D. Yu. Popov

Growth ordering and anomalous birefringence in ugrandite garnets

Abstract Intermediate members of the grossular−andradite and grossular−uvarovite series are known to display anomalous birefringence, which is inconsistent with the ideal cubic space group of garnet Ia3̅d. To determine the reason for such birefringence, the crystal structures of at least 15 samples were refined by different authors including the present ones. The crystals with the value of anomalous birefringence of ∆n > 0.001 are normally characterized by partial ordering of the octahedral cations (Al/Fe and Al/Cr for grossular−andradite and grossular−uvarovite series, respectively). This reduces the symmetry to the orthorhombic space group Fddd or even to the triclinic space group I1̅ . As is seen from the distribution of occupancies over octahedral sites in the triclinic space group, eight occupancies are grouped into two quartets of similar occupancies, leading to pseudo-orthorhombic crystal structures. The variety of structures with different degrees of pseudo-orthorhombicity is due to the action of the growth dissymmetrization mechanism. Simulations of the optical indicatrix in the point-dipole approximation confirm cation ordering as the main cause of the anomalous birefringence.

An application of the point-dipole model to the problem of optical anomalies in grandite garnets

Abstract The origin of anomalous birefringence in grossular-andradite (grandite) garnets is considered. Using the point-dipole model we have calculated the anomalous optical indicatrix for four crystals (three from the paper by Takéuchi et al. (1982) and one studied previously by the present authors). The calculated indicatrix is in good agreement with the observed one. This suggests that the origin of anomalous birefringence in grandite garnets is due to Al/Fe ordering. Interrelations between the observed optical indicatrix, the structure of the growing face, the site occupancy in the crystal structure and the calculated optical indicatrix suggest a growth origin of Fe/Al ordering in grandite garnets and confirm the growth dissymmetrization hypothesis.

Transformation of tricyclic makaluvamines from the marine sponge Zyzzya fuliginosa into damirones

Makaluvamines H and C isolated from the marine sponge Zyzzya fuliginosa were transformed into damirones A and B, respectively, by alkaline hydrolysis. The crystal structure of damirone A was established by X-ray diffraction analysis.

3β-Methoxyolean-18-ene (miliacin) from the marine fungus Chaetomium olivaceum

A pentacyclic tritepenoid, 3β-methoxyolean-18-ene (miliacin), was isolated for the first time from the marine fungus Chaetomium olivaceum. Its structure was determined on the basis of 2D NMR spectroscopy and X-ray diffraction data.

Crystal Structures of LiCsTiF6 and Cs2TiF6 and Internal Mobility of Complex Anions

Single crystals of LiCsTiF6(I) and Cs2TiF6(II) are characterized by X-ray diffraction analysis (DAR-UMBk and SMART 1000 CCD diffractometers, MoKα radiation, graphite monochromator; anisotropic least squares refinement to R = 0.038 and Rw = 0.046 (I) and R1 = 0.017 and wR2 = 0.046 (II)). Packing modes of structural units in the crystals are considered. Types of internal motions of complex TiF62– ions are established, and their activation energies in the crystals of LiCsTiF6 and Cs2TiF6 are estimated in the temperature range 200–500 K.

Optical properties of oxides and fluorides with a rutile-type structure in terms of the point-dipole model

The polarizabilities of ions in the MgF2, ZnF2, TiO2, and SnO2 compounds have been calculated based on the point-dipole model. It is shown that cation polarizabilities produce a stronger effect on the bire-fringence of AX2(X = F, O) compounds than anion polarizabilities.

Crystal Structure of NH4NaTiF6and Internal Mobility of the Complex Anions

Single crystals of NH4NaTiF6are studied by X-ray diffraction analysis (automated diffractometer, MoKαradiation, graphite monochromator, sin θ/λ ≤ 1.0 Å–1, 1566 reflections with I> 3σ(I ), anisotropic least squares method to R= 0.045 and Rw= 0.040). The compound belongs to the NaRbSnF6structural type. The types of internal motions of the complex ions NH4+and TiF62–are determined in the temperature range 200–500 K and compared with the motion of ionic groups in (NH4)2TiF6and Na2TiF6crystals.

Crystal Structure and Dynamics of Water Molecules and Fluoride Ions in NaKSbF5·1.5H2O and NaRbSbF5·1.5H2O

AbstractThe atomic structures of crystal hydrates were determined at 173 and 293 K for NaKSbF5·1.5H2O (I) and at 293 K for NaRbSbF5·1.5H2O (II). Crystals I and II are isostructural, space group

Synthesis and Structure of Sb-Containing Compounds of Binuclear Phenols

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