Volume 215, Issue 3

Owing to some recent confusion in the literature the room temperature structure of LaAlO 3 was redetermined by X-ray diffraction using a Guinier camera equipped with an image plate system. Different samples of pure LaAlO 3 as well as a sample doped with 0.2 percent by weight ThO 2 were used in the investigation. The space group of the structure, which is a rhombohedrally distorted perovskite structure, due to a rotation of the oxygen octahedra, is clearly R3c. The lattice constants of pure LaAlO 3 are a = 5.3655(2) Å and c = 13.112(1) Å with a c/a-ratio of 2.444(1). The influence of doping on the lattice parameters and the tilting of the O 6 -octahedron is discussed.

The term pseudo-symmetry means a spatial arrangement that feigns a symmetry without fulfilling it. In crystal structures pseudo-symmetry is a more common feature than often recognized. In case of small deviations a variety of phenomena results: polytypism and stacking faults, disorder and twinning, commensurate and incommensurate super-structures, phase transitions and unusual reflection conditions. Most of these effects complicate crystal structure determination considerably. A series of examples with focus on the new class of ternary bismuth subhalides illustrates the different crystallographic consequences of pseudo-symmetry in crystal structures.

A simple extremal principle for the determination of the phases of the structure factors of a crystal is described. An empirical test is provided. An analytical theory of the extremal functions for phase determination is developed.

Hydroxylclinohumite Mg 9 [SiO 4 ] 4 (OH, F) 2 , a new mineral from Zelentsovskaya mine near Magnitka (Zlatoust district, Southwestern Urals), is the OH-dominant equivalent of clinohumite Mg 9 [SiO 4 ] 4 (F,OH) 2 . The crystal structure of hydroxylclinohumite has been refined from single-crystal X-ray data to R = 0.026 for the 1912 observed unique reflections [F o > 4σ(F o ): a = 4.7480(3) Å, b = 10.2730(7) Å, c = 13.6894(2) Å, α = 100.72(1)°, V = 656.1(1) Å 3 , P2 1 /b (No. 14), Z = 2]. Twinning on {001}, which apparently doubles the c axis, was detected and a ratio of 0.94:0.06 between the volumes of the two twin components has been obtained by least-squares refinement. The crystal structure of hydroxylclinohumite corresponds to that of clinohumite except for the positions of the hydrogen atoms. The crystal chemistry of the clinohumite subgroup is discussed and a general formula M 2+ 8 (M 2+ , M 4+ ) (SiO 4 ) 4 (OH, F, O) 2 (M 2+ and M 4+ mainly Mg and Ti, respectively) is proposed for it.

Complexes of lanthanide picrates with bis(pentamethylene)urea (BPMU) with composition Ln(pic) 3  · 3(BPMU) (pic = picrate; Ln = La-Er,Y) were synthesized and characterized by CHN analyses, EDTA titrations of lanthanide ions, infrared absorption spectroscopy, X-ray powder patterns, absorption spectra of the Nd and Er compounds and emission spectra of the Sm and Eu compounds. X-ray single-crystal analysis of the Pr compound was carried out and showed that the coordination polyhedron is a distorted monocapped square antiprism with C 2v symmetry. This compound presents yellow crystals with symmetry of the monoclinic space group P2 1 /c with a = 12.906(4) Å, b = 21.80(1) Å, c = 22.96(1) Å, β = 105.46(3)°.

The structure of the title compound, C 29 H 47 BrO 6 , has been determined by direct methods. The crystal is monoclinic, space group P2 1 with a = 16.404(5) Å, b = 7.135(5) Å, c = 12.536(5) Å, β = 103.32(3)° and Z = 2. The structure presents a hydrogen bond between the H 2 O molecule and the 23-hydroxyl group of the spirostan moiety. All ring junctions are trans and the molecule is twisted to only a small extent about the length of the spirostan moiety. The C=O distance of the acetate group is smaller than expected.

