Band 209, Heft 2

Structural data, retrieved from the Inorganic Crystal Structure Database, have been used to study the effective shape and sizes of six coordinated Cu 2+ and Mn 3+ ions in oxides and fluorides. The distortions of the coordination octahedra for these two metal ions, essentially due to the structural implications of the Jahn-Teller effect, are assumed to be interpretable in terms of a nonspherical (ellipsoidal) effective shape of the metal ions. For octahedra with approximate tetragonal D 4 h symmetry (rotational ellipsoid) a reasonable one-parameter correlation function is shown to be 2/ r 4 2 + 1/ r 2 2 = 3/ r 0 2 , where r 4 is the ionic size perpendicular to and r 2 the size along the rotation axis. The single parameter r 0 to be determined, estimated to 0.66(1) Å for Cu 2+ and 0.59(1) Å for Mn 3+ , represents the size of the spherical case. From the correlation function, possible estimates of the effective sizes for square planar four coordination (and linear two coordination) can be obtained by extrapolation, giving 0.54 Å and 0.48 Å for four coordinated Cu 2+ and Mn 3+ , respectively. According to the correlation function the radii decrease with the square root of the coordination number. For ions with effective ellipsoidal shapes like Cu 2+ and Mn 3+ , it is shown that i.a. not only the use of average bond distances but also the previously used linear relation between average distances and distortion-parameters are strongly dependent on the degree of non-sphericity. A modified linear relation, between average bond distance and distortion, with improved linear correlation is suggested. As an alternative, the root harmonic mean square ( rhms ) value, 1/[unk], that gives an estimate of the parameter r 0 , is suggested to be used for judging the relevance of an observed bond length distribution ( R 1 ,…, R 6 and r i estimated R i – r ligand ) for six-coordinated ions.

A two-wavelength method for the distinction of elements of similar atomic numbers in generally solved crystal structures, called ( δ -synthesis, and its application to structures with no center of symmetry is discussed. As an example the determination of the distribution of copper and nickel in the mineral mückeite (CuNiBiS 3 ) is presented. Copper occupies a tetrahedrally coordinated and nickel a tetragonal planar coordinated site. During these studies δ -synthesis proved to be a method which is very robust against specimen-dependent errors of measurement.

The reconstructive high-temperature transformation of NaAlSiO 4 nepheline to carnegieite, stuffed derivatives of the SiO 2 phases tridymite and cristobalite, has been studied by light microscopy and X-ray precession photographs. X-ray precession patterns of single crystals heattreated between 1543 K and 1593 K yielded an oriented transformation of NaAlSiO 4 nepheline to carnegieite with 〈111〉 CA || 〈001〉 NE and 〈 hh 2 h 〉 CA || 〈100〉 NE (cubic setting of carnegieite (CA), and orthohexagonal setting of nepheline (NE)). Two equivalent carnegieite orientations occur, both having 〈111〉 CA || 〈001〉 NE in common, corresponding to transformation twins with {111} ca as twin plane. The oriental relationships between nepheline and carnegieite are similar to those observed in the high-temperature transformation of tridymite to cristobalite. However, the transformation mechanisms are believed to be different. Thermally activated Na and weakening of the tetrahedral Al–O-bonds produce quickly migrating non-bridging O-atoms which act as conversion nuclei, accelerating strongly the transformation process in comparison to SiO 2 .

The successive phase transitions of a BaZnGeO 4 crystal have been investigated by various techniques such as X-ray diffraction, DTA, dielectric, pyroelectric, piezoelectric and optical measurements from 100 K to 1300 K. The pyroelectric and optical studies have been reported for the first time in this paper. The material gives rise to a pyroelectric current from 100 K to 520 K. Five phase transitions have been detected in this temperature range by pyroelectric current measurements. The optical measurements allow the observation of four phase transitions. The birefringence decreases with rising temperature between 100 and 1300 K. A mechanism is proposed for the observed pyroelectricty.

In continuation of an earlier study on monoclinic and orthorhombic space groups, the effects of Anisotropic Anomalous Scattering (AAS) on the systematically extinct (‘forbidden’) axial reflections of all tetragonal space groups are investigated under the limiting conditions of purely σ -polarized radiation and one generally positioned ‘edge atom’ in the asymmetric unit. The compilation of the results, Polarized Structure Factors and ensuing characteristic intensity functions, I ([unk], Ψ ), offers a concise survey over the different cases of relevant symmetry, and contributes to the understanding of FRED (Forbidden Reflection near Edge Diffraction). One interesting application of FRED is partial structure determination, i.e. the derivation of possible ‘edge atom’ coordinates, from the intensity variations I ([unk], Ψ ), of a small number of ‘forbidden’ axial reflections. The method is invariant against the normally unknown AAS so that nothing but the occurrence of AAS and a few intensity measurements at selected azimuthal settings are required. Characteristic cases for both h 00 and 00 l reflections are discussed.

