Band 198, Heft 3-4

Lattice parameters and crystal structures of the low-temperature quartz-type (‘low-quartz’) forms (space group P 3 1 21) of SiO 2 and GeO 2 were refined from single-crystal X-ray diffraction data under hydrostatic pressures up to 10.2 GPa for SiO 2 and 5.57 GPa for GeO 2 . R W ( F ) values range from 2 to 5%. Hexagonal unit-cell parameters for SiO 2 : a = 4.921(1), c = 5.4163(8) Å at ambient conditions; a = 4.604(1), c = 5.207(1) Å at 10.2(1) GPa. For GeO 2 : a = 4.9844(2), c = 5.6477(2) Å at normal pressure; a = 4.750(1), c = 5.548(5) Å at 5.57 GPa. Volume decrease, 16% for SiO 2 and 11% for GeO 2 is accomplished mainly by tetrahedral tilting, the rest arising from tetrahedral angle distortion. GeO 2 -quartz is an almost perfect high-pressure model for SiO 2 -quartz: At 10 GPa the geometry of the SiO 2 -quartz structure approaches that of GeO 2 at ambient pressure (e.g. similar values for c / a , atomic parameters, tetrahedral tilt angle, tetrahedral distortion). These values then further change for GeO 2 with increasing pressure reflecting increasing structural distortion. The Si – O – Si angle decreases with pressure from 144.2(2) to 130.3(1)°, the Ge – O – Ge angle from 130.0(1) to 123.4(3)°. The Si … Si distance between vertex-connected tetrahedra shrinks from 3.0627(4) to 2.9152(8) Å, the respective Ge … Ge distance from 3.1515(3) to 3.054(1) Å. Both distances, at maximum pressures, fall slightly below smallest reported values for silicates and germanates at ambient conditions. The variations in the angle T – O – T and the nonbonded distance T … T are nearly independent of changes in the T – O bond lengths. The shortest O … O distance between unconnected tetrahedra decreases from 3.345(2) to 2.793(2) Å in SiO 2 , and from 3.023(3) to 2.809(9) Å in GeO 2 . For quartz this distance remains longer than the longest tetrahedral edge, which increases from 2.640(3) to 2.690(2) Å. For GeO 2 however, it shrinks to the second-shortest oxygen – oxygen distance in the structure; only two symmetry equivalent tetrahedral edges are shorter, 2.725(4) Å, at the maximum pressure. The two symmetry equivalent second-shortest intertetrahedral O … O distances in GeO 2 decrease from 3.192(2) to 2.926(6) Å, becoming shorter than the largest tetrahedral edge, which increases from 2.903(2) to 2.943(5) Å. Extrapolating these developments, the oxygen atoms would reach positions at the lattice points of a cubic body-centred lattice (Sowa, 1988). Increasing tetrahedral distortion with pressure is displayed mainly by angle distortion. The quantity, DI(OTO) (Baur, 1974), increases from 0.6 to 2.6% for SiO 2 and from 2.5 to 4.2% for GeO 2 . The longer Si – O bond remains constant at 1.614(2) Å, whereas the shorter one decreases from 1.605(2) to 1.599(2) Å. These values reflect the stiffness of the respective bonds. The changes in the Ge – O bond lengths are correspondingly small but are less regular with increasing pressure. The variations of bond angles O – T – O and respective tetrahedral edges are nearly independent of changes in the T – O bond lengths. Tetrahedral distortion mechanisms are seen to differ in comparing changing pressure or temperature.

