Influence of the shoot density of Halodule wrightii Ascherson from rocky and sandy habitats on associated macroalgal communities

This study evaluated the influence of the shoot density of the shoal grass Halodule wrightii on the composition of the associated algal community, in rocky and sandy habitats on the coast of Ceará in northeastern Brazil. The phycological community included 18 species in 10 families, members of Rodophyta (72.2%), Chlorophyta (22.2%) and Phaeophyceae (5.5%). The largest proportion were epilithic (50%), followed by epiphytes on H. wrightii (38.4%), epipsammics (8%), and epiphytes on other algae (4%). Epiphytes on H. wrightii occurred mainly associated with rhizomes, but also tendrils of H. musciformis occurred attached to the leaves. The phycological community varied according to the density of H. wrightii, independently of particular characters of the meadows, although both habitat and other environmental variables seemed to influence the macroalgae composition and diversity. The rocky habitat was more diverse than the sandy habitat, but in the sandy habitat the shoal grass was important for algal settlement in areas where hard substrates were scarce or absent

Structure and conductivity of the NASICON analog Na/sub 3/SC/sub 2/(PO/sub 4/)/sub 3/

Neutron and x-ray diffraction data of Na/sub 3/SC/sub 2/(PO/sub 4/)/sub 3/ are presented that show the transition from an ordinary-conducting, high temperature phase to a still higher temperature, superionic conducting phase. There are at least four polymorphic modifications. At room temperature, the refined neutron diffraction data confirm the Cc space group reported by Efremov and Kalinin but with a = 16.0449(24), b = 8.9225(15), c = 9.0656(13)A, ..beta.. = 126.918(21), R/sub weighted pattern/ = 17.5%, and R/sub expected/ = 7%. Between 25/sup 0/C and 64/sup 0/C, a second polymorph appears. The structure is monoclinic but has not yet been refined. At 64/sup 0/C, a third polymorph C appears. It is rhombohedral R3c. Using hexagonal axes, a = 8.9273(2), c = 22.3668A, R/sub wp/ = 6.28% and R/sub e/ = 3.83% at 100/sup 0/C. At 166/sup 0/C, the high temperature polymorph D appears. It is the superconducting phase of NASICON(Sc). It, too, is rhombohedral R3c with a = 8.9274(1), c = 22.5493(6)A, R/sub wp/ = 5.81% and R/sub e/ = 3.87% at 225/sup 0/C. The diffraction data are correlated with ionic conductivity measurements as a function of temperature

/sup 7/Li NMR relaxation in superionic. beta. -lithium aluminum

NMR measurements have been carried out on stoichiometric ..beta..-LiAl samples as a function of temperature and frequency. The /sup 7/Li spin-lattice relaxation was found to be predominantly dipolar. Data were analyzed using several diffusion models. The results show that a fraction of the constitutional Li vacancies in ..beta..-LiAl is bound to the vicinity of a Li antisite defect. The remaining free vacancies are responsible for the fact that LiAl is a superionic Li conductor. The spin-spin relaxation results support this picture of two different vacancy environments. The high temperature results demonstrate still another type of Li diffusion related to thermally generated defects. 41 refs., 6 figs., 2 tabs

Proton density modulation of D atoms in PdD/sub 1-x/

Recent resistivity and neutron diffraction measurements have provided evidence for ordering of D(H) atoms on the octahedral interstitial sites of PdD/sub 1-x/. This order--disorder transition is responsible for the 50 K anomaly which has been reported in many of the physical properties. Neutron diffraction measurements on a PdD/sub 0/ /sub 76/ single crystal revealed satellite reflection at (4/5,2/5,0) and equivalent positions. These satellites can be accounted for by a multi-domained tetragonal unit cell with a/sub t/ = a/sub c/ root 5/2 and c/sub t/ = c/sub a/. This ordered state can be described as a deuteron density wave along a cubic direction. This density is modulated such that four fully occupied planes (Pd and D) are followed by a vacant plane (Pd only)

Chemical spectroscopy

The purpose of chemical spectroscopy with neutrons is to utilize the dependence of neutron scattering cross-sections on isotope and on momentum transfer (which probes the spatial extent of the excitation) to understand fundamental and applied aspects of the dynamics of molecules and fluids. Chemical spectroscopy is divided into three energy ranges: vibrational spectroscopy, 25-500 MeV, for which much of the work is done on Be-filter analyzer instruments; low energy spectroscopy, less than 25 MeV; and high resolution spectroscopy, less than 1 MeV, which typically is performed on backscattering spectrometers. Representative examples of measurements of the Q-depenence of vibrational spectra, higher energy resolution as well as extension of the Q-range to lower values at high energy transfers, and provisions of higher sensitivities in vibrational spectroscopy are discussed. High resolution, high sensitivity, and polarization analysis studies in low energy spectroscopy are discussed. Applications of very high resolution spectroscopy are also discussed. (LEW

Lattice dynamics of the mixed-conducting intermetallic compound,. beta. -LiAl

The intermetallic compound, ..beta..-LiAl, that crystallizes in the uncommon Zintl structure is a mixed-conducting electrode and has many unusual properties pointing to the existence of unusual bonding in the semi-metallic compound. In order to elucidate the nature of the bonding in LiAl, we have studied the lattice dynamics of ..beta..-LiAl by inelastic neutron scattering. Results for the phonon dispersion curves have been obtained for the principal symmetry directions. A force constant fit to the results indicates that the Al-Al force constants are unusually large. Pair potentials were constructed by conventional pseudopotential calculations. The pair interactions favoring the Zintl structure were used to compute phonon dispersion curves. Good agreement between theory and experiment can be obtained for the acoustic branches