Volume 204, Issue 1

This article gives a complete account of the research in synchrotron radiation powder diffractometry (SRPD) conducted at DESY/Hasylab between 1981 and 1992 and incorporates an extensive bibliography on SRPD in general. A detailed description of the experimental station for powder diffraction established in fall 1988 at beamline B2 of HASYLAB is given and selected examples of research conducted to date are reviewed to demonstrate its capabilities and potential. Aspects of instrument availability and beamtime allocation are entertained in the concluding remarks. References have been selected such that maximum coverage of the field is achieved with a minimum of citations and the list is hopefully complete with respect to the retrievable literature pertinent to the development of synchrotron powder diffractometry at Hasylab. Papers covering methodical aspects of data analysis and more general aspects of SRPD, however, have not been included.

The crystal structure of MnGeO 3 ilmenite was refined using X-ray intensity data collected at 1 atm, 2.03 GPa, 3.25 GPa, 3.89 GPa and 4.99 GPa, all at room temperature. Compression of the unit cell is anisotropic, with c approximately twice as compressible as a , and c/a decreasing nearly linearly with increasing pressure. The isothermal bulk modulus determined from the cell volume data is 175(3) GPa if K 0 ′ is assumed to be 4.0. The mean Mn–O distance is more than twice as compressible as the mean Ge–O distance. Within the Mn octahedral site, the longer of the two independent Mn–O bonds decreases by 1.8% in the pressure range studied whereas the shorter bond decreases by 0.8%. Similarly, the longer of the two independent Ge–O bonds is more compressible than the shorter Ge–O bond in the Ge octahedral site. Distortions of the Mn and Ge octahedra do not change significantly with pressure, and the cation sites remain fully ordered at all pressures studied.

Well shaped crystals of Cr 2 (SO 4 ) 3 and Al 2 (SO 4 ) 3 could be prepared by chemical transport reactions using HCl (Cr 2 (SO 4 ) 3 ) or SOCl 2 (Al 2 (SO 4 ) 3 ) as transport agent. Cr 2 (SO 4 ) 3 and Al 2 (SO 4 ) 3 are isostructural to the rhombohedral modification of Fe 2 (SO 4 ) 3 and crystallize in the trigonal space group R [unk] with a = 8.1245(7) (8.0246(4)) Å and c = 21.944(3) (21.357(1)) Å, Z = 6 (hexagonal setting). The crystal structures were refined to a residual R = 0.035 using 828 unique reflections with | F |≥3 σ| F | for Cr 2 (SO 4 ) 3 and to R = 0.026 (772 unique reflections with | F |≥3 σ| F |) for Al 2 (SO 4 ) 3 . Main building units of the two compounds are MO 6 -octahedra and SO 4 − -tetrahedra which are linked via common corners to a 3-dimensional network.

Elastic and thermoelastic constants of beryl (Be 3 Al 2 Si 6 O 18 ), topaz (Al 2 F 2 SiO 4 ), diaspore (AlOOH), sanidine (K 0.89 Na 0.11 AlSi 3 O 8 ) and periclase (MgO) have been determined from ultrasonic resonance frequencies of thick plates. The mean elastic stiffnesses of diaspore and corundum and also of sanidine and α-quartz are closely related. Correlations between thermoelastic properties and thermal expansion are discussed. Crystals with a close-packed structure of oxygen ions exhibit much larger absolute values for the temperature derivative of the mean elastic stiffness than crystals with smaller packing density such as certain tecto- and ino-silicates.

The title compounds have closely related structures and intergrown crystals are very common. Both compounds have the same orthorhombic space group Cmc 2 1 and both have – within the error limits – two translation periods in common: a = a ′ = 376.1(1) pm, b = b ′ = 1376.0(2) pm, c = 1319.0(1) pm, c ′ = 1212.3(2) pm, Z = 8, Z ′ = 4 for Mn 3 As 2 (II) and Mn 5 As 4 , respectively. Both structures were determined from one intergrown “single-crystal”. The conventional residuals are R = 0.044(669 structure factors, 61 variable parameters) and R ′ = 0.048 (553 F's, 55 variables). The difference between the two compounds arises through the absence of one Mn position in Mn 5 As 4 . The structures are intermediate between those of NiAs and Ni 2 In. They are also closely related to the earlier determined structure of Mn 3 As 2 (I).

The crystal structure of BiTeI has been redetermined with 1924 independent reflections, using three-dimensional intensities measured on a computer controlled Philips PW 1100 single crystal diffractometer. The structure belongs to the rhombohedral space group P 3 and the cell constants, obtained by least-squares calculation from direct θ -value measurements on the diffractometer, are a = 4.336 and c = 6.84 Å and do not considerably deviate from those mentioned in the literature. The positional and thermal parameters, with anisotropic temperature factors, were refined by full-matrix least-squares calculations to a final R = 0.0745. Each Bi atom is six-coordinated by three Te and three I atoms at distances 3.88(5) Å and 3.05(2) Å respectively thus forming a distorted octahedron. The faces of the octahedron are two equilateral triangles (Te-Te-Te, I-I-I) with sides 4.336 Å and six isosceles triangles (three I-Te-I and three Te-I-Te) with sides I–Te 3.29(3) Å and I–I or Te–Te 4.336 Å. The octahedra are linked to each other by a common Te–I edge along the a and b axes in infinite layers.