Two new compounds belonging to the family of bis(dithiooxalato)nickelate(II) complex anion with the general formula [(C 6 H 5 ) 4 X] 2- [Ni(S 2 C 2 O 2 ) 2 ], where X = P (NIDTP) and As (NIDTAS), have been synthetized. Crystal structure analyses of these compounds show that both crystallize in the triclinic system, space group is P [unk] with Z = 2. Crystal data for NIDTP are: a = 11.050(5), b = 13.263(7), c = 18.385(8) Å, α = 111.97(5), β = 90.49(5), γ = 106.10(4)°, V = 2382(2) Å 3 , F (000) = 1012, D x = 1.364, D 0 = 1.37(1) Mg·m −3 , R = 0.040 and R w = 0.046 for 6732 observed reflections; for NIDTAS: a = 10.446(5), b = 17.525(8), c = 14.07(4) Å, α = 91.20(9), β = 100.8(1), γ = 88.28(4)°, V = 2528(7) Å 3 , F (000) = 1084, D x = 1.400, D 0 = 1.40(1) Mg·m −3 , R = 0.112 and R w = 0.124 for 2174 observed reflections. While the asymmetrical unit of the compound NIDTP contains one discrete complex [Ni(S 2 C 2 O 2 ) 2 ] 2− anion, in general position, and two [(C 6 H 5 ) 4 P] + cations, the asymmetrical unit of NIDTAS consists of two centrosymmetric [Ni(S 2 C 2 O 2 ) 2 ] 2− anions and two [(C 6 H 5 ) 4 As] + cations. In both cases the organic cations and anions are linked by electrostatic interactions and hydrogen contacts of type C – H … O.

C 24 H 28 O is tetragonal, space group P 4 1 with a = b = 11.639(1) Å, c = 14.244(2) Å, V = 1929.5(1) Å 3 , Z = 4, D c = 1.144 Mg m −3 , F (000) = 720, M r = 332.49, μ = 0.524 mm −1 , λ (Cu Kα ) = 1.5418 Å, T = 293 K. The final R is 0.082 ( wR = 0.073) for 1106 observed reflections. The results of the structure determination agree with the circular dichroism analysis through the octant rule. Besides, circular dichroism and configuration of 3,3-dibenzyl bornane-2-one and of others 3,3-dialkyl bornane-2-one derivatives are also discussed.

The observed stacking disorder in scholzite can be explained by a stacking ambiguity of layers parallel to (100). This is due to equivalent surroundings of the calcium atoms on the boundary planes of two neighbouring layers in two possible relative positions to each other. Consequently, a series of possible polytypes of this compound was derived, among which the structure of parascholzite is a likely candidate, from which the space group and the lattice parameters could be found in agreement with these predictions. It was demonstrated that the OD structure does not consist of layers of one kind but of two kinds.

The crystal and molecular structure of ammonium citrate dihydrate [(NH 4 ) + Bi(C 6 H 4 O 7 ) − ] · 2H 2 O has been established by means of X-ray diffraction on a single crystal using Cu K α -radiation. The crystals are monoclinic, space group C 2/ c with a = 1680.5(2), b = 1254.4(3), c = 1040.1(2) pm, β = 91.27(2)°, Z = 8. The structure was solved by the Patterson method, and was refined by least-squares techniques yielding final R ( R W ) values of 0.042 (0.034) based upon 1855 reflections. The citrate acts as a chelating and as a bridging ligand, thus forming a three-dimensional network with carboxy groups coordinating to bismuth in a mono- as well as a bidentate manner. The essential structural characteristics is the anion [(Bi – O – C 6 H 4 O 6 ) − ] that strikingly contains a four -basic citrate the hydroxy oxygen atom of which is covalently bonded to bismuth. Two water molecules and the ammonium ion are located in channels parallel to the z -axis.

A reinvestigation of the crystal structure of diglycine hydrochloride, (NH 2 CH 2 COOH) 2 HCl, was carried out. The structure is orthorhombic, space group P 2 1 2 1 2 1 , with a = 5.2992(6), b = 8.083(2), c = 17.973(2) Å and Z = 4. Three-dimensional intensity data collected on a CAD 4 diffractometer using Cu Kα radiation was used for the structure elucidation and refinement. The final R value is 0.055 for 779 reflections. All the hydrogen atoms have been located in the present study. The glycine molecules are held together by a network of N – H … Cl, N – H … O and O – H … O hydrogen bonds. One of the glycine molecules exists as a zwitterion whereas the other one in the cationic form.

Rietveld refinements have been used to determine the cation distributions in three (Mg,Co) orthovanadates (X-ray data) and in Co 1.5 Ni 1.5 (VO 4 ) 2 (neutron powder diffraction data). The compounds crystallize in the orthorhombic space group Cmca ( Z = 4), with two distinct octahedral metal sites having point symmetries 2/ m and 2 . The refinements show that in all solid solutions the cation distributions are almost random, with a slight preference of Co 2+ for the more symmetric site.