Volume 217, Issue 10

Fundamental limitations, with respect to nanocrystalline materials, of the traditional elaboration of powder diffraction data like the Rietveld method are discussed. A tentative method of the analysis of powder diffraction patterns of nanocrystals based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the “ apparent lattice parameter ”, alp ) is introduced. We examine the application of our methodology using theoretical diffraction patterns computed for models of nanocrystals with a perfect crystal lattice and for grains with a two-phase, core-shell structure. We use the method for the analysis of X-ray and neutron experimental diffraction data of nanocrystalline diamond powders of 4, 6 and 12 nm in diameter. The effects of an internal pressure and strain at the grain surface are discussed. The results are based on the dependence of the alp values on the diffraction vector Q and on the PDF analysis. It is shown, that the experimental results lend a strong support to the concept of a two-phase structure of nanocrystalline diamond.

Two non-stoichiometric chromium rich Cr 5+ x S 8 ( x  = 0.20 and 0.26) samples were prepared under high pressure and high temperature conditions. In the crystal structures three different metal atom sites are fully and the fourth site is only partially occupied. The results of single crystal and Rietveld refinements are in good agreement. The CrS 6 octahedra share common corners and common faces. Across common faces extraordinary short Cr—Cr distances are observed suggesting metal-to-metal bonding. On the basis of the distortion of the CrS 6 octahedra Cr 4+ is located on two of the four Cr sites. With increasing Cr content the amount of Cr 4+ is lowered and the distortion of two of the four CrS 6 octahedra is significantly reduced.

Bismuth germanate single crystals containing low angle and very low angle boundaries were selected as specimen for annealing experiments at 1000 °C for varying lengths of time in the range: 8 h to 80 h. Specimen with larger surface along (111) and (100) planes were investigated. The tilt angles of the boundaries were found to be in the range: 16 arc sec to 88 arc sec. Half widths of the diffraction curves of different subgrains were in the range: 15 arc sec to 39 arc sec. High resolution X-ray diffractometry and topography were employed for structural characterization and secondary ion mass spectrometry (SIMS) was used for study of impurity distribution. Annealing led to significant improvement in the crystalline perfection of specimen and all but one of the boundaries could be removed. The very low angle boundaries, which could not be annealed out, had a tilt angle of 41 arc sec. The half widths of diffraction curves of the two remaining subgrains were: 10 arc sec and 17 arc sec. The growth of subgrains as a result of annealing was investigated. SIMS investigation of annealed specimen, in which a very low angle boundary persisted, showed the boundary was decorated with silicon impurity, which presumably acted as a pining center. Detailed diffractometric and topographic experiments helped in delineating the two subgrains around the very low angle boundary. The morphological features of the very low angle boundary were studied in detail.

The high pressure behaviour of 3R-NbS 2 has been investigated by angle-dispersive X-ray powder diffraction using diamond anvil cells up to 14 GPa. The compression behaviour of the structure is highly anisotropic. The compressibility perpendicular to the layers is 2.5 times higher than within the layers. A fit of a 3 rd -order Birch-Murnaghan equation of state gave a volume at zero pressure V 0 =174(1) Å 3 and a bulk modulus b 0 =57(1) GPa, with a pressure derivative b '=8.6(5).

Crystal structures of both isostructural com-pounds were solved from single crystal X-ray diffraction data (space group P 2 1 / n ; Z = 4; Rb-compound: a = 9.812(1) Å, b = 25.793(3) Å, c = 11.777(2) Å, β = 104.68(1)°, R gt ( F ) = 0.0458, wR ref ( F 2 ) = 0.0918; Cs-compound: a = 9.9058(8) Å, b = 25.919(2) Å, c = 11.883(1) Å,β = 105.542(7)o, R gt (F) = 0.0450,wR ref (F 2 ) = 0.0951. The main structural feature is the nitrosylpentacyanoferrate(II)-complex [Fe(CN) 5 NO] 2– . The iron atoms are coordinated to five cyanide and one nitrosyl ligand. The coordination polyhedron of the Y 3+ cation is a distorted tetragonal antiprism built up from three nitrogen atoms of three complexes and five water molecules. Both rubidium and cesium ions are surrounded by seven cyanide groups of five complexes and one water molecule to form a distorted bicapped trigonal prism. One metastable state (SI) can be excited by irradiation with light in the spectral range of 400–500 nm at temperatures below 180 K. The radiationless thermal decay is detected by Differential Scanning Calorimetry by heating above T = 180 K. Evaluating the exothermal heat flow using the Arrhenius law yields an activation energy of E A = 0.64(2) eV and a frequency factor of Z = 20(8)·10 12 s –1 for the rubidium compound and E A = 0.56(2) eV, Z = 30(8)·10 10 s –1 for the cesium compound. The contribution of the cations on the decay temperature of the metastable states is discussed on the basis of the atomic distances and coordinations.