The crystal structures of the title compounds R 3 PAu(S 2 CO-c-C 6 H 11 ), R = Et, Ph and c-hexyl, have been determined at room temperature. Monoclinic crystals of the R = Et compound have space group C 2/ c with unit cell dimensions a = 26.754(4) Å, b = 7.264(1) Å, c = 18.928(3) Å, β = 102.04(1)°, Z = 8 and D x = 1.811 Mg m −3 . Crystals of the R = Ph structure are triclinic with space group P [unk] and unit cell dimensions a = 11.372(2) Å, b = 13.053(6) Å, c = 9.488(3) Å; α = 109.45(3)°, β = 99.19(2)°, γ = 106.28(3)°, Z = 2 and D x = 1.722 Mg m −3 . The R = c-hexyl compound crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with unit cell dimensions a = 8.121(1) Å, b = 11.108(4) Å, c = 30.212(5) Å, Z = 4 and D x = 1.591 Mg m −3 . The structures were refined by a full-matrix least-squares procedure on 1908 reflections to final R = 0.039 for the R = Et compound, 3664 reflections and R = 0.055 for R = Ph and 1501 reflections and R = 0.040 for R = c-hexyl. The Au atom in each of the structures is linearly coordinated by the S atom, derived from a monodentate xanthate ligand, and the P atom. Different orientations of the xanthate ligands results in two structural forms. For the R = Et and Ph compounds the O atom is in close proximity of the Au atom whereas for the R = c-hexyl compound the S atom is orientated towards the Au atom. The different structures are rationalized in terms of the steric demands of the phosphine ligands. A close Au … Au interaction of 3.120(1) Å is found in the R = Et compound; no such contacts are observed in the R = Ph and c-hexyl structures.

Bond-length-bond-strength relationships which are valid for different oxidation states of the elements have been established for oxides of As, Se, Br, Pb and Bi. According to new structural data, such relationships of single oxidation states in oxides of Ga(III), Ge(IV), In(III), Tl(I) and Tl(III) have been recalculated as well as those for oxides of Sn, Sb, Te, and I.

The elastic constants of triclinic TlH 3 (C 2 O 4 ) 2 · 2 H 2 O and their temperature derivatives have been determined from ultrasonic resonance frequencies of thick plane-parallel plates. The elastic constants differ only slightly from those of the isotypic potassium salt. However, a distinct difference in respect to the deviation from Cauchy's relations occurs similar to the situation in many other isotypic potassium and thallium salts. The thermoelastic constants show an almost isotropic behaviour in contrast to the very large elastic anisotropy.

The 4 phases of HMX and its transitions were investigated using X-ray diffraction. The phases were heated stepwise in a temperature range from 170 K to 510 K, measuring a diffraction pattern after each step. The thermal expansion of the different phases and the volume changes during the phase transitions were obtained. Anomalies and strongly anisotropic expansions were observed with β - and γ -HMX. The highest thermal expansion was found with γ -HMX, followed by δ -, β - and α -HMX. The highest volume change was observed with the β → δ transition, followed by the α → δ and the γ → δ transition.

The structure of Li 2 Mg 2 [Si 4 O 11 ] was determined by single crystal X-ray diffraction. It is triclinic, space group P [unk], with a = 8.645(1) Å, b = 7.401(1) Å, c = 6.884(1) Å, α = 104.71(1)°, β = 101.08(1)°, γ = 99.41(1)°, V = 407.5(6) Å 3 and Z = 2. Least -squares refinements based on 1462 observed reflections with intensities I ≥ 3 σ ( I ) resulted in R = 0.054 and R w = 0.036. The main structural elements are chains of corner-linked [SiO 4 ] tetrahedra and ribbons consisting of edge-linked [MgO 6 ] octahedra and [LiO n ] polyhedra. The [SiO 4 ] tetrahedral chain can be described as a loop-branched dreier single chain. This structure type has not been reported for other silicates. Tetrahedral chains and octahedral ribbons are arranged in layers of tetrahedra and octahedra, respectively, which are alternately stacked parallel to (110) and are linked by shared oxygen atoms. This structural arrangement, known from the structures of pyroxenes, pyroxenoids and amphiboles, is compared in detail with that of the closely related pyroxenoid-like dreier single chain silicates. The topology of the tetrahedral-octahedral linkage which occurs at the apical oxygen atoms of the chains is similar to that in serandite, while the linkage with the basal oxygen atoms of the chains is similar to that in wollastonite. Thus, Li 2 Mg 2 [Si 4 O 11 ] represents a new structural member of the family of the pyroxenoid-like single chain silicates.

Te(OH) 6 · K 8 P 8 O 24 · 2 H 2 O is the first example of an adduct between telluric acid and a cyclooctaphosphate. We report its chemical preparation and its atomic arrangement. The title compound is triclinic P [unk], Z = 1 with the following unit cell dimensions: a = 11.315(9), b = 10.67(1), c = 7.547(3) Å α = 108.72(5), β = 100.30(2), γ = 66.80(5)° Crystal structure was determined and refined to final R = 0.028 for 7265 independent reflections. The atomic arrangement is mainly characterized by the coexistence of independent units of [P 8 O 24 ] 8− ring anions and of Te(OH) 6 groups, both centrosymmetric. The hydrogen bond scheme is described. In addition, the internal symmetries and the geometries of the rare [P 8 O 24 ] 8− ring anions presently investigated are discussed.