A rapid, solventless method is described for the decoration of carbon nanotubes with metal nanoparticles. The straightforward two-step process utilizes neither reducing agents nor electric current and involves the dry mixing of a precursor metal salt (e.g., a metal acetate) with carbon nanotubes (single- or multi-walled) followed by heating in an inert atmosphere. The procedure is scalable to multigram quantities and generally applicable to various other carbon substrates (e.g., carbon nanofiber, expanded graphite, and carbon black) and many metal salts (e.g., Ag, Au, Co, Ni, and Pd acetates). As a model system, Ag nanoparticle-decorated carbon nanotube samples were prepared under various mixing techniques, metal loading levels, thermal treatment temperatures, and nanotube oxidative acid treatments. These nanohybrids were characterized by a variety of microscopic and spectroscopic techniques. For example, X-ray diffraction and scanning electron microscopy indicated that the average size of the Ag nanoparticles has little to do with the thermal treatment temperature but can be easily controlled by varying the Ag loading. Raman spectroscopy illustrated both the metal-nanotube electronic interactions and the surface enhancement effect from the Ag nanoparticle attachment. High-resolution transmission electron microscopy captured the in situ salt-to-metal conversion events on the nanotube surface. The mechanistic implications from the characterization results are discussed.

Hydrozincite, Zns(OH)e(COs) , is monoclinic with a 0 = 13.62, b 0 = 6-30, c o = 5.42 A, fl = 95 ° 50', space group C2/m, two formula units per cell. The structure has been determined from Patterson projections and refined by the least-squares method with partial three-dimensional intensity data.The structure is composed of zinc in both octahedral and tetrahedral coordination, in the ratio 3:2. The octahedral zinc atoms form part of a C6 type sheet with holes, which are distributed on a rectangular 6.3 × 5-4 A net. Zinc atoms in tetrahedral coordination occur above and below these holes. The complex sheets parallel to (100) are held together by CO a groups, occurring normal to the sheets. Two oxygen atoms of the CO s group are bonded to an octahedral and a tetrahedral zinc atom each, while the third oxygen atom is hydrogen bonded to three OH groups. The average tetrahedral Zn-O distance, 1.95 /~, is significantly smaller than the average octahedral Zn-O distance, 2.10 ~k.The OH:CO a ratio is variable in natural and synthetic samples, the C03-deficient phases being highly stacking-disordered. On the basis of this structure these phenomena are understandable, since missing carbonate groups will facilitate mistakes in the layer sequence.

Abstract. Cryolite, Na3A1F6[=2Na+(Nag.sAlg.~)F3] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF6] and [A1F6] octahedra and the cavities are occupied by Na § ions. At 295 K, it is monoclinic (~ phase), space group P21/n with a=5.4139(7), b=5.6012(5) and c = 7.7769 (8) A and fl = 90.183 (3), Z = 2. A high temperature single crystal X-ray diffraction study in the range 295-900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at TO~885 K, in contrast to a previous report that it becomes cubic at ,-~ 823 K. The space group of the high temperature 1~ phase is Immm with a = 5.632 (4), b= 5.627 (3) and c= 7.958 (4)/~, Z=2 at 890 K. Above To, the coordination number of the Na § ion in the cavity increases from eight to twelve and the zigzag Nal-A1 octahedral chains parallel to c become straight with the Nal -F--A1 angle = 180 ~ The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [A1F6] group and transforms according to the F4 + irreducible representation of Immm. Coupled to the [A1F6] rotation is a second primary order parameter corresponding to the displacement of the Na2 + ion in the cavity from its equilibrium position. This order parameter transforms according to the X~ irreducible representation of Immm. Following Immm ~ P21/n phase transition, four equivalent domains of P21/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral A1-F and Na--F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above To indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The fl phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the ~ phase. This view is consistent with the entropy of phase transition, ASt .... (11.43 JK-1 mol-1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK -1 tool-l), where 4 is the number of domains formed during the phase transition. The dynamic nature of the/~ phase is independently confirmed from a considerable narrowing of the 27A1 nuclear magnetic resonance (NMR) line-shape above To .

Cristobalite, a high temperature phase of silica, SiO2, undergoes a (metastable) first-order phase transifrom a loss of translation vectors 89 (110) (F---,P). These domains are macroscopic and static in e-cristobalite, and microscopic and dynamic in /?-cristobalite. The order parameter t/, couples with the strain components as e~/2, which initiates the structural fluctuations, thereby causing the domain configurations to dynamically interchange in the/?-phase. Hence, the e-/? cristobalite transition is a fluctuation-induced first-order transition and the ]3-phase is a dynamic average of e-type domains.