Volume 222, Issue 9

Subgroups of crystallographic groups play an important role in many branches of mathematics, physics and crystallography such as representation theory, the theories of phase transitions, manifolds and in the comparative study of crystal structures [14]. In this work, the index 2 subgroups of a huge family of crystallographic groups called triangle groups are derived using black and white tilings. The focus of the work will be in determining the index 2 subgroups of triangle groups in the hyperbolic plane.

Monoclinic BiB 3 O 6 is an excellent nonlinear optical material with many advantages compared to other borate crystals. As one part of understanding the properties of BiB 3 O 6 , the harmonic frequencies of BiB 3 O 6 are calculated by applying the numerical-difference technique based on first-principles gradient-corrected hybrid B3PW density functional theory electronic structure calculations within the Gaussian-orbital-based CO-LCAO scheme. The complete 13 A and 14 B vibrational modes are assigned, graphically visualized and classified according to the Bi—O and B—O motions. Comparisons with previous experimental reports are discussed in detail.

The crystal structure of RuSr 2 GdCu 2 O 8 was successfully refined in the orthorhombic system ( Pbam space group) using synchrotron powder diffraction data collected on a high purity and low defect containing sample. The microstructures of selected samples submitted to different thermal treatments were investigated by means of TEM analysis: the progressive annealing in an O 2 atmos phere suppresses the formation of nano-domains and extended planar defects, whereas annealing under vacuum favours the formation of extended planar defects and nano-domains as well. Lattice parameters increase with the anneling time, whereas an abrupt decrease of the c parameter occurs after de-oxygenation. A dependence of magnetization and superconductivity on the thermal treatment was evidenced and related to the structural and microstructural changes.

The crystal chemistry of GdScO 3 and, for the first time, of DyScO 3 , SmScO 3 and NdScO 3 has been investigated using single crystals. The structure of the Czochralski grown crystals was refined from single crystal X-ray diffraction data, their chemical compositions were analysed by inductively coupled plasma optical emission spectrometry (ICP OES). Orthorhombic distorted perovskite structure types with the space group Pnma could be confirmed in all cases. The B-site of the lanthanoid scandates shows with 2.090 to 2.116 Å typical bond lengths for octahedrally coordinated scandium. The distortion of the B-site is rather small. The A-site is occupied by the lanthanoid (Ln) and the Ln—O bond distances vary from 2.233 to 3.722 Å. This indicates its high distortion and makes an assignment of the coordination number difficult. However, an 8-fold coordination for the A-site has to be assumed. The Ln-scandates show a continuous structural evolution with the size of the lanthanoid. A discontinuity within the intermediate members of the Ln-scandates — which had previously been described — was not observed. The ICP OES investigation of these samples results in non-stoichiometric chemical compositions: the chemical analyses show always a depletion in the lanthanoid. Site occupancy refinements based on single crystal diffraction data support the idea of the Ln-depletion on the A-site (inducing O-defects on the O2-position). Several substitution mechanisms are discussed, but the authors favour that vacancies on the A-site coupled with oxygen vacancies cause the Ln-deficiencies. The crystallochemical formula of the investigated Ln-scandates may be written as: (∀ 0.056 Dy 0.944 )ScO 2.916 ,(∀ 0.048 Gd 0.952 )ScO 2.928 , (∀ 0.045 Sm 0.955 )ScO 2.933 and (∀ 0.033 Nd 0.967 )ScO 2.951 .

Podlesnoite, BaCa 2 (CO 3 ) 2 F 2 , a fluorocarbonate mineral recently discovered at the Kirovskii mine, Khibiny alkaline massif, Kola Peninsula, Russia, is orthorhombic, Cmcm , a = 12.501(8) Å, b = 5.846(3) Å, c = 9.443(5) Å, V = 690.1(7) Å 3 , Z = 4, D calc = 3.614 g/cm 3 . The crystal structure was solved from 561 unique reflections with I > 2σ( I ) by direct methods and refined to R ( F ) = 0.0278, Rw ( F 2 ) = 0.0577. Podlesnoite is a representative of a new structure type slightly related to aragonite. The most specific feature of the podlesnoite structure is a honeycomb-like framework built by the columns of edge-sharing CaO 6 F 2 polyhedra; they are stretched parallel to the c axis. Interatomic bond-distances within these polyhedra vary from 2.2912(16) to 2.4883(15) Å. The CO 3 groups are parallel to the ac plane, the C—O bond distances range from 1.276(2) to 1.296(4) Å. The bond distances within BaO 6 F 4 polyhedra are 2.771(2) to 2.915(2) Å.

