[en] The X-ray crystal structures of a series of new compounds (H₃O)₂[(Mn(H₂O)_1.5)₃(Re₆Se₈(CN)₆)₂]·19H₂O (1), (Me₄N)₂[(Co(H₂O)_1.5)₃(Re₆S₈(CN)₆)₂]·13H₂O (2), (Me₄N)₂[(Co(H₂O)_1.5)₃(Re₆Se₈(CN)₆)₂]·3H₂O (3), (Et₄N)₂[(Mn(H₂O)₂)₃(Re₆Se₈(CN)₆)₂]·6.5H₂O (4), (Et₄N)₂[(Ni(H₂O)₂)₃(Re₆S₈(CN)₆)₂]·6.5H₂O (5), and (Et₄N)₂[(Co(H₂O)₂)₃(Re₆S₈(CN)₆)₂]·10H₂O (6) are reported. All six compounds are isostructural crystallizing in cubic Im3-barm space group with four formulae per unit cell. For compounds 1, 3-5 the following parameters were found: (1) a=19.857(2) A, R1=0.0283; (3 at 150 K) a=19.634(1) A, R1=0.0572; (4) a=20.060(2) A, R1=0.0288; (5) a=19.697(2) A, R1=0.0224. The structures consist three-dimensional cyano-bridged framework formed by cyano cluster anions [Re₆Q₈(CN)₆]^4-, Q=S, Se and transition metal cations, M²⁺=Mn²⁺, Co²⁺, Ni²⁺. Water molecules and large organic cations Me₄N⁺ and Et₄N⁺ are included in cavities of this framework. Porosity of the framework, its ability to accommodate different cations and water molecules by little changes in the structure, as well as distortion of coordination framework under loss of water of crystallization is discussed

[en] The wavelength dependence of the neutron absorption cross section of hydrogen in [ReH₃(C₂H₄)₂-{P(i-Pr)₂Ph}₂] was investigated by recording the decrease in Bragg peak intensity from a standard NaCl crystal caused by attenuation of the incident beam by the Re sample. Data were collected on the Single Crystal Diffractometer (SCD) at the Intense Pulsed Neutron Source (IPNS, Argonne National Laboratory) which enabled concurrent measurements at a selection of wavelengths using the time-of-flight technique. (orig.)

[en] The structure of Tl₂SeO₄ at 293, 100 and 30 K has been determined. Space group at 293 and 100 K is Pmcn, Z=4, lattice parameters at 293 K are a=6.0838(12)(100 K: 6.0336(19)), b=10.965(3) (100 K: 10.903(5)) and c=7.9385(15) A(100 K: 7.921(2)). At 30 K space group is P2₁2₁2₁, Z=4, with lattice parameters a=6.2333(17), b=10.533(3) and c=7.828(2). Measurement was carried out with a Stoe IPDS (293 K) and a Nonius CCD using an Oxford He-cryostat for cooling (100 and 30 K). Final agreement factors at 293 K after refinement with the program SHELXL97 are R(F)=0.061 (100 K: 0.097), wR(F**2)=0.051 (100 K: 0.156) for 566(100 K: 678) unique reflections and R(F)=0.039, wR(F**2)=0.097 for 1162 unique reflections at 30 K. The high-temperature phase is isotypical to the β-K₂SO₄ structure, characterized by isolated selenate tetrahedra and two different cation sites with coordination number 9 and 11. In the low-temperature phase, the tetrahedra are rotated and the coordination numbers of the Tl⁺ ions are reduced to 8 and 10, respectively. Structural changes are similar to the ones observed in high- and low-temperature phases of Tl₂MoO₄. A comparison to other Tl₂TX₄ compounds shows that the structural instabilities are closely connected to shortest Tl-O distances

[en] The first asymmetric bis(silyl) niobocene hydrides Cp₂Nb(SiHMe₂)H(SiXMe2) (2: X = F (a), Cl (b), Br (c), I (d)), having interligand interactions Nb-Hu Si-X, were prepared by selective halogenation of the Si-H bond in Cp₂Nb(SiHMe₂)₂H. X-ray studies of 2b,c and a combined X-ray/neutron diffraction study of 2b establish a slightly stronger interaction of the niobium-bound hydride with the halosilyl ligand

[en] This study examines the relationship of structure precision, as expressed by the e.s.d.'s of atomic coordinates, to the R factor and chemical constitution of a given crystal structure. On the basis of the work of Cruickshank [Acta Cryst. (1960), 13, 744-777], it is shown that anti σ(C-C), the mean e.s.d. of a C-C bond length in a structure, or anti σ(C), the mean isotropic e.s.d. of a C atom, can be estimated by expressions of the form anti σ = kRN^1/2_c. Here, N_c is taken as ΣZ²_i/Z²_C, with the atomic numbers Z_i summed over all atoms in the asymmetric unit and Z_C = 6. It is also shown that anti σ(E), the mean isotropic e.s.d. of a non-C atom, can be estimated by anti σ(E) kRN^1/2_c/Z_E. Values of k were determined by regression analyses based on subsets of 25 984 and 20 334 entries in the Cambridge Structural Database (CSD) that contain atomic coordinate e.s.d.'s. 95% of coordinate e.s.d.'s for C atoms can be estimated to within 0.005 A of their published value and 78% to within 0.0025 A. These predicted anti σ values provide useful estimates of precision for those 39 000 structures for which coordinate e.s.d.'s are not available in the CSD. Details of the diffraction experiment, which might provide an improved estimating function in Cruickshank's (1960) treatment, are not available in any CSD entries. However, values of N_r (the number of reflections) and N_p (the number of parameters) used in refinement were added manually for 817 entries, and the variation of anti σ(C-C) with decreasing N_r/N_p ratios is examined: there is a rapid increase in anti σ(C-C) as N_r/N_p decreases below circa 6.0. A method for approximating s, the r.m.s. reciprocal radius for the reflections observed, is presented, but it is found that a function of the form anti σ(C-C) = kRN^1/2_c/ anti s(N_r - N_p)^1/2 [directly analogous to Cruickshank's (1960) equation] had only slightly improved predictive ability for this data set by comparison with functions based upon R and N^1/2_c alone. Possible reasons for this apparent anomaly are discussed. (orig.)

