Band 157, Heft 3-4

The crystal structure of the alums, (NH 3 CH 3 )Al(SO 4 ) 2 · 12H 2 O and (NH 4 )Al(SO 4 ) 2 · 12H 2 O, has been determined and refined from X-ray and neutron diffraction data. The compounds crystallize cubic in space group Pa 3 ( Z = 4) with a 0 = 12.322(3) Å and a 0 = 12.242(1) Å respectively. The positional and thermal parameters of all atoms including hydrogens have been refined by full matrix least-squares analysis resulting in R w values from the X-ray and neutron data of 0.030 and 0.029 respectively for the methylammonium alum and of 0.024 and 0.014 respectively for the ammonium alum. The atoms of the cation groups (NH 3 CH 3 ) + and (NH 4 ) + are distributed on 8( c ) and 24( d ) positions in two orientations of equal probability on and around the [111] direction related to each other by an inversion centre. The disorder in the cation groups is explained by a quantized rotation. Disorder in the sulfate groups has been determined to 4.2% for the methylammonium alum and to about 17% for the ammonium alum. The disordered sulfate tetrahedra are distorted and in a reversed orientation along the threefold axis. The system of the hydrogen bonds is discussed.

The excitation and emission spectra of NaBr with high thallium content have been examined. The data obtained have revealed that the luminescence behaviour of heavily doped NaBr: Tl phosphor differs markedly from that of the lightly doped one. It has been shown that besides generally accepted isolated substitutional Tl + ions, the charged complex ions of the type (TlBr n ) − also act as luminescence centres.

Distortions of octahedra and octahedral sheets in 1 M micas have been systematically investigated on the basis of recently published structure data. A survey on the variations of individual interatomic distances has revealed that there is a marked difference in the modes of the distortions between two crystallographically independent octahedra M(1)O 4 (OH, F) 2 and M(2)O 4 (OH, F) 2 . There is a general trend that vacancies or cations with small charge are preferentially located at M(1); the octahedra about M(1) tend to be larger compared with those about the other site. Such a mode of cation ordering may be accounted for in terms of the variation of shared edge lengths which change to relieve the mutual repulsion between octahedral cations. Variations in dimensions of the octahedral sheet along directions perpendicular and parallel to the sheet are related to each other, and the rate of change in thickness of the octahedral sheet is largely affected by the variation of the lateral dimensions of the tetrahedral sheet. The relative stability of mica structures has been discussed in connection with the variation of the size and shape of octahedra. A structural mechanism was proposed for thermal decomposition of mica structures.

The crystal and molecular structure of 2-hydroxybenzimidazole, C 7 H 6 N 2 O, has been determined. The crystals are monoclinic, space group P 2 1 , with a = 5.292(2), b = 5.071(3), c = 23.42(1) Å, and β = 94.52(4)°, Z = 4. The structure and configuration were determined from Mo Kα single crystal diffractometer data by direct methods and refined to an R value of 0.052. The dimerization of the molecules is present in this structure.

X-ray investigations on single crystals of synthetic fairchildite, K 2 Ca(CO 3 ) 2 , confirmed the space group P 6 3 / mmc with a = 5.294(1) and c = 13.355(2) Å. The structure was determined from three-dimensional Patterson and Fourier syntheses and refined by least-squares to R w = 0.046. One of the two crystallographically different CO 3 groups exhibits an orientational disorder in the oxygens around the C atom. The two positions Me(1)[7] and Me(2)[8] are assumed to be statistically occupied by K and Ca atoms.

The crystal structure of Phosphate-Tetraamminecobalt(III) Trihydrate, [Co(PO 4 )(NH 3 ) 4 ] · 3 H 2 O, has been determined by means of three-dimensional X-ray data. The crystals are monoclinic, a = 9.730(4) Å, b = 7.301(2) Å, c = 6.880(2) Å, and β = 92.15(3)°, Z = 2, space group P 2 1 , The structure was solved by direct methods, and refined by least squares calculations (795 independent reflections, 729 observed reflections) to a final R -value of 4.4%. The cobalt atom is surrounded by four nitrogen and two oxygen atoms at the corners of a somewhat distorted octahedron, thus the phosphate group is acting as a chelate group with the formation of a four-membered ring. The non-aqueous H-atoms have been located.

The crystal structure of benzoic acid, C 6 H 5 CO 2 H, has been redetermined by X-ray diffraction at room temperature. Extensive neutron-diffraction measurements have also been made: by single-crystal methods at room temperature and 130 K; and, at 130 K and 5 K, by powder-profile analysis on C 6 D 5 CO 2 H. The structure consists of centrosymmetric dimers [formula (1)], in which two molecules are linked by a pair of hydrogen bonds between their carboxyl groups. Better precision attaches to the X-ray results. Full-matrix refinement, on 1011 independent reflexions, converged at R = 3.7%. This refinement was indeed based on a model that was formally ordered, so far as concerns all atoms except the acidic hydrogen. However the structural results implied an averaged molecule, with the C – O distances 1.258, 1.268(2) Å and the C – C – O angles 118.7, 117.8(1)°; and the acidic hydrogen appeared as two “half atoms” on the hydrogen bond, 0.9 Å from each oxygen atom. These findings are most simply interpreted as due to disorder: the two configurations, A and B (of Fig. 1), occur randomly and in nearly equal proportions. Owing to difficulties inherent in the crystal texture of benzoic acid, the neutron results were less satisfactory. Large single crystals were affected by twinning. Though the powder method avoids this difficulty, the structure, further confused by modulation, is rather too complicated for profile refinement. At 5 K however, the structure may be ordered, consisting wholly of dimers in the A -configuration.

