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The results of detailed structural studies of trigonal lamellar particles of both gold and silver are presented. The particles have been characterized both in sol by means of optical spectroscopy and powder X-ray diffraction and ex sol using high resolution electron microscopy in both plan view and profile imaging modes. The results of these studies have indicated that the particles have a trigonal outline and are shortened along a ≺111≻ direction to give a plate-like morphology. The presence of small numbers of parallel {111} twin planes has also been confirmed and used to explain the presence of the formally forbidden ⅓{422} reflections observed in plan view. The precise structural requirements for the observation of such reflections has also been confirmed using multislice calculations. Possible growth mechanisms for these particles are also discussed.

We have measured the low-temperature ͑T ͒ dependence of the anisotropic penetration depth l͑T ͒ of magnetically aligned powders of HgBa 2 Ca n21 Cu n O 2n121d (n 1 and 3) down to 1.2 K. For both members the T dependence of the in-plane penetration depth l ab ͑T ͒ is strongly linear, whereas the outof-plane component l c ͑T͒, for n 1 and 3, varies as T 5 and T 2 , respectively. For comparison, we also report l c ͑T͒ data for grain-aligned YBa 2 Cu 3 O 72d (d 0.0 and 0.43) which vary as T and T 2 , respectively. The results are discussed in terms of d x 2 -y 2 -wave symmetry of the order parameter in cuprates.[S0031-9007 (97)04087-8] PACS numbers: 74.25.Nf, 74.72.Bk, 74.72.GrThe high-T c superconductors (HTSC) containing CuO 2 planes are layered materials with anisotropic physical properties. Evidence is now growing that the energy gap in HTSC has "d-wave" symmetry with nodes in the order parameter in certain directions in k space [1,2]. Detailed knowledge of the behavior of the superfluid density and the symmetry of the energy gap are important for understanding various properties of the superconducting state including the pairing mechanism in these materials. One of the best probes of the superfluid density and the energy gap morphology at the Fermi surface is the penetration depth ͑l͒. Many early studies of the temperature ͑T ͒ dependence of l on single crystals, films, and aligned powders of yttrium barium copper oxide (YBCO) gave evidence for non-s-wave pairing, usually in the form of a T 2 behavior for l͑T ͒ at low temperatures rather than exponential dependence expected for s wave. However, this was not widely accepted until the work of Hardy et al. [3] on high quality YBCO crystals which showed a linear temperature dependence in the in-plane penetration depth from 1.3 to 20 K. We have found that in some cases, structural and chemical defects near the surface of YBCO crystallites have significant effects on l at low temperatures and that heat treating the crystallites after grinding in air, or grinding under argon, helped obtain higher quality surfaces and thus intrinsic information from aligned powder composites [4]. Most studies of l have concentrated on YBCO [1,3,5] and Zn doped YBCO [5,6]. Relatively few studies have examined other materials such as La 22x

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We report the discovery of a new, chemical route for 'activating' the hydrogen store MgH2, that results in highly effective hydrogen uptake/release characteristics, comparable to those obtained from mechanically-milled material.