Band 151, Heft 1-2

A structure analysis of those polytypes of cadmium iodide which have been identified was carried out and a convention for the Zhdanov notation of this structure was proposed. The possibility of dividing all CdI 2 polytypes into four groups characterized by different types of symmetry of 10. L and 10. L reflection intensities is given. In order to establish regularities in structure of different groups of polytypes, a new method of symbolical description for polytypic structures in crystals of MX 2 compounds is proposed. The description of structure based on defining three-ion layers I–Cd —I as fundamental units of CdI 2 crystals structure enables a unique description of these structures. It was also established that the relative intensity of chosen reflections depends on the layer sequence for some hexagonal polytypes. The parameter α , describing the ratio of the “ + ” and “ + ” symbols in the Hägg notation, can be calculated from the number of layers in the unit cell for rhombohedral polytypes.

The crystal structure of the mineral görgeyite, K 2 SO 4 · 5 CaSO 4 · H 2 O, has been determined from visually estimated photographic intensities using the direct methods program MULTAN, and refined by block-diagonal least-squares structure factors to an R value of 0.096. The mineral is monoclinic and contains four formula units in a cell of dimensions a = 17.51(1), b = 6.822(5), c = 18.21(1) Å, β = 113.3° and space group C 2/ c . Two independent calcium atoms, one of which sits on the diad axis, are nine-fold coordinated by oxygen atoms in a closely similar arrangement, with average Ca–O distances of 2.52 Å in the range 2.40–2.68 Å. A third calcium atom is eight-fold coordinated with a mean Ca–O distance of 2.46 Å in a range of 2.37–2.64 Å. The potassium atom is also eight-fold coordinated. The three independent sulphate ions are approximately regular with mean S–O distances of 1.472(8), 1.477(27), and 1.470(7) Å respectively but their behaviour in providing oxygen coordination is different for each. The hydrogen atom of the water molecule has not been positively located though it is likely that a hydrogen bond is present.

A three-dimensional X-ray diffraction analysis of BaS 2 O 3 · H 2 O was based on 523 symmetry-independent reflexions. The structure was refined by full-matrix least-squares to an R (excluding zero-weight data) of 0.065 for 492 reflexions. The structural details of the earlier studies are confirmed. An analysis of the thermal motions of the individual nonhydrogen atoms was made on the basis of thermal vibration ellipsoids and molecular geometry.

A pyroelectric radiation detector using tourmaline has been constructed and examined. Its voltage response (500° K, 3 Hz) is determined to 2.1 · 10 2 [ V / J ]. The current response has a magnitude of 4.6 · 10 −8 [ A / J ]. A method is described for the determination of the pyroelectric coefficient γ ( T ) of unknown pyroelectrics using a reference crystal. Because of its high temperature stability tourmaline can be used for high-power detector application or for high-temperature measurements as a pyroelectric thermometer.

An anorthite-rich plagioclase An 66 from Lake County, Oregon, USA, was investigated by means of neutron diffraction. The distribution of aluminium and silicon atoms in the TO 2 -network was directly determined by neutron diffraction, whereby the method of the complementary Fouriersynthesis was applied. With increasing Si-content the Al-atoms at T 1 ( m o), T 2 (oo) and T 2 ( mz ) were substituted by Si-atoms. T 1 (o z ) is occupied in the region of An 100-66 by Al-atoms. A reversible phase transition between I [unk] and C [unk] was observed at about 1200°C.

Both polymorphs of sapphirine, namely sapphirine-lTc and sapphirine-2M, were found in granulites near Wilson Lake (Labrador). Both minerals can be described as consisting of equivalent layers with monoclinic symmetry, translation periods a = 9.78, c = 9.93Å, with β = 110.2°, the width of a layer being b 0 = 7.20Å. The layers are built up by octahedral walls connected by tetrahedral chains. Adjacent layers are related by translation vectors t 1 = a /2 + b 0 − c /4, or t 2 = a /2 + b 0 + c /4. Sapphirine-1Tc corresponds to the sequence t 1 t 1 t 1 … (or t 2 t 2 t 2 …), whereas sapphirine-2M corresponds to the sequence t l t 2 t 1 t 2 … The structures of the two polymorphs from Wilson Lake were refined with the aim to determine the distribution of magnesium, iron, and aluminium cations in octahedral sites and of aluminium and silicon cations in tetrahedral sites. The structural data indicate for both polymorphs a substantially similar distribution, which corresponds to that obtained by P. B. Moore in his study of the sapphirine from Fiskenaesset.