[en] A new organic salt, bis[3,4;3',4'-bis(ethylenedithio)-2,2',5,5'-tetrathiafulvalenium] dicyanoargentate(I) monohydrate [(BEDT-TTF)₂Ag(CN)₂.H₂O] (I), has been synthesized and its crystal structure and physical properties determined. The structure of (I) is different from those of the superconducting (β), metallic (β'') and insulating (β' and α') phases of anhydrous (BEDT-TTF)₂X, where X is a triatomic monoanion, but is similar to the newly found superconducting salts κ-(BEDT-TTF)₂I₃ and (BEDT-TTF)₂Cu(NCS)₂. Crystal data for (I): [C₁₀H₈S₈]₂[Ag(CN)₂].H₂O, M_r=947.2, monoclinic, P2₁, a=12.593(3), b=8.642(1), c=16.080(3) A, β=109.33(2)deg, U=1651.2(5) A³, Z=2, D_x=1.905 g cm^-3, Mo Kα X-radiation (graphite monochromator), λ=0.71069 A, μ=16.03 cm^-1, F(000)=950, T=298 K, R(wR)=0.033 (0.034) for 3884 unique observed [F>2σ(F)] data. The anion array consists of linear chains of three-coordinate Ag atoms linked by H₂O to form layers parallel to the BEDT-TTF layers, with which they form alternating stacks parallel to c. (I) is metallic over the temperature range 150-300 K. The EPR spectrum at 295 K consists of a single line at g=2.007(3), with a peak-to-peak width H_pp of 70 G (0.007 T). Between 50 and 300 K an activated spin susceptibility with E_a=0.01 eV is observed. (orig.)

[en] Complete text of publication follows. Three different stoichiometries of 4,4^,-bipyridine and pyromellitic acid were crystallised. The 1:1, 1:2 and 2:1 phases were examined with X-rays at room temperature, the positions of the acidic hydrogens were undetermined. The 1:1 and 1:2 phases were examined on the I.L.L diffractometer D19. The 1:1 phase at 215 K due to a phase transition at 200 K. The structure refined in P-1 to RI = 4.33%. The 2:1 phase caused problems for the X-rays. The space group was uncertain, either Cc or C2/c; Cc refinement was unstable, C2/c refined to only 12%. In contrast the neutron data, at 20 K, completed the structure and resolved the pseudo symmetry, refining to R1 = 3.38% in Cc. This is in agreement with a later 20 K X-ray structure recently recorded in Durham. The data set for the 2:1 phase has been collected during thermal tests on the neutron Laue diffractometer LADI at the I.L.L. This data set will yield interesting results and demonstrate the potential of neutron Laue diffractometry for the study of small molecules. (author)

[en] A new conducting organic charge-transfer salt, β-(BEDT-TTF)₂AuBr₂ has been synthesised and its crystal structure determined. Single crystal four-probe conductivity has been measured at ambient and applied pressures from 300-1.4K and EPR spectroscopy from 300-4.2K. β-(BEDT-TTF)₂AuBr₂ crystallises in P1 (Z=1) with a 9.020(1), b = 5.707(9), c 16.320(1)A; α 97.60(1), β 92.12(1), γ 102.89(1)⁰; V = 811.8(2)A³. While the electronic properties are two-dimensional, the molecular packing of the BEDT-TTF molecules is slightly different from that found in other β-phase BEDT-TTF salts with triatomic counterions. β-(BEDT-TTF)₂AuBr₂ is metallic to 1.4K at pressures up to 5.6 kbar although at all pressures studied there is appreciable hysteresis in the cooling and warming cycles of the conductivity between 6 and 14K. Abrupt transitions in the temperature dependence of both the peak-to-peak EPR linewidth and integrated signal intensity occur near 20K. The possibility of a low temperature structural modification is discussed. (author)

[en] We report magnetic and high-pressure conductivity measurements for two β-phase salts of BEDT-TTF. β-(BEDT-TTF)₂ AuI₂ shows a high, temperature independent Pauli susceptibility of 3.4(3)x10^-4 emu/mole. The superconducting transition (≅ 3.1 K from resistivity measurements at 1 bar) is depressed at -1.0 K/kbar. The β-phase AuBr₂^- salt shows some structural differences in comparison to other β-phase salts. Its resistivity remains metallic down to 1.4 K, and we find resistive anomalies at ≅ 110 K and ≅ 5-13 K in the pressure range 1 bar to 5.6 kbar. (orig.)

[en] The application of neutrons to the study of structure/property relationships in organic non-linear optical materials (NLOs) is described. In particular, charge-transfer effects and intermolecular interactions are investigated. Charge-transfer effects are studied by charge-density analysis and an example of one such investigation is given. The study of intermolecular interactions concentrates on the effects of hydrogen-bonding and an example is given of two structurally similar molecules with very disparate NLO properties, as a result of different types of hydrogen-bonding. (author)