Volume 206, Issue 2

The combination of the random approach with the representations method proved of large usefulness. Very difficult structures can be routinary solved via the package SIR92 which integrates the two strategies. A new method is proposed which actively uses the information contained in the psi-zero triplets in order to drive phases towards the correct values.

It has become practice in direct methods to use a multisolution approach to the solution of the phase problem based on the initial refinement of random phases. The maximization of a function of phases proved to be very effective in obtaining correct phase sets. The expansion of this method leads to a method which is known under the name XMY, X minus Y and which is now an integral part of MULTAN90. Different weighting schemes are investigated in order to explore the possibility of still improving the actual XMY-method.

The structure of the ergoline derivative 1,1-Diethyl-3-[(2R,3R,5R,8S,10S)-2,3-dihydro-2,6-dimethyl-8-ergolinyl]-urea has been determined by X-ray single crystal diffraction at three temperatures: 295, 50 and 15 K. Large displacement parameters of the diethyl-urea side group atoms cause rather improper bond distances at room temperature which, after cooling, adopt normal values. An inspection of the displacement parameters expressed in U eq 's, shows that even the temperature reduction from 50 to 15 K causes an average decrease of U eq 's by 33%. At room temperature, the individual displacement parameters depend strongly on the atomic distances from the molecular center of gravity. This dependence disappears almost completely at 15 K.

An iterative technique is described for determining the double Patterson function of a non-centrosymmetric structure. A knowledge of the double Patterson leads directly to estimates of the phases of 3-phase structure invariants. It has been used successfully on small one-dimensional structures and further development is in progress. Parts of the iterative technique, namely a maximum entropy approximation to the double Patterson and an exact relationship among 3-phase structure invariants, can be applied to three-dimensional structures without requiring a large and powerful computer.

Frozen-in order parameter fluctuations and the spread of the ferroelastic transition temperature over a wide temperature range are found in strongly barium-diluted lead phosphate crystals, (Pb 1− x Ba x ) 3 (PO 4 ) 2 . For x -values near 0.09 the phase boundary of the transformation R [unk] m → C 2/ c in the phase diagram lead phosphate-barium phosphate approaches T c = 0 K. Small Ba-inhomogeneities corresponding to order parameter fluctuations lead to the coexistence of ferroelastic and paraelastic phases. The superposition of static monoclinic and rhombohedral signals are analyzed by neutron powder diffraction. The crystal structure of a sample with barium content x = 0.08 is refined for 8 K, 300 K and 700 K using the Rietveld technique. The evolution of the structural deformation on a local and a macroscopic length scale is studied using spectroscopic, X-ray and optical birefringence methods. In a (Pb 0.92 Ba 0.08 ) 3 (PO 4 ) 2 powder sample the short range monoclinic deformation vanishes above 700 K.

The crystal structures of the monoclinic dimorphs of natural britholite-(Ce) and britholite-(Y) have been solved in space group P 2 1 using both annealed and unannealed samples. Like the P 6 3 britholite atomic arrangement reported recently, the structure of monoclinic britholite is similar to the apatite atomic arrangement. The monoclinic britholite dimorph differs from its hexagonal counterpart principally in the ligation of the REE equivalent of the apatite Ca(1) site. Whereas in P 6 3 britholite each Ca(1) equivalent has either three short or three long REE-O(3) bonds, in the P 2 1 dimorph the Ca(1) equivalents have either one long and two short REE-O(3) bonds or one short and two long REE-O(3) bonds. Thus 3/m is removed from the P 6 3 / m apatite symmetry elements, yielding P 2 1 symmetry. The symmetry reduction explains the common observation of biaxial optical characteristics of britholite samples.

The compound [Cr 2 (NH 2 ) 3 (NH 3 ) 6 ]I 3 was synthesized by the reaction of CrI 2 with supercritical ammonia in high pressure autoclaves at 773 K and P(NH 3 ) = 600 MPa. Its crystal structure was determined on the basis of X-ray single crystal data: space group P 6 3 / mmc , a = 7.670(2) Å, c = 18.096(4) Å, Z = 3, R / R w = 1 = 0.016/0.018 for 396 observed structure factors with I ≥ 3 σ ( I ). The compound contains dinuclear [Cr 2 (NH 2 ) 3 (NH 3 ) 6 ] 3+ -cations with a short d(Cr–Cr) = 2.65 Å. The entire structure is described by a motif of a common close-packing of iodine and nitrogen. Chromium occupies face-sharing nitrogen octahedra desribed by the stacking sequence ⋯ |A Cr B Cr A C cr B Cr C|⋯.

Large optical quality, single crystals of triclinic ammonium hydrogen succinate have been grown from aqueous solutions. An X-ray structure analysis yields space group P [unk], a = 7.484(2), b = 8.876(2), c = 4.726(1) Å, α = 91.25(2), β = 93.26(2), γ = 100.26(3)°, Z = 2, ϱ calc = 1.456(1), ϱ obs = 1.454(2) g cm −3 . The structure contains strong hydrogen bonds between adjacent succinate ions which form a periodic bond chain along [110]. Another strong bond chain results from hydrogen bonds and ionic interactions between NH 4 ions and the carboxylic groups along [001]. The succinate ion exhibits no intrinsic symmetry as found in most other acid salts of succinic acid. In the final refinement one hydrogen atom had to be split into two positions related by a centre of symmetry. Complete elastic and thermoelastic tensors have been determined from ultrasonic resonance frequencies of thick plates and their temperature dependence, respectively. Further, the tensor of thermal expansion has been studied in a wide temperature range. The existence of a second-order phase transition has been observed by thermal expansion and DSC measurements at about 169 K.

The title compound is the condensation product of the aminoacid d -penicillamine with glyoxylic acid, followed by esterification with methanol/HCl. It crystallizes from ether/petroleum ether orthorhombically in the space group P 2 1 2 1 2 1 with a = 1033.4(6), b = 1678.2(6) and c = 730.7(3) pm.