Magnesiumphthalocyanine (MgPc; blue pigment) has attracted attention as a material useful for laser printers because of its near IR absorption. Two kinds of six-coordinate MgPc complexes have been grown from solution and their structures have been analyzed: MgPc/(H 2 O) 2 (N-methyl-2-pyrrolidone) 2 (crystal 1) and MgPc/(2-methoxyethanol) 2 (crystal 2). The crystal data of crystal 1 are: MgPc/(H 2 O) 2 (NMP) 2 , M r = 773.15, monoclinic, P2 1 /n, a = 6.876(2) Å, b = 21.427(4) Å, c = 12.994(6) Å, β = 102.20(2)°, V = 1871(1) Å 3 , Z = 2, D x = 1.375, D m = 1.38 Mgm -3 , graphite-monochromated MoK α , λ = 0.7107 Å, F(000) = 812.00, μ = 1.07 mm -1 , T = 296 K, R = 0.074 for 1606 reflections. The crystal data of crystal 2 are: MgPc/(CH 3 OCH 2 CH 2 OH) 2 , Mr = 689.03, monoclinic, P2 1 /n, a = 12.37(1) Å, b = 8.078(6) Å, c = 17.974(9) Å, β = 109.70(7)°, V = 1690(2) Å 3 , Z = 2, D x =1.354, D m = 1.37 Mgm -3 , graphite-monochromated MoK α , λ = 0.7107 Å, F(000) = 720.00, μ = 1.08 mm -1 , T = 296 K, R = 0.080 for 1407 reflections. In both complexes, the MgPc skeleton is found to be deformed as characterized by the molecular symmetry of C i in contrast to D 4h assumed for MgPc in the free space.

Pyroelectric, dielectric and piezoelectric coefficients of metastable 4-methylbenzophenone, point group 3, have been determined by standard methods. Furthermore, thermal expansion and elastic properties of this phase and of the stable monoclinic phase have been studied. The pyroelectric effect is comparable with that of tourmaline, the piezoelectric effects to those of a-quartz. The differences between the two polymorphs with respect to thermal expansion and to elastic and thermoelastic properties are small, but nevertheless significant.

As is also found for paraffin lamellar crystals heated near the rotator or melt transition, the major change in electron diffraction patterns imposed by lamellar disorder in binary or multicomponent solid solutions is the attenuation and intensity modification of low-angle (00l) reflections. Using a Gaussian model for atomic occupancies near the interface, a systematic basis is found for the interfacial disorder thickness, describing the phenomenological ‘void’ distribution at the lamellar interface. (Vibrational spectroscopy has shown that this is actually a distribution of non-planar chain conformations.) Single crystal electron diffraction data from various co-soluble paraffin chain assemblies, used previously to determine their crystal structures (but also including a new determination for n-C 36 H 74 / n-C 38 H 78 / n-C 40 H 82 1: 1: 1), are re-analyzed so that further distinctions can be made among simple binary and ternary solid solutions, multicomponent waxes, and low molecular weight polyethylene. Expressions of the latter polyethylene structure, involving ‘bridging molecules’ can be found in certain natural insect and plant waxes.

The asymmetric unit of bis(dicyclohexylammonium) bis(oxalatotriphenylstannate) [Cc: a = 14.669(4) Å, b = 12.764(2) Å, c = 32.198(9) Å, β= 95.14(1)°, Z = 4] consists of two [(cyclo-C 6 H 11 ) 2 NH 2 ] + / [(C 6 H 5 ) 3 Sn(O 2 CCO 2 )] - ion-pairs that are linked by hydrogen bonds into a pseudo centrosymmetric dinuclear entity. The tin atoms in the stannates show cis-C 3 SnO 2 trigonal bipyramidal coordination geometries. The structure can also be described either as two c-glide related substructures of pseudo C1 symmetry, or as an occupancy modulation that keeps half the asymmetric units of a 1: 1 disordered P2 1 /c parent structure for which a´ = a/2, b´ = b/2 and c´ = c. The pseudo symmetry suggests a mechanism for stacking faults and twinning, which have been taken into consideration in refining the structure. The two dicyclohexylammonium counterions are locally disordered.