The growth and dissolution of Ge single crystals through disproportionation of GeI 2 vapor are initiated by a sudden supersaturation change. The crystals have been observed in a quadratic fused silica pyramid whose flat transparent walls allow the automatic determination of the crystal contours from shadow pictures. The prominent macroscopic changes of the shape occur along the 〈111〉 directions. The steady-state growth form is thus mainly composed of {100} faces and the dissolution form of {111} faces. In a few cases, small as-grown {110} faces were also observed. Growth and dissolution proceed laterally via steps which run along the 〈110〉 directions. In the case of growth they move on the (001) face in the four 〈110〉 directions, and on the (111) face slowly in the 〈[unk][unk]2〉 directions and fast in the 〈11[unk]〉 directions. In the case of dissolution low and high steps move on the (111) face in the 〈[unk][unk]2〉 and 〈11[unk]〉 direction respectively.

The role of directed methods in the protein field is analyzed. A criterion is formulated which suggests the necessary conditions for the success or the failure of the ab initio direct procedures. The distribution of quantity z = 〈 α 〉/ σ α is central in our considerations. Most of the experimental protein data do not satisfy such a criterion, therefore their ab-initio solution is a quite improbable event.

Sixteen structure types derive from the aristotypic rutile, TiO 2 , structure. Seven of these are based on an ordering of cations: the CoReO 4 , MgUO 4 , AlWO 4 , CuUO 4 , trirutile, CuSb 2 O 6 and FeNb 2 O 6 types; three on an ordering of anions: the InOOH, the manganite and HgFOH types; and six more on various distortions from the aristotype: the CaCl 2 , CuF 2 , V 1 − x Al x O 2 (or VO 2 -T), V 0.8 Cr 0.2 O 2 , β -NbO 2 and α -NbO 2 types. The manganite and AlWO 4 types could also be considered as distortion variants (as respectively the MoO 2 and VO 2 -M2 types). The trirutile type is defined as a derivative of rutile, with a tripled c cell constant and with space group P 4 2 / m 2 1 / n 2/ m . Because other ‘polyrutile’ types cannot easily be defined as unique structure types the use of these terms is not recommended. Instead the example first described should be used for naming the type (e.g. MgUO 4 type or CoReO 4 type). The derivatives due to distortions of the aristotypic rutile type are distinguished based on their underlying net of bonds. A structure type should be defined on the basis of its symmetry and the topology of the net formed by its strong primary bonds. Therefore, one should count among the rutile type derivatives only those which are based on a 6,3 net with the same bonding topology as the rutile type proper. This includes the CuF 2 type and the CuUO 4 type, but not the marcasite type (FeS 2 ), the TeO 2 type or the KrF 2 type. Relationships between crystal structures can be viewed on three levels: 1. crystal structures of the same stoichiometry, having the same type of space group symmetry and based on the same pattern (net) of bonds are isostructural; 2. crystal structures of the same stoichiometry, crystallizing in a subgroup of a more highly symmetric aristotype and based on the same bonding pattern are derivatives of the aristotype; 3. crystal structures of the same stoichiometry, crystallizing in the same type of space group as an aristotype or in a subgroup, but not based on the same bonding pattern are merely formally related to the aristotype.

The new compounds Zn 3 Pt 9 B 4 , Ga 2.7 Ir 9 B 5 , and Ga 3 Pt 9 B 4 were prepared by reaction of the elements at 1373 K. Their crystal structures were determined by means of single crystal X-ray methods. In the case of Zn 3 Pt 9 B 4 , powder neutron diffraction was applied in addition. Zn 3 Pt 9 B 4 [hexagonal, P [unk]2 m ; a = 911.11(6) pm; c = 286.00(3) pm; Z = 1; 491 reflexions; 20 parameters; R = 0.023] and Ga 2.7 Ir 9 B 5 [hexagonal, P [unk]2 m ; a = 904.65(9) pm; c = 287.03(2) pm; Z = 1; 412 reflexions; 20 parameters; R = 0.039] crystallize with a new structure. Trigonal transition metal prisms partly centered by boron atoms are arranged to form channels with pentagonal cross sections containing rows of zink and gallium atoms respectively. In the structure of Zn 3 Pt 9 B 4 one of the boron sites has an occupation parameter of 0.5. Ordering of the vacancies in Ga 3 Pt 9 B 4 [hexagonal, P [unk]2 c ; a = 875.03(1) pm; c = 637.42(1) pm; Z = 2; 991 reflexions; 26 parameters; R = 0.052] results in a variant of the structure with doubled lattice constant c and a zig-zag chain of gallium atoms within the channel.

