Volume 213, Issue 12

The present status of the methodology of full dynamical surface structure determination by low energy electron diffraction (LEED) is reviewed with respect to both experiment and theory. Restriction is to today widely used experimental and computational techniques including the powerful approach by Tensor LEED on the theoretical side. Special emphasis is on more recent developments to tackle increasingly complex surface structures. So, we describe new structural search procedures which aim to arrive at the global rather than only a local R-factor minimum in parameter space as the best fit between experiment and theory. Also, we illuminate the application of LEED to disordered adsorbates and the related development of holographic image reconstruction from diffuse diffraction patterns. The most recent extension of this direct method to ordered structures is included as well, showing that the resulting structural information is most valuable if not essential for finding the correct atomic model of the surface. Examples are given in each case and a selection of particularly demanding structure determinations is presented as well.

A new double lithium rubidium sulphate, Li 3 Rb(SO 4 ) 2 · H 2 O, was found. Single crystals were obtained by drowning out in a tetramethoxysilane gel medium. The compound crystallises in the space group P 1 with parameters a = 5.1356(4) Å, b = 4.9853(4) Å, and c = 8.2712(9) Å, α = 90.032(7)°, β = 105.729(6)°, γ = 90.004(7)° and Z = 1. The crystal structure was determinated by X-ray diffraction; in addition the new double sulphate was characterized by IR-spectroscopy. The system Rb 2 SO 4 –Li 2 SO 4 –H 2 O at 323 K is given.

The crystal structure of ζ ′-Ag 2 Ga has been determined by single crystal X-ray diffraction at 293 K and 150 K. The structure is hexagonal with the non-centrosymmetric space group P [unk]2 m (no. 189). The cell dimensions are a = 0.77680(16) nm, c = 0.28759(6) nm at 293 K and a = 0.77460(16) nm, c = 0.28704(5) nm at 150 K. ζ ′-Ag 2 Ga has the InMg 2 type of structure with nine atoms (six Ag and three Ga) on four different atomic sites (3 f , 3 g , l b and 2 c ). The Ag atoms fully occupy the 3 f and 3 g positions and the Ga atoms the l b and 2 c positions. The structure parameters (atomic positions and anisotropic thermal parameters) were refined using data collected on a Siemens SMART CCD SYSTEM. The composition of the single crystals was determined by Energy Dispersive X-ray Spectroscopy (EDS) in a Scanning Electron Microscope (SEM).

The crystal structure of chloromenite, Cu 9 O 2 (SeO 3 ) 4 Cl 6 , a = 14.170(3) Å, b = 6.262(1) Å, c = 12.999(3) Å, β = 113.05(1)°, V = 1061.3(4) Å 3 , Z = 2, space group I 2/ m , has been solved by direct methods and refined to an R ( F ) index of 0.051 ( wR ( F 2 ) = 0.128). The structure is based on unbranched zweier single chains { uB , 1 1 ∞ } [O 2 Cu 6 ] of corner sharing [OCu 4 ] tetrahedra running parallel to the b axis. The geometry of structurally similar [ uB , l 1 ∞ }[O 2 M 6 ] (M = Cu, Zn) chains of oxocentered [OM 4 ] tetrahedra (M = Cu, Zn) in inorganic compounds is discussed.

The orthorhombic crystal structure of Cu 4 O 2 [(As,V)O 4 ]Cl, a = 5.440(1) Å, b = 11.154(2) Å, c = 10.333(2) Å, V = 627.0(3) Å 3 , Z = 4, Pbcm , has been solved by direct methods and refined to an R ( F ) index of 0.049 ( wR ( F 2 ) = 0.048). The structure is based on unbranched vierer chains { uB , l 1 ∞ }[O 4 Cu 8 ] running parallel to the c axis, which are composed of oxocentered tetrahedra [OCu 4 ] linked via opposite edges. The chains are bent by the large chloride ions in zigzag fashion so that the periodicity of the chains is equal to four. [(As,V)O 4 ] groups are located between the puckered chains of oxocentered tetrahedra and crosslink them into layers parallel to the (100) plane, which are held together by the Cu–O As/v bonds.

In the binary system MnSO 4 /H 2 SO 4 , two new acid sulfates were synthesized and their structures have been determined: Mn(HSO 4 ) 2 , Mg(HSO 4 ) 2 type, P 2 1 / n , Z = 2, a = 5.014(1) Å, b = 8.426(2) Å, c = 7.452(1) Å, β = 100.05(1)°, R 1 = 0.035, and Mn(HSO 4 ) 2 (H 2 SO 4 ) 2 , Cd(HSO 4 ) 2 (H 2 SO 4 ) 2 type, P 2 1 / c , Z = 4, a = 5.166(1) Å, b = 15.397(3) Å, c = 7.963(1) Å, β = 104.74(2)°, R 1 = 0.039. Mn(HSO 4 ) 2 · H 2 O was isolated from 85% sulfuric acid, Mg(HSO 4 ) 2 · H 2 O type, P 2 1 / n , Z = 4, a = 8.720(2) Å, b = 5.084(1) Å, c = 15.663(3) Å, β = 101.11(3)°, R 1 = 0.026. In all compounds, the manganese atoms are octahedrally coordinated by oxygen atoms originated from either HSP 4 − or H 2 SO 4 tetrahedra. Hydrogen bonding systems consist of layers or chains. In the structure of Mn(HSO 4 ) 2 , the HSO 4 − tetrahedra form infinite zigzag chains along [010]. The structure of Mn(HSO 4 ) 2 · H 2 O is characterized by layers of HSO 4 − tetrahedra and H 2 O molecules connected by hydrogen bonds. Layers consisting of HSO 4 − and H 2 SO 4 tetrahedra are formed in the structure of Mn(HSO 4 ) 2 (H 2 SO 4 ) 2 .

