Search Results

You are looking at 1 - 10 of 77 items

  • Author: Ute Ch. Rodewald x
Clear All Modify Search

Forty eight new compounds RT2Zn20 were prepared by annealing cold-pressed pellets of the elemental components in an argon atmosphere. They crystallize with the cubic CeCr2Al20 type structure (Fd3̅̅m , Z = 8), which was refined from single-crystal diffractometer data of TbFeiZn20 (a = 1411.1(1) pm ), YRu2Zn20 (a = 1422.6(1) pm ), DyRu2Zn20 (a = 1422.1(1) pm), GdCo2Zn20 (a = 1406.0(1) pm ), DyRh2Zn20 (a = 1418.2(1) pm ), and TmNi2Zn20 (a= 1401.6(1) pm) to conventional residuals varying betw een R = 0.011 and R - 0.024. The com pounds have a tendency for tw inning, thus m im icking hexagonal sym metry, with the cubic [111] axis as the axis w ith the pseudohexagonal symmetry. M inor inconsistencies in the cell volum es of these com pounds indicate slight deviations from the ideal com position. N evertheless, the five atom ic sites of this structure w ere found to be fully occupied w ithin the error lim its w ith the exception of one zinc site of TmNi2Zn20. The coordination for the site of the rare earth atom s is a Frank-K asper polyhedron with coordination num ber (CN) 16. The transition metal atom s occupy a site w ith icosahedral zinc coordination (CN 12). Two of the three zinc sites are in pentagonal prism atic coordination of zinc atom s, capped by rare earth and/or transition metal atom s (CN 12), w hile the third zinc site has 12 zinc neighbors form ing a hexagonal prism , w hich is capped by tw o rare earth atom s (CN 14).

Abstract

The new metal-rich phosphide Sm15Ir33P26 was obtained from a reaction of the elements in a lead flux (1 : 2 : 2 : 30 molar ratio, 1370 K, slow cooling rate). Sm15Ir33P26 adopts a new structure type which was characterized by powder and single crystal X-ray data: C2/m, a = 4720.6(8), b = 392.87(9), c = 1329.2(2) pm, β = 95.478(1)°, wR2 = 0.0376, 3757 F 2 values and 227 variables. The iridium and phosphorus atoms build up a complex three-dimensional [Ir33P26]δ polyanionic network in which the samarium atoms fill diverse larger cavities with coordination numbers of 15–17. The seventeen crystallographically independent iridium atoms have between 3 or 4 phosphorus neighbors in distorted trigonal-, square-planar, or tetrahedral coordination. All phosphorus atoms are isolated and most of them fill tri-capped trigonal prisms. The Sm15Ir33P26 structure is an intergrowth variant of ThCr2Si2, SrPtSb, CeMg2Si2, and TiNiSi-related slabs.

Abstract

Well-shaped needles of the metal-rich phosphide Ce13Ir34.4P24 were synthesized by a reaction of the elements in a lead flux (1 : 2 : 2 : 60 molar ratio, 1370 K, slow cooling rate of 2 K/h). Ce13Ir34.4P24 was characterized by X-ray powder and single crystal data: new structure type C2/m (mS144–1.2), a = 4512.2(9), b = 398.51(8), c = 1331.1(2) pm, β = 105.01(2)°, wR2 = 0.0326, 3554 F 2 values and 186 variables. The structure is built up from a complex three-dimensional [Ir34.4P24]δ polyanion in which the seven crystallographically independent cerium atoms fill distorted hexagonal cavities. Geometrically the Ce13Ir34.4P24 structure can be described as an intergrowth variant of ThCr2Si2, SrPtSb, and CeMg2Si2 related slabs, similar to Lu3Ir7P5, Sm15Ir33P26, and Ce4Ir14P9.