A modulated structure of an An 52 (Labrador, Canada) was determined according to an extended theory of satellites. The theory explains the whole extinction rules of satellite reflections of plagioclase feldspars. The superstructure, which is interpreted as the periodic antiphase domains, consists of two centrosymmetric structure elements ϱ 1 and ϱ 2 with M 2 T 8 O 16 (M: Ca, Na and T: Al, Si), c ∼ 7 Å. The structure-factors for main and satellite reflections are expressed as F main ∝ F 1 + F 2 and F sat ∝ F 1 − F 2 , respectively, where F 1 and F 2 are the Fourier transforms of ϱ 1 and ϱ 2 . Based on the model, ϱ 1 and ϱ 2 were refined. The R -factor attained 8% for 4910 reflections ( R a = 6.6% for 1640 a reflections and R e = 18.2% for 3270 e reflections), including unobserved reflections. Al/Si-distributions in the region of An 100 –An 50 are discussed using the results of the refinement.

The crystal structure of synthetic LiBO 2 · H 2 O 2 , Li 2 [B 2 (O 2 ) 2 (OH) 4 ], was determined from single-crystal X-ray diffractometer data. The compound is monoclinic, space group P 2 1 / c ; the lattice constants are a = 704.0(5) pm, b = 788.0(4) pm, c = 508.5(2) pm and β = 97.98(5)°; the unit cell contains Z = 2 formula units Li 2 [B 2 (O 2 ) 2 (OH) 4 ]. The measured density is D m = 1.97 and the calculated density is D c = 1.997 Mg · m −2 . The crystal structure has been solved from 533 observed reflections by means of the Multan method and refined by least-squares calculations to an index R of 4.5% and R w of 3.8%. The structure contains the peroxoanion [B 2 (O 2 ) 2 (OH) 4 ] 2− found already in the structure of triclinic NaBO 2 · 3 H 2 O · H 2 O 2 , Na 2 [B 2 (O 2 ) 2 (OH) 4 ] · 6H 2 O. The Li atom is coordinated by six O atoms with distances between 197.0 and 225.3 pm. Connections between the B – O rings are provided only by the Li + ions, contrary to the hydrated sodium peroxoborate.

Partitions of the three-dimensional space by Dirichlet domains with cubic symmetry have been studied. Within the symmorphic cubic space groups a total of 91 different types of Dirichlet domains were found and their fields of existence were accurately determined. The occurence of the same type of Dirichlet domain in various space groups was investigated. In these space groups the P - und F -lattices exhibit very simple partitions with few different types of Dirichlet domains only, whereas in the I -lattices more complicated partitions occur.

The crystal structures of tetraalkaline-hexazidonickelat(II)-dihydrates M 4 Ni(N 3 ) 6 · 2H 2 O (M = K, Rb, Cs), space group P [unk], and of dicesium-tetrazidonickelat(II)-monohydrate Cs 2 Ni(N 3 ) 4 · H 2 O, space group Pbca , have been determined with the aid of a four circle automatic X-ray diffractometer (STOE) using graphite monochromated molybdenum Kα -radiation. The absorption was corrected numerically and the structures were solved by direct methods. In all four structures the nickel(II)-ions are surrounded by six azide groups, which form regular octahedra. The alkaline ions have nine neighbours which form nonregular prisms with pyramides on their prism faces. In M 4 Ni(N 3 ) 6 · 2H 2 O the Ni(N 3 ) 6 -octahedra are isolated, M(1)- and M(2)-ions are surrounded by six azide groups and three H 2 O, M(3)- and M(4)-ions by nine azide groups. In Cs 2 Ni(N 3 ) 6 · H 2 O the Ni(N 3 ) 6 -octahedra are connected via azide groups, Cs(1)-ions are surrounded by nine azide groups, Cs(2)-ions by seven azide groups and two H 2 O.

The perfection of single crystals of CdTe grown from the vapor phase has been studied by X-ray topography. Results show that the main defects are dislocations with Burgers vector of the [unk] [110] type. Occasionally those of the [unk] [11[unk]] type are observed. The majority of the dislocations are curved and tangled. The dislocation density is about 10 3 cm −2 . Small precipitates of Te as well as twin lamellae are present in a part of the crystals.

The crystal structure of a new synthetic sulfosalt, Tl 3 SbS 4 , has been determined. Synthetic Tl 3 SbS 4 crystallizes in the triclinic space group P 1 with a = 6.285(1) Å, b = 6.364(1) Å, c = 11.647(1) Å, α = 94.61(1)°, β = 98.51(2)°, γ = 103.93(1)°, Z = 2. The structure was determined by three dimensional Patterson synthesis and refined to a final R -value of 0.07 for all observed reflections. The six independent Tl atoms in the Tl 3 SbS 4 structure are surrounded by six, seven or eight S atoms in form of an irregular polyhedron. The two Sb atoms are coordinated by four S atoms in form of an almost ideal tetrahedron. The SbS 4 tetrahedra are isolated. The structure is built up on a three dimensional network of Tl polyhedra and SbS 4 tetrahedra.