X-ray powder measurements were used to investigate the phase transitions and crystal structures of K 2 NbCl 6 and Rb 2 NbCl 6 . In both cases, the metrical changes of the unit cell are accompanied by a compressive distortion of the NbCl 6 2− groups from O h to D 4h symmetry. This static manifestation of the Jahn-Teller effect is ascribed to electro static interactions with the cations. Thus, Rb 2 NbCl 6 trans forms from Fm -3 m ( Z = 4) to I 4/ mmm ( Z = 2) due to a cation-supported compression in the c direction, whereas K 2 NbCl 6 expands along c when converting from Fm -3 m to P 4/ mnc ( Z = 2). On further cooling a second transition leads to Cccm ( Z = 4) where b as the shortest axis is approximately parallel to the direction of the octahedral contraction. The oxidation state of Nb 4+ ( d 1 ) is checked via bond-length bond-strength correlations and magnetic measurements.

Powder synchrotron X-ray diffraction studies confirm that 7-oxabicyclo[2.2.1]heptane, (oxanorbornane), forms three solid phases on cooling and four on warming, as previously indicated by spectroscopic and DSC meas urements. The two highest temperature phases are orien tationally disordered with phase I being face-centred cubic ( a = 8.5469(1) Å at 240 K) and phase II primitive cubic ( a = 10.60558(4) Å at 194 K). Phase III, which appears over a narrow temperature range on warming, is monoclinic, space group P 2 1 / a , Z ′ = 4, with a = 21.858(4) Å, b = 10.964(2) Å, c = 9.333(1) Å β = 90.491(8)° at 185 K. Phase IV, which occurs below 175 K, is monoclinic, space group C 2/ c , Z ′ = 3, with a = 13.6372(2) Å, b = 10.2593(1) Å, c = 24.0466(3) Å, β = 102.5212(4)° at 50 K. The structures of phases III and IV are ordered and have been solved from the powder diffraction data by direct-space methods, then refined by the Rietveld method, restraining the molecules to the known structure, which has m 2 m ( C 2v ) symmetry.

The structure of a new semihydrate solvated form of the antibiotic azithromycin is reported. The new form was obtained by partial dehydration of the crystal of the monohydrate- i -PrOH, when the sample was left in laboratory conditions (22–25 °C, 55–65% relative humidity) over a long period of time (about 40 months). The new semihydrate solvate form retains the crystalline structure of the monohydrate (unit cell parameters and space group) but shows changes in the position of the hydrogen atoms of some OH groups of the azithromycin molecules, leading to a new and different hydrogen bonding network. Moreover, in one of the two azithromycin molecules in the asymmetric unit (the one labelled as “b”), the dimethyl amine group (in the desosamine sugar) shows a significant positional change. This fact provides information about the role of the solvent in the building of the solvated forms of the azithromycin drug, and allows the mobility of this group to be associated with an inversion operation and not a rotation .

Fe(III) complexes were prepared by the reaction of two different ONNO type Schiff bases N,N′-bis (salicylidene)-1,3-propanediamine (LH 2 ) and N,N′-bis (salicylidene)-2,2′-dimethyl-1,3-propane diamine (LDMH 2 ) in N − 3 containing medium. The X-ray diffraction studies revealed that the complexes are of dinuclear type connected with μ -1,1-azido bridges. It was observed from the thermogravimetric analyses of the complexes that the azide group decomposed around 200 °C with exothermic reaction. This exothermic decomposition did not affect the rest of the molecule. Following the decomposition of the azide group the remainder of the molecule decomposed with carbonization process. It was seen that the molecule decomposed into Fe 2 O 3 above 650 °C in oxygen containing medium. [FeLDMN 3 ] 2 complex was found to have N 4 O 2 distorted octahedral sphere with Fe…Fe distance of 3.414 Å.