Needle-shaped crystals of the metal-rich phosphide Ce4Ir13.55P9 were synthesized from the elements in a lead flux (starting composition 1 : 2 : 2 : 60) at 1370 K followed by slow cooling. Ce4Ir13.55P9 crystallizes with a new orthorhombic structure type: Pnma, a = 1269.1(2), b = 399.1(1), c = 3349.9(7) pm, wR2 = 0.0722, 2025 F2 values and 139 variables. Two of the 14 crystallographic iridium sites show small defects. All phosphorus atoms have slightly distorted tricapped trigonal prismatic metal coordination by cerium and iridium. The iridium and phosphorus atoms build up a threedimensional [Ir13.55P9]δ− polyanion in which the cerium atoms fill distorted hexagonal cavities. Within the polyanion the phosphide anions are isolated, and one additionally observes a broad range of Ir-Ir bonding (Ir-Ir distances 278 - 298 pm). From a geometrical point of view the Ce4Ir13.55P9 structure can be considered as an intergrowth structure of distorted ThCr2Si2- and SrPtSb-related slabs

SrRhIn2, SrPdIn2, Srlrln2, and SrPtIn2 have been synthesized by reaction of mixtures of the elements in glassy carbon crucibles in a high-frequency furnace. The new compounds were investigated by X-ray diffraction on powders as well as single crystals; a = 437.3(2), b = 1091.9(5), c = 798.0(2) pm for SrRhln2, a = 453.54(7), b = 1079.8(2), c = 790.4(1) pm for SrPdIn2, a = 434.83(8),b= 1102.6(2) ,c = 798.6(2) pm for Srlrln2, a = 447.5( 1) , b = 1091.0(3), c = 787.6( 1) pm for SrPtIn2. They adopt the MgCuAl2 structure, a ternary ordered version of Re3B. Chemical bonding analysis leads to the description of a filled Srln2 structure in which the In-in-bonding is modified by the insertion of transition metal atoms into the planar strontium layers, thus favoring strong indium-transition metal bonding.

Abstract

The new compounds Tm2NiAs2 and Yb2NiAs2 crystallize with a hexagonal structure similar to that reported for Zr2NiAs2. It was refined for Tm2NiAs2 from single-crystal X-ray data in the polar space group P63mc\ a = 408.4(1), c = 1374.2(3) pm, R = 0.044 for 176 structure factors and 16 variable parameters. The compounds Ln2NiAs2 (Ln = Tb -Er) are confirmed to crystallize with an analogous structure from X-ray powder data. In contrast to the earlier structure refinement of Zr2NiAs2 in the higher-symmetric space group P 63/mmc, where the honeycomb layers of nickel and arsenic atoms are described as planar, these layers are puckered in Tm2NiAs2. These compounds are expected to undergo a displacive phase transition, with the centrosymmetric space group P63/mmc to be correct at high temperature. Since the room temperature structure is polar, these compounds might be classified as ferroelectric from a symmetry point of view. The equiatomic title compounds LnNiAs and UNiAs crystallize with a related hexagonal structure corresponding to a superstructure of the AIB2 type with lattice constants varying between a = 416.2(1), c = 1636.1(4) pm for LaNiAs and a = 401.9(2), c = 1488.5(5) pm for YbNiAs. The crystal chemistry of these hexagonal structures is briefly discussed. Using oxidation numbers chemical bonding in the lanthanoid containing compounds may be rationalized with the formulas (Ln+3)2Ni(As-3)2 and Ln+3NiAs-3, thus suggesting semiconducting behavior.

The title compound was prepared by reaction of elemental calcium with the calcium metaperrhenate Ca(ReO4)2. Its crystal structure was determined from single-crystal X-ray data: Amm2, a = 560.31 (5)pm, b = 1572.4( 1)pm, c = 719.91 (6)pm ,Z = 2 ,R = 0.033 for 930 structure factors and 46 variable parameters. The calcium atoms occupy three atomic sites, all with seven oxygen neighbors. Of the two different rhenium atoms one has square-pyramidal oxygen coordination with an average oxidation number +6.25. The other rhenium site (oxidation number +7) was refined as a split position with trigonal-bipyramidal (75%) and tetrahedral oxygen coordination (25%). One oxygen site remains unoccupied, whenever the tetrahedral rhenium site is occupied, resulting in the composition Ca5Re3O14.75. A test for superconductivity of this black compound down to 1.5 K was negative.

