Volume 193, Issue 3-4

A generalized susceptibility of dislocations is calculated in ferroelectrics and ferromagnetics and also in the vicinity of a structural phase transition point by which natural frequencies for dislocations are defined, when the fariation of the order parameter η is not related with long-range fields. Two existing vibration branches are shown: “quasiacoustic”, whose frequency tend to zero at q → 0, where q is the wave vector of the bending wave, propagating along the dislocation, and “quasioptical”, splitting out from the edge of the optical zone of the crystal vibrations. A noticeable contribution to the damping of the dislocation vibrations is shown to be related with energy dissipation at vibrations of the order parameter η .

A structural model of amorphous CdAs films in the composition range from 45 to 82 at.% As, based on the microparacrystalline theory, has been developed. An analysis of the X-ray diffraction data has been carried out by comparison of both experimental intensity ki ( k ) ( k = 4 π sin θ / λ ) and radial distribution 4 π r 2 (ϱ – ϱ o ) functions with those of calculated using the concept of paracrystallinity. Two structural regions with different composition ranges are distinguished. The first structural domain extends from 45 to about 67 at.% As. In this composition range, the distorted tetrahedral structure, intermediate between CdAs and Si III type structure, is adequate to account for the experimental data. The second structural domain extends from about 74 to 82 at.% As. In this composition range, the ki ( k ) and 4 π r 2 (ϱ – ϱ o ) functions look much like those of glassy CdAs 2 . The tetrahedral structure model based on the tetragonal CdAs 2 structure, which is characterized by As spiral chains linked together by Cd atoms, proves to be suitable for arsenic-rich compositions. It has been shown that the “ α * relation”, empirically established by Hosemann et al., is satisfied for all investigated amorphous cadmium arsenide films as well as CdAs 2 glass.

About 250 inorganic structure types, reported with a monoclinic centred unit cell, were tested for overlooked trigonal symmetry and the metrics of the different “best” monoclinic cells describing an hR Bravais lattice were studied. Three-fold axes were detected in the compounds listed below, for which the crystallographic data were converted to the corresponding trigonal space groups: PtTe: hR 12, R [unk] m , a = 3.963, c = 19.98 Å, Z = 6 Pt 3 Te 4 : hR 21, R [unk] m , a = 3.988, c = 35.39 Å, Z = 3 Pt 2 Te 3 : hR 30, R [unk] m , a = 4.003, c = 50.89 Å, Z = 6 Li 8 Pb 3 : hR 33, R [unk] m , a = 4.757, c = 32.05 Å, Z = 3 LiFe 6 Ge 4 : hR 33, R [unk] m , a = 5.045, c = 19.66 Å, Z = 3 LiFe 6 Ge 5 : hR 72, R [unk] m , a = 5.048, c = 43.64 Å, Z = 6 CaGa 6 Te 10 : hR 105-3, R 32, a = 14.42, c = 17.65 Å, Z = 6 La 3.266 Mn 1.1 S 6 : hR 165-9.3, R 3 m , a = 14.08, c = 21.80 Å, Z = 15 It is recommended that these data be tested in future refinements. The recently refined “monoclinic” structure of Cs 4 PbCl 6 is shown to be identical with earlier determined trigonal Cs 4 PbCl 6 . The structures of the Pt tellurides may be interpreted as an intergrowth of Pt 2 Te 2 (PtTe-type) and PtTe 2 (CdI 2 -type) layers, whereas the structure of LiFe 6 Ge 5 contains 14 Å thick MgFe 6 Ge 6 -type slabs (alternating Zr 4 Al 3 - and CeCo 3 B 2 -type layers), stacked with a shift. In LiFe 6 Ge 4 similar, but thinner slabs are found. CaGa 6 Te 10 is a normal valence compound where all Ga atoms centre Te 4 tetrahedra to form an anionic tetrahedron complex where each individual GaTe 4 tetrahedron is connected to the surrounding tetrahedra via 6 Ga-Te-Ga links.

The crystal and molecular structure of deuterated 4-methyl Pyridine C 6 D 7 N has been determined at 4.5 K using pulsed neutron powder diffraction. It crystallises in the space group I 4 1 / a with a = 7.5338(1) Å, c = 18.4960(1) Å and Z = 8. The structure is the same as that determined by Ohms et al. [J. Chem. Phys. 83 (1985) 273 – 279] for the hydrogenous material at 120 K, and there appears to be no phase transition down to 4.5 K. Our results contradict the conclusions of Biswas [Indian J. Physics 35 (1961) 261 – 268], who assumed a monoclinic space group P 2 l / c in interpreting X-ray powder diffraction data at 93 K.

[Co( bbdh )(OH 2 )(ONO 2 )](NO 3 ) · H 2 O was obtained from Co(NO 3 ) 2 · 6H 2 O and bbdh { bbdh = l,6-bis(benzimidazol-2-yl)-2,5-dithiahexane) in ethanol. It crystallizes in the monoclinic space group P 2 1 / n with a = 1 335.2(7), b = 1 666.2(5), c = 1068.7(3) pm, β = 105.65(4)°, Z = 4. The structure was refined by a full-matrix least-squares procedure to final R = 0.031 for 3591 independent reflections with I > 3 σ ( I ). The Co 2+ ion is octahedrally coordinated by the bbdh ligand, one oxygen atom of a nitrate group and one oxygen atom of a water molecule, the two nitrogen atoms of bbdh being in trans and the two sulfur atoms in cis arrangement. The Co – S distances are 250.59(8) and 252.68(8) pm, d(Co – N) = 208.6(2) and 209.8(3) pm. The distance to the oxygen atom of the coordinated nitrate ion is 209.5(2) and to the water molecule 204.2(2) pm. The non-coordinating nitrate ion is connected to the water ligand by a hydrogen-bond and the non-coordinating water molecule is hydrogen-bonded to the pyrrolic nitrogen atom of a benzimidazole group.

The crystal structure of the title compound, C 18 H 13 BrCl 2 N 4 OZn, has been determined from three-dimensional X-ray diffraction data. The structure is monoclinic, space group P 2 l / c , with a = 8.039(1), b = 19.448(2), c = 13.300(3) Å, β = 115.55(2)°, V = 1876.01 Å 3 and Z = 4. The structure was solved by direct methods and refined by least-squares to a final R = 0.044 ( R w = 0.038, R g = 0.041) for 1644 observed unique reflections [ F o > 5 σ ( F o )]. The organic ligand acts as a tridentate chelating agent, while zinc is in a distorted trigonal bipyramidal coordination [Zn – Cl: 2.231(3)–2.246(3), Zn–N: 2.147(10)–2.149(7), Zn–O: 2.229(7) Å]. The IR and UV spectra are in accordance with the established geometrical features of the structure.

The presence of non-crystallographic symmetry in crystals affects the intensity distribution as well as cumulative distribution function N ( z ), which has been derived for cases of many parallel repetitions of atoms at random using the series expansion distribution function. It is seen that the change in the value of N ( z ), from that obtained by using normal distribution, is quite significant which is illustrated with the help of an hypothetical example.