52 research outputs found
Compressibility of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4âxAlxOxN7âx] (x = 2; M = Sr, Ba)
The compressibilities of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4âxAlxOxN7âx] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42â
GPa were generated using the diamondâanvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order BirchâMurnaghan equation-of-state to the pâV data results in V0 = 302.91â
(6)â
Ă
3, B0 = 176â
(2)â
GPa and BâČ = 4.4â
(2) for SrYb[Si4N7]; V0 = 310.4â
(1)â
Ă
3, B0 = 161â
(2)â
GPa and BâČ = 4.6â
(2) for SrYb[Si4âxAlxOxN7âx]; and V0 = 317.3â
(5)â
Ă
3, B0 = 168â
(2)â
GPa and BâČ = 4.7â
(2) for BaYb[Si4âxAlxOxN7âx]. While the linear compressibilities of the a and c axes of BaYb[Si4âxAlxOxN7âx] are very similar up to 30â
GPa, distinct differences were observed for SrYb[Si4N7] and SrYb[Si4âxAlxOxN7âx], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role
High pressure x-ray diffraction study of the volume collapse in Ba24Si100 clathrate
International audienceThe high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu et al., Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical
The hydration structure of Cu2+: More tetrahedral than octahedral?
A comprehensive multi-technique approach has been used to address the controversial question of the preferred geometric form of the Cu2+ aqua-ion hydration shell. A combination of H/D isotopic substitution neutron scattering and X-ray scattering has been used to refine atomistic models of 0.5 m and 2.0 m solutions of Cu(ClO4)2, that have also been constrained to simultaneously reproduce detailed local structure information about the cation environment obtained by X-ray Absorption spectroscopy. The adoption of the Empirical Potential Structure Refinement (EPSR) technique as a single unified analytical framework minimises the chances for biasing the result in favour of a specific pre-conceived outcome. The results are consistent with an average coordination for each Cu2+ ion of 4.5 ± 0.6 water molecules that matches the more recent picture of five-fold coordination in a 2.0 m solution, but interestingly this combined study highlights that the preferred local geometry of the ion sites is found to have a mixed character of tetrahedral, trigonal bipyramidal and octahedral components. A further point to note is that this new model adds support to a largely ignored result in the literature relating to the linear electric field effect induced g-shifts observed in the electron paramagnetic resonance spectra of glassy Cu2+ complexes (Peisach and Mims, Chem. Phys. Lett., 1976, 37, 307â310) that first highlighted the importance of tetrahedral distortions in the cation's hydration shell structure
High-pressure phase transformations, pressure-induced amorphization, and polyamorphic transition of the clathrate Rb6.15Si46
International audienceThe type-I clathrate Rb6.15Si46 with partly empty cage sites has been studied up to 36 GPa using Raman spectroscopy, synchrotron x-ray diffraction in diamond-anvil cells, and ab initio total-energy and lattice-dynamics calculations. A first phase transition is observed at 13±1âGPa and a "volume collapse" transition within the clathrate structure is then observed at 24±1âGPa. Pressure-induced amorphization into a high-density amorphous (HDA) state occurs above P=33±1âGPa. The HDA form transforms into a low-density amorphous polymorph during decompression. During the compression study using angle dispersive synchrotron x-ray diffraction techniques we measured bulk modulus parameters for rocksalt-structured TaO, included adventitiously in the clathrate sample [K0=293(3)âGPa and KâČ0=5.4(3)]
High Pressure X-ray Diffraction Study of MgMn2O4 Tetragonal Spinel
The phase stability of the MgMn2O4 spinel has been studied by means of
high-pressure X-ray diffraction for pressures up to 30 GPa. Two samples with
different inversion degrees have been considered. Both spinels undergo a phase
transition towards an orthorhombic structure (CaMn2O4-type). For the more
inverted sample the transition pressure is at least 1 GPa lower with respect to
that of the less inverted spinel. Also the volume contraction, relative
compressibility and density trends are different for the two samples. These
variations have been explained according to differences in the cation
distribution. and electronic properties of the samples.Comment: 12 pages; 4 Figures presented at the SRMS-
The electronic structure, surface properties, and in situ N2O decomposition of mechanochemically synthesised LaMnO3
The use of mechanochemistry to prepare catalytic materials is of significant interest; it offers an environmentally beneficial, solvent-free, route and produces highly complex structures of mixed amorphous and crystalline phases. This study reports on the effect of milling atmosphere, either air or argon, on mechanochemically prepared LaMnO3 and the catalytic performance towards N2O decomposition (deN2O). In this work, high energy resolution fluorescence detection (HERFD), X-ray absorption near edge structure (XANES), X-ray emission, and X-ray photoelectron spectroscopy (XPS) have been used to probe the electronic structural properties of the mechanochemically prepared materials. Moreover, in situ studies using near ambient pressure (NAP)-XPS, to follow the materials during catalysis, and high pressure energy dispersive EXAFS studies, to mimic the preparation conditions, have also been performed. The studies show that there are clear differences between the air and argon milled samples, with the most pronounced changes observed using NAP-XPS. The XPS results find increased levels of active adsorbed oxygen species, linked to the presence of surface oxide vacancies, for the sample prepared in argon. Furthermore, the argon milled LaMnO3 shows improved catalytic activity towards deN2O at lower temperatures compared to the air milled and sol-gel synthesised LaMnO3. Assessing this improved catalytic behaviour during deN2O of argon milled LaMnO3 by in situ NAP-XPS suggests increased interaction of N2O at room temperature within the O 1s region. This study further demonstrates the complexity of mechanochemically prepared materials and through careful choice of characterisation methods how their properties can be understood
Impact of abutment geometry on early implant marginal bone loss. A double-blind, randomized, 6-month clinical trial
Objectives
The objective of this study was to analyze the impact of the abutment width on early marginal bone loss (MBL).