The crystal structures of [N(CH 3 ) 4 ] 2 HgCl 4 between 191 K and 350 K have been studied by single crystal X-ray diffraction. Below T c 1 = 277 K the structure is monoclinic P 2 1 / a with lattice parameters a = 12.320 (7), b = 9.034 (4), c = 15.402 (4) Å, and β = 89.64 (1)° at T = 191 K. Above T c 1 the structure is orthorhombic Pnma with lattice parameters 12.419 (7), b = 9.068 (4), and c = 15.684 (2) Å at room temperature, and with lattice parameters a = 12.421 (9), b = 9.125 (1), and c = 15.777(4) Å at T = 350 K. At room temperature each TMA cation disorderly occupies two orientations (TMA is tetramethylammonium). At low temperature the structure is fully ordered, with additional shifts and rotations of all ions, in accordance with the results of Asahi et al ., J. Phys. Soc. Jpn. 60 (1991) 921. Refinements using restrictions of the non-crystallographic tetrahedral point group for the TMA cations and refinements without restrictions are used to show that TMA has tetrahedral symmetry at all temperatures. The HgCl 4 anions are distorted, but the distortions are independent of the temperature. The phase transition at T c 1 is described as an order/disorder transition accompanied by major rotations and shifts of the ions. New evidence for a previously reported phase transition at T c 2 = 320 K (Zangar et al. Phase Transitions 12 (1988) 141) is obtained from NMR. This transition is shown to be a non symmetry breaking structural transition, that correspond to a gradual loss of the orientational order of the TMA cations.

The crystal structure of bismutocolumbite, Bi(Nb 0.79 Ta 0.21 )O 4 with orthorhombic (sp. gr. Pnna ) stibio-tantalite structure type at ambient conditions has been de-termined at 2.59 and 9.56 GPa, and 23 o C. Crystal data and results of structure refinement using powder X-ray diffrac-tion and Rietveld analysis for Bi-rich stibiocolumbite, (Sb 0.52 Bi 0.48 )(Nb 0.71 Ta 0.29 )O 4 , are reported. At the pressure ~2.9 GPa bismutocolumbite exhibits phase transition which is accompanied by the change of its symmetry: sp. gr. Pnna is replaced by the sp. gr. Pn 2 1 a . This kind of transformation is anticipated at ambient pressure in bismutocolumbite- stibiocolumbite solid solution series in the compounds with Sb/(Bi+Sb) ratio higher than 0.52 which characterises the studied crystals of stibiotantalite. Similarly with scheelite-like BiVO 4 , in bismutocolumbite the reduction of the unit cell volume under HP is mainly a result of Bi-O polyhedral compression, which is accompanied by a rearrangement of (Nb,Ta)O 6 octahedra within octahedral sheets parallel to (010). (Nb,Ta)O 6 octahedra remain rigid with smaller change in size at high pressure.

The redetermination of the crystal structure of Wilm's salt, tetraammonium hexachlororhodate(III) nitrate, (NH 4 ) 4 [RhCl 6 ]NO 3 from single crystal data, necessitated a change in space group symmetry description, as a conse-quence of comparative structure refinements, from the polar space group R 3 m to the chiral space group R 32 for the room temperature structure. In contrast to the former structure determination it is shown, that in the correct space group a structure model can be derived, in which all parts of the structure – including the H atom positions of the ammonium ions – appear completely ordered on their sites. The ammonium cations and the hexachlororhodate as well as the nitrate anions form a three-dimensional framework via weak N-H···Cl and N-H···O hydrogen bonds. Reproducing the original synthesis conditions and analysing the products by powder diffraction methods, the by-products have been identified as ammonium nitrate and diammonium aquapentachlororhodate(III). Doing DTA experiments on the title compound, two endothermic effects at 54°C and 128°C have been detected that may be related to phase transitions from the ordered room temperature structure to disordered high temperature sub structures. According to the results of thermo gravimetry the title compound decomposes at temperatures above 140°C. The temperature dependant IR spectroscopy in the range of RT-60 C it is probable that the above mentioned thermal effect at 54°C is associated with an order disorder phase transition introduced by the ammonium cations. The UV/Vis spectroscopy show that at 8 K a significant distortion of the [RhCl 6 ] 3– octahedron from highest 32 point symmetry is plausible.