The crystal structures of Ba(OD)Cl and Ba(OD)Br have been determined by neutron powder diffraction at 295 K and 1.5 K and Rietveld refinement methods { R I = 9.3, 8.2 [Ba(OD)Cl], 5.7, and 4.9% [Ba(OD)Br]}, that of Ba(OH)Cl by single-crystal X-ray diffraction at 295 K ( R = 4.1% for 1260 reflections). They are isostructural to laurionite Pb(OH)Cl {space group Pnma , Z = 4, a = 738.97(3), b = 443.69(3), and c = 913.90(5) pm [Ba(OH)Cl], a = 738.07(4), b = 447.62(2) and c = 897.93(5) pm [Ba(OD)Cl] and a = 759.65(2), b = 438.77(1), and c = 1030.22(2) pm [Ba(OD)Br] at 295 K}. Whereas hydrogen bonds are not present in the bromide, in the case of Ba(OH)Cl weak hydrogen bonds to adjacent Cl − ions are formed which are strengthened on going down to lower temperatures due to reorientation of the OH − ions from trifurcated H-bonds to nearly linear ones. The hydrogen bonds of Ba(OD)Cl have been established to be stronger than those of Ba(OH)Cl. This becomes apparent by both the smaller cell volume [296.65(4) pm 3 instead of 299.6(1) pm 3 ] and the different transition temperature (170 K–300 K instead of 100 K–180 K).

By means of chemical vapour transport reactions (transport agent: 1 atm Cl 2 at room temperature) dark red, transparent crystals of RhPO 4 (1323 K → 1223 K) and orange crystals of RhP 3 O 9 (1223 K → 1123 K) were obtained with edge length up to 1 mm. The structures of both compounds were refined from x-ray single crystal data (RhPO 4 : α -CrPO 4 structure type, Imma , Z = 12, a = 10.3967(8) Å, b = 13.1124(9) Å, c = 6.3929(5) Å, 1249 independent reflections with | F | ≥ 3 σ ( F ), 50 variables, R = 0.026; RhP 3 O 9 : C-MP 3 O 9 structure type, Cc , Z = 12, a = 13.002(8) Å, b = 19.065(9) Å, c = 9.296(4) Å, β = 127.04(3)°, 4541 independent reflections with | F | ≥ 3 σ ( F ), 119 variables, R = 0.033). The crystal structures of RhPO 4 and RhP 3 O 9 are described and compared to isotypical compounds. Temperature dependent x-ray powder photographs gave no evidence for a phase transition for RhPO 4 neither then for RhP 3 O 9 up to 1223 K.

The title compound, C 10 H 19 N 3 O 2 Cu, is monoclinic, space group P 2 1 /n, Z = 2. Cell dimensions are a = 7.6071(6) Å, b = 20.623(1) Å, c = 7.9777(7) Å and β = 101.7359(7)°. A total of 3510 reflections were measured with an Enraf Nonius CAD-4 automatic diffractometer using Cu K α radiation. The structure was refined by full-matrix least squares to a conventional R = 0.047 for 2957 unique reflections. The coordination around the central copper atom is octahedral with the copper atom on the equatorial plane of four nitrogen atoms. The Cu–N bond lengths have a mean value of 2.0646 Å. The two remaining apices of the octahedron are occupied by two nitrogen atoms of the β -dione groups at distances Cu–N = 2.5143 Å.

The linear electro-optic tensor [ r ijk σ ] (unclamped) and the tensor of linear electrostriction [ d ijk ] were determined for the isotypic title compounds (point group [unk]2 m ) at room temperature and λ = 633 nm. Small electro-optic coefficients (one seventh to one tenth of those of KDP), small electrostrictive components (like d 111 of α -quartz) and high electro-optic anisotropy ( r 123 / r 231 ≫ 1) are the main features of these complex thiosulphates.

Both isomers of 3,5-dioxo-3,5-diisopropoxy-1,3,5-thiadiphosphorinane have two molecules in the asymmetric unit. The trans isomer crystallises in P [unk] with a = 9.635(3) Å, b = 10.516(3) å, c = 14.146(4) Å, α = 93.20(2), Å β = 100.28(2) Å, γ = 90.18(2)° (at 178 K), final R ( F ) 0.041. The cis isomer crystallises in P 2 1 / n with a = 8.459(4) Å, b = 11.410(4), Å c = 29.458(12) Å, β = 93.12(3)° (at 293 K), final R ( F ) 0.046. The rings adopt a chair conformation. All four rings display closely similar dimensions. The P–C–P angles are wide (120°). In the cis isomer the bulky substituents are both axial, whereas in the trans isomer one substituent is forced to be equatorial.