Material and Methods
A balanced, randomized, double-blind clinical trial with two parallel experimental arms was conducted without a control group. The arms were âcylindricalâ abutment and âconcaveâ abutment. Eighty hexagonal internal connection implants, each with a diameter of 4âĂâ10âmm, were placed in healed mature bone. The main variable was the peri-implant tissue stability, which was measured as MBL at 8âweeks and 6âmonths.
Results
The final sample consisted of 77 implants that were placed in 25 patients. 38 (49.4%) were placed using the cylindrical abutment, and the other 39 (50.6%) were placed using the concave abutment. The early global MBL of â0.6â±â0.7âmm in the cylindrical abutment group was significantly higher than it was in the concave abutment group, in which the early global MBL was â0.4â±â0.6âmm (p = .030). The estimated effect size (ES) was negative for the cylindrical abutment (ES = â1.3730, CI â2.5919 to â0.1327; t-value = â2.4893; p = .0139), therefore implying a loss of mean bone level, and it was positive for the concave abutment (ES = 2.8231; CI: 1.4379 to 4.2083; t-value = 4.0957; p = .0002), therefore implying an increase in the average bone level.
Conclusions
The concave abutments presented significantly less early MBL at 6âmonths post-loading than classical cylindrical abutments did
Radio continuum observations of Class I protostellar disks in Taurus: constraining the greybody tail at centimetre wavelengths
We present deep 1.8 cm (16 GHz) radio continuum imaging of seven young
stellar objects in the Taurus molecular cloud. These objects have previously
been extensively studied in the sub-mm to NIR range and their SEDs modelled to
provide reliable physical and geometrical parametres.We use this new data to
constrain the properties of the long-wavelength tail of the greybody spectrum,
which is expected to be dominated by emission from large dust grains in the
protostellar disk. We find spectra consistent with the opacity indices expected
for such a population, with an average opacity index of beta = 0.26+/-0.22
indicating grain growth within the disks. We use spectra fitted jointly to
radio and sub-mm data to separate the contributions from thermal dust and radio
emission at 1.8 cm and derive disk masses directly from the cm-wave dust
contribution. We find that disk masses derived from these flux densities under
assumptions consistent with the literature are systematically higher than those
calculated from sub-mm data, and meet the criteria for giant planet formation
in a number of cases.Comment: submitted MNRA
Combined spatially resolved operando spectroscopy: New insights into kinetic oscillations of CO oxidation on Pd/Îł-Al 2O3
Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/Îł-Al 2 O 3 during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first âŒ1 mm of the bed, which gives rise to oscillations in CO 2 and O 2 concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O 2 for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies
Grain growth across protoplanetary discs: 10-micron silicate feature versus millimetre slope
Young stars are formed within dusty discs. The grains in the disc are
originally of the same size as interstellar dust. Models predict that these
grains will grow in size through coagulation. Observations of the silicate
features at micron wavelengths are consistent with growth to micron sizes
whereas the slope of the SED at longer wavelengths traces growth up to mm
sizes. We here look for a correlation between these two grain growth
indicators. A large sample of T-Tauri and Herbig-Ae/Be stars was observed with
the Spitzer Space Telescope at 5-13 micron; a subsample was observed at mm
wavelengths. We complement this subsample with data from the literature to
maximise the overlap between micron and mm observations and search for
correlations. Synthetic spectra are produced to determine which processes may
produce the dust evolution. Dust disc masses in the range <1 to 7 x 10^-4 MSun
are obtained. Most sources have a mm spectral slope consistent with grain
growth. There is a tentative correlation between the 10-micron silicate feature
and the mm slope of the SED. The observed sources seem to be grouped per
star-forming region in the micron-vs-mm diagram. The modelling results show
that the 10-micron feature becomes flatter and subsequently the mm slope
becomes shallower. Grain size distributions shallower than that of the ISM
and/or bright central stars are required to explain specific features. Settling
of larger grains towards the disc midplane affects the 10-micron feature, but
hardly the mm slope. The tentative correlation between the strength of the
10-micron feature and the mm slope suggests that the inner and outer disc
evolve simultaneously. Dust with a mass dominated by mm-sized grains is
required to explain the shallowest mm slopes. Other processes besides grain
growth may also be responsible for the removal of small grains.Comment: 23 pages, 14 figures, accepted by A&
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