Band 137, Heft 5-6

The original samples of the magnetite series Fe 3 O 4 – γ Fe 2 O 3 of G.H ägg have been investigated with x-ray diffraction and Mössbauer spectroscopy. X-ray diffraction studies showed that the intensities of the superstructure lines increase with increasing deviation from the stoichiometric ratio. The patterns of all samples exhibited a cubic-primitive lattice. Tetragonal symmetry was not observed. Mössbauer absorption spectra gave evidence for vacancy distributions over tetrahedral as well as octahedral iron sites of magnetite. Broadening of the octahedral iron lines indicated that the relaxation times of the hopping electrons at constant temperature increases as the deviation from the stoichiometric ratio increases.

Most of the SiC polytypes known uptil now are found to be based on either of the basic structure 6 H , 15 R or 4 H . A new polytype 147 R with structure [(3332) 4 32] 3 has been discovered which is based on 33 R instead of either of the basic structures. In addition a small region of the same crystal is found to be a high-period polytype (588 R or 196 H ) and is based on 147 R . The implication of these observations regarding the growth mechanism of polytypic crystals has been discussed.

The refinement of the crystal structure of a lunar pigeonite (Mg 0.69 Fe 0.23 Ca 0.08 )SiO 3 from the lunar rocks of the Apollo-14 mission is reported. The most remarkable discrepancies from other pigeonites concern the z coordinates which are strongly influenced by (001) augite exsolutions in pigeonites.

The crystal structure of 1M-BaLiMg 2 AlSi 3 O 10 F 2 has been determined from single-crystal diffractometer data and refined to an R value of 7.1% by least-squares methods. The synthetic barium mica belongs to the monoclinic space group C 2/ m with a = 5.2858(2), b = 9.1575(6), and c = 10.0375(5) Å, β = 100.124(4)° and Z = 2. There is no Li,Mg ordering in the octahedral sites or Al,Si ordering in the tetrahedral sites. The average bond lengths are (Al,Si)–O = 1.645, (Mg,Li)–O(F) = 2.066, inner Ba–O = 2.975, outer Ba–O = 3.192 Å. Compared to other micas, the large divalent barium ion reduces the interlayer separation and restricts rotation within the tetrahedral layer.

The crystal structure of barium azide monohydrate Ba(N 3 ) 2 · H 2 O has been determined from three-dimensional photographic x-ray data and was refined by the method of least-squares to a final R = 10.3%. The unit cell is monoclinic, a = 7.29 Å, b = 10.84 Å, c = 6.96 Å, β = 104°42′, space group Cc – C s 4 with four molecules per cell. Each barium atom is surrounded by seven end nitrogen atoms and two H 2 O, in the form of a distorted triangular prism with three more atoms above the centers of each of the three faces of the prism. The azide groups are linear and symmetric with an average N–N distance of 1.173 Å.

A monoclinic metastable phase of anorthite composition, CaAl 2 Si 2 O 8 , has been synthesized as an artificial product. Crystals are twinned mostly on a submicroscopic scale. In the cell whose dimensions are a = 8.228(1), b = 8.621(1), c = 4.827(1) Å, β = 90.00(5)°, there are two formula units. The space group is P 2 1 . The structure has been worked out with the minimum-function method based upon intensities measured on an automated diffractometer. Final atomic coordinates give an R of 0.06 with isotropic thermal parameters. The structure is a framework in which there can be recognized rings of four tetrahedra like the ones of felspar. The calcium atoms are statistically distributed in two point positions with the occupancies of 0.403(4) and 0.564(5) respectively. The structure has a pronounced pseudosymmetry of Pmnn . The deviation from this higher symmetry is primarily explained by nearly complete order in the Si and Al sites in the framework. A comparison of this structure with that of anorthite has revealed that the mean T–O–T angles at two- and three-coordinated oxygen atoms are smaller than the corresponding angles of anorthite. A mechanism of twinning is suggested in terms of mistakes in the Si and Al placements which may occur in the process of crystal growth. There is no evidence that the synthetic “orthorhombic CaAl 2 Si 2 O 8 ”, described by D avis and T uttle (1952), is distinct from the present monoclinic phase.

A minimum residual method (the method of key shifts) is proposed for the structure determination of superstructures. The method is based on the fact that there are in most cases only a few important changes (key shifts) of atomic parameters in the extension from a substructure to the corresponding superstructure. The method starts from the substructure. We change each atomic parameter of each atom by particular amounts in the plus and minus directions, and calculate the corresponding R values. The key shifts can then be detected from the steepest descents of the R values. Once an approximate superstructure has been found out by this method, i.e. approximately correct phases have been assigned to the reflections, the structure can be refined by ordinary Fourier and least-squares methods. The method has successfully been applied to the structure determination of freieslebenite, PbAgSbS 3 and jordanite, Pb 28− x As 12 S 46− x .

Crystals of KIO 3 show the space-group symmetry P 1 at room temperature. The lattice constants being: a = 8.923 Å, b = 8.942 Å, c = 7.709 Å, α = 54.4° β = 125.3°, γ = 90.6°. In this structure, which belongs to the perowskite typ, two types of IO 6 -octahedral chains could be distinguished, the straight ones alternating with those which are somewhat twisted. This arrangement gives use to a pseudo-monoclinic symmetry, which becomes true if the crystals are heated to 72.5°C, the space group being Cm . At 212°C they transform to a rhombohedral modification possessing the space group R [unk].

The crystal structure of CuBi 5 S 8 has been determined using three-dimensional intensity data. The unit cell, with dimensions a = 13.214 ± 0.001, b = 4.025 ± 0.003, c = 14.087 ± 0.001 Å and β = 115.6° ± 0.03° contains two formula units. The space group is C 2 h 3 – C 2/ m . The structure was derived from the Patterson, Fourier and difference Fourier diagrams. The final discrepancy index R is 8.5%. The coordinations around the three independent Bi atoms are octahedral, trigonal pyramidal and square pyramidal. Two Cu atoms in the unit cell are distributed statistically in the fourfold position h (0, y ,1/2) and their coordination polyhedra are distorted tetrahedra.