Abstract

The gallide YRh2Ga was synthesized by melting of the elements in an arc-furnace followed by annealing in a sealed silica tube in an induction furnace. YRh2Ga crystallizes with a new structure type: P63/mmc, a=552.2(1), c=3119.5(6) pm, wR=0.0957, 497 F 2 values, and 34 variables. It is the n=1 member of the RE 2+n T 3+3n X 1+2n structure series with Laves phase (MgNi2 type in the present case) and CaCu5 (CeCo3B2 type in the present case) related slabs in the Parthé intergrowth concept.

New stannides CaTSn2 (T = Rh, Pd, Ir) and Ca2PhSn5 were prepared as single phase materials by a reaction o f the elements in glassy carbon crucibles under flowing purified argon. The four compounds were investigated by X-ray diffraction both on powders and single crystals and their structures were refined from single crystal data. The stannides CaTSn2 (T = Rh, Pd, Ir) adopt the MgCuAl, structure with space group Cmcm: a = 434.1(1), b = 1081.7(3), c = 748.8(2) pm, wR2 = 0.040Ö, 451 F2 values for CaRhSn2, a = 442.7(2), b = 1113.8(4), c = 745.6(2) pm, wR2 = 0.0318, 471 F ; values for CaPdSn2, and a = 429.5(1), b = 1079.5(3), c = 758.6(2) pm, wR2 = 0.0465, 455 F2 values for CaIrSn2 with 16 variables for each refinement. Chemical bonding analysis leads to the description o f a distorted filled CaSni substructure in which the tin-tin bonding is modified by the insertion o f transition metal atoms into the planar calcium layers, favoring strong tin-transition metal bonding. 119Sn Mössbauer spectra show single signals for CaTSn2 (T = Rh, Pd, Ir) which are subjected to quadrupole splitting. The electron count o f the CaTSn2 compounds correlates with the ll9Sn isomer shift. Ca2Pt3Sn3 crystallizes with the Yb2Pt3Sn5 type structure: Pnma, a = 734.8(1), b = 445.50(7), c = 2634.8(5) pm, wR2 = 0.0636, 1406 F2 values and 62 variables. The platinum and tin atoms in Ca2Pt3 Sns build a complex three-dimensional [Pt3Sn5] polyanion in which the calcium cations fill distorted pentagonal and hexagonal channels. According to semi-empirical band structure calculations the strongest bonding interactions are found for the Pt-Sn contacts, follow ed by Sn-Sn bonding. The 119Sn Mössbauer spectrum of Ca2Pt3Sn5 shows two superimposed signals at δ = 2.10(3) and δ= 2.18(6) mm/s

The platinum-rich intermetallic compounds GdPt2In and GdPt2Sn were synthesized by arc-melting of the elements and subsequent annealing. The structures were refined from single crystal X-ray diffractometer data: ZrPt2Al type, space group P63/mmc, a = 455.1(1), c = 899.3(3) pm, wR2 = 0.0361, 166 F2 values, 9 variables for GdPt2In, and a = 453.2(1), c = 906.5(2) pm, wR2 = 0.0915, 166 F2 values, 9 variables for GdPt2Sn. The platinum and indium (tin) atoms build up threedimensional [Pt2In] and [Pt2Sn] networks with short Pt-In (Pt-Sn) distances and Pt2 dumb-bells (290 and 297 pm in GdPt2In and GdPt2Sn). The gadolinium atoms have coordination number 14 with 8 Pt and 6 In (Sn) neighbors. Magnetic susceptibility measurements on GdPt2In show Curie-Weiss behavior with an experimental magnetic moment of 8.06(2) μB/Gd atom. GdPt2In orders ferromagnetically at 27.7(2) K