The low-temperature structure of hemimorphite, Zn 4 Si 2 O 7 (OH) 2 · H 2 O, was investigated by neutron diffraction at 20 K using the “Time-Of-Flight” (TOF) method with a four-circle single-crystal diffractometer and a position-sensitive detector at the Intense Pulsed Neutron Source (IPNS) at the Argonne National Laboratory, IL. The reversible second-order phase transition, which had previously been determined at 98(2) K by optical measurements, was confirmed by two TOF neutron histograms acquired at 20 K and 120 K. Whereas the 20 K histogram showed superlattice reflections hkl with k , l = n /2 and intensities up to 30 σ ( I ), these reflections were absent in the 120 K data set. In contrast to the room-temperature structure of hemimorphite ( Imm 2, a ∼ 8.37 Å, b ∼ 10.73 Å, c ∼ 5.12 Å), the additional reflections of the low-temperature structure lead to a supercell with a = 8.354(1) Å, b = 21.519(4) Å, c = 10.240(1) Å. According to the general reflection conditions, the low-temperature superstructure of hemimorphite was refined in space group Abm 2 (= Acm 2 = Aem 2) with Rw ( F 2 ) = 0.058. Whereas the room-temperature structure shows dynamically disordered hydroxyl and H 2 O groups, the low-temperature structure is characterized by an ordered arrangement of non-equivalent hydroxyl groups and rotated H 2 O molecules linked by an enhanced H bond system along the channels of the structure, thus leading to a doubled c lattice parameter. A partially ordered arrangement of the channels across the structure (using the [1/2, 1/4, 1/4] or [1/2, 1/4, −1/4] translation vectors) results in doubling of the b lattice parameter. According to space group symmetry and additional non-space group reflection conditions 0kl with k + l = 4 n and h 00 with h = 2 n , an order/dis-order (OD) structure model with microdomains and twinning is proposed.

The crystal and molecular structures of four diimine adducts formed between R 2 SnCl 2 [R = i Pr, Cy, CH 2 Ph; and R 2 = Me, Ph] and 1,10-phenanthroline have been determined at room temperature. The colorless crystals of R = i Pr are monoclinic, space group P 2 1 / c with unit cell dimensions a = 8.261(2) Å, b = 15.125(3) Å, c = 15.804(2) Å, β = 104.82(1), Z = 4 and D x = 1.586 Mg m −3 . The yellow crystals of R = Cy are monoclinic, space group P 2 1 / n with unit cell dimensions a = 10.474(1) Å, b = 16.167(2) Å, c = 14.341(3) Å, β = 101.46(1)°, Z = 4 and D x = 1.496 Mg m −3 . Orange crystals of R = CH 2 Ph are monoclinic, space group P 2 1 / n with unit cell dimensions a = 10.356(3) Å, b = 17.540(2) Å, c = 13.817(9) Å, β = 105.93(5)°, Z = 4 and D x = 1.519 Mg m −3 . The colorless crystals of R 2 = Me, Ph are monoclinic, space group P 2 1 / n with unit cell dimensions a = 9.118(2) Å, b = 11.023(2) Å, c = 18.712(2) Å, β = 93.03(1), Z = 4 and D x = 1.634 Mg m −3 . The structures were solved by direct methods and each refined by a full-matrix least-squares procedure to final R = 0.045 using 2200 reflections for R = iPr; to R = 0.043 using 2644 reflections for R = Cy; to R = 0.045 using 2576 reflections for R = CH 2 Ph; and to final R = 0.036 for 2538 reflections for R 2 = Me, Ph. The solid state structures of R 2 SnCl 2 (phen) feature distorted octahedral tin atom geometries with a trans disposition of the R substituents. A systematic trend to longer Sn–Cl and Sn–N distances with decreasing Lewis acidity is found, however, with a relatively large spread of values which is often correlated in the case of Sn–Cl with C1…H interactions. The structures of R 2 SnCl 2 (phen) have also been investigated employing ab initio methods at the Hartree-Fock SCF level of theory so that a correlation may be made between the R 2 SnCl 2 (phen) structures in the presence and absence of crystal packing effects. The calculated structures are generally more symmetric suggesting that there is no inherent chemical reason for a lack of symmetry in the solid state. Further, generally there is an elongation of Sn-ligand distances in the gas phase structures. These two key observations point to the importance of crystal packing effects on molecular geometry as well on the derived interatomic parameters.