Antimony(III) oxide iodide 2MC-Sb 5 O 7 J shows ferroelastic behavior characteristic of the species 6/ m F 2/ m . The defect structure of the crystals revealed by X-ray topography is described. The almost total absence of dislocations can be interpreted from the ferroelastic behavior. A visibility criterion for X-ray topographic contrast of the domain walls is given and explained by simple geometric considerations. The orientation of the domain walls is calculated from the spontaneous strain tensor. The relation between mechanical twinning and ferroelasticity is explained in the present case.

The crystal structures of two lithium nickel borides were determined by single crystal X-ray investigations. Li 3 Ni 20 B 6 (cubic, Fm3m, a = 1047.8 pm, Z = 4) crystallizes in the Cr 23 C 6 structure type with partly random distribution of lithium and nickel. The centers of the cubo-octahedra are statistically occupied by 50 % Li and 50 % Ni, while the Friauf-polyhedra are filled by 90% Li and 10% Ni. Li ~3 Ni 16 B ~8 (tetragonal, P 4/ m b m , a = 821.6 pm, c = 741.4 pm, Z = 2) is related to the Cr 23 C 6 type by some of its building elements. A part of the structure can be deduced from the Ti 3 Co 5 B 2 type.

The crystal structures of the newly prepared ternary borides SrNi 12 B 6 and BaNi 12 B 6 have been determined by single-crystal X-ray methods and refined to R -values of 0.031 and 0.052 respectively. Both compounds crystallize in space group R[unk]m–D 5 3d with lattice constants a = 959.0 pm, c = 745.5 pm, Z = 3 for SrNi 12 B 6 and a = 968.9 pm, c = 747.0 pm, Z = 3 for BaNi 12 B 6 . The structure contains isolated boron atoms, surrounded by distorted trigonal prisms of Ni atoms with one additional Ni upon one of the faces. The prisms are arranged in connected layers consisting of six- and three-membered rings. Of the compounds SrNi 2 B 2 , BaNi 2 B 2 , and Ba 2 Ni 9 B 6 lattice constants are reported.

Chalcophyllite, Cu 9 Al(AsO 4 ) 2 (SO 4 ) 1.5 (OH) l2 · 18 H 2 O, crystallizes in space group R [unk] with a hex = 10.756 and c hex = 28.678Å. The structure was determined by Patterson and Fourier methods and refined by the least-squares technique to a final R index of 0.056. The dominant structural feature is the arrangement of Cu and Al polyhedra into complex sheets to which the As tetrahedra are attached by one face. The sheets are connected to each other by the hydrogen bonding system, where the sulphate groups are immersed. An interpretation of twinning with (100) as twin plane is given, and the partial occupancy of SO 4 tetrahedron is discussed.

Two basic aluminium sulphate minerals, aluminite and metaaluminate, Al 2 (OH) 4 SO 4 · n H 2 O, n = 8 and 5, respectively, have been investigated by means of X-ray powder diffraction data. The unit cell dimensions were determined from Guinier photographs measured by a computer-controlled drum densitometer (Malmros and Werner, 1973), and evaluated by a trial-and-error indexing program. The monoclinic cells found for aluminite were a = 11.444(2) Å, b = 15.849(1) Å, c = 7.4397(7) Å, β = 106.85(1)°; for meta-aluminite they were a = 7.930(3) Å, b = 16.879(4) Å, c = 7.353(5) Å, β = 106.74(7)°. Some general aspects of the accuracy required for indexing of powder patterns are also discussed and demonstrated by the minerals studied.

The precession method is extended to the back-reflection region and used to record distant upper levels. Nets fulfilling the equi-inclination condition ζ = 2 cos [unk] are photographed without a central blind spot. The back-reflection cone-axis technique records nets with ζ [unk] 2 cos [unk]. Due to the mechanical design of the camera, cone-axis patterns are taken by an uninterrupted precession motion; isolated nets are taken by precession oscillations. The crystal can be rotated about the dial axis and set at will as in frontreflection cameras. In addition, a 90° auxiliary arc permits the dial axis to be tilted by as much as 90° in a plane parallel to the precession axis. Consequently back-reflection photographs of isolated nets and cone-axis photographs can be taken in any direction using a single mounting of the crystal. Also, the instrument is easily converted to operate as a frontreflection precession camera. Distortions of back-reflection cone-axis rings may be used to orient the crystal. Computer programs are available for (i) correcting misorientations, (ii) calculation of cell parameters and calibration of the camera, and (iii) indexing cone-axis patterns.

The crystal structure of Zn 4 O(BO 2 ) 6 has been redetermined and refined to an R value of 0.044. The new parameters and the interatomic distances and angles are given.