover rocks. Furthermore, multiple sulphur (δ(34)S, Δ(33)S) isotopes support the identification of microbial sulphate reduction and sulphide oxidation in the subsurface, the latter is also evident through the presence of thick aggregates of sulphide-oxidizing Thiothrix bacteria.

In this work, the biogeochemistry of marine sediments from the Kiel Bight, coastal SW Baltic Sea, is studied based on the abundance and isotopic composition of organic carbon and different forms of sedimentary sulphur. Active bacterial sulphate reduction, partly under sulphate-limiting conditions, is evident from paired δ(34)S and δ(18)O values of pore water sulphate. The resulting pore water sulphide is partly precipitated as acid-volatile iron sulphide and subsequently forms sedimentary pyrite, partly serves in later diagenetic sulphurisation of organic matter, or remains dissolved in the pore water, all evident from the respective δ(34)S values. Microbial sulphate turnover is associated with an apparent isotopic fractionation between dissolved sulphate and dissolved sulphide (Δ(34)S) that varies between 46 and 66‰.