113 research outputs found
Crystal structure of the high-pressure phase of the oxonitridosilicate chloride Ce4[Si4O3â +â xN7â ââ x]Cl1â ââ xOx, xâ0.2
The structural compression mechanism of Ce4[Si4O3â
+â
xN7â
ââ
x]Cl1â
ââ
xOx, xâ 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6â
GPa using the diamondâanvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6â
GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. The main structural mechanisms, leading to a volume reduction of about 5% at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2, Ce3 split position, and a bending of some of the inter-polyhedral SiâNâSi angles in the arrangement of the corner-sharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure
Incommensurate Phase in Î-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive NâHâ â â O and CâHâ â â O Hydrogen Bond Networks
The intermediate incommensurate modulated structure of Î-cobalt (III) sepulchrate trinitrate is ordered yet retains amplitude of molecular rotations common to high temperature disordered as well as low temperature high ZâČ structure. Contributions from bifurcated NâHâ
â
â
O bonds as well as very short Hâ
â
â
H contacts in addition to CâHâ
â
â
O bonds leads to severe frustrations in crystal packing than that in the lock-in phase
Phase stability of lanthanum orthovanadate at high-pressure
When monoclinic monazite-type LaVO4 (space group P21/n) is squeezed up to 12
GPa at room temperature, a phase transition to another monoclinic phase has
been found. The structure of the high-pressure phase of LaVO4 is indexed with
the same space group (P21/n), but with a larger unit-cell in which the number
of atoms is doubled. The transition leads to an 8% increase in the density of
LaVO4. The occurrence of such a transition has been determined by x-ray
diffraction, Raman spectroscopy, and ab initio calculations. The combination of
the three techniques allows us to also characterize accurately the pressure
evolution of unit-cell parameters and the Raman (and IR)-active phonons of the
low- and high-pressure phase. In particular, room-temperature equations of
state have been determined. The changes driven by pressure in the crystal
structure induce sharp modifications in the color of LaVO4 crystals, suggesting
that behind the monoclinic-to-monoclinic transition there are important changes
of the electronic properties of LaVO4.Comment: 39 pages, 6 tables, 7 figure
Hexagonal Si-Ge Class of Semiconducting Alloys Prepared Using Pressure and Temperature
Multi-anvil and laser-heated diamond anvil methods have been used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500â1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery atambient pressure of hexagonal Ge-Si solid solutions (P6/mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be preparedfor all Ge-Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge-Sisolid solution family, and is of interest for optoelectronic applications
Chemical analysis of trace elements at the nanoscale in samples recovered from laser-heated diamond anvil cell experiments
Abstract: High pressure and high temperature experiments performed with laser-heated diamond anvil cells (LH-DAC) are being extensively used in geosciences to study matter at conditions prevailing in planetary interiors. Due to the size of the apparatus itself, the samples that are produced are extremely small, on the order of few tens of micrometers. There are several ways to analyze the samples and extract physical, chemical or structural information, using either in situ or ex situ methods. In this paper, we compare two nanoprobe techniques, namely nano-XRF and NanoSIMS, that can be used to analyze recovered samples synthetized in a LH-DAC. With these techniques, it is possible to extract the spatial distribution of chemical elements in the samples. We show the results for several standards and discuss the importance of proper calibration for the acquisition of quantifiable results. We used these two nanoprobe techniques to retrieve elemental ratios of dilute species (few tens of ppm) in quenched experimental molten samples relevant for the formation of the iron-rich core of the Earth. We finally discuss the applications of such probes to constrain the partitioning of trace elements between metal and silicate phases, with a focus on moderately siderophile elements, tungsten and molybdenum
High-Pressure Synthesis of ÎČ-Ir4B5 and Determination of the Compressibility of Various Iridium Borides
"A new iridium boride, beta-Ir4B5, was synthesized under high-pressure/high-temperature conditions of 10.5 GPa and 1500 degrees C in a multianvil press with a Walker-type module. The new modification beta-Ir4B5 crystallizes in a new structure type in the orthorhombic space group Pnma (no. 62) with the lattice parameters a = 10.772(2) angstrom, b = 2.844(1) angstrom, and c = 6.052(2) angstrom with R1 = 0.0286, wR2 = 0.0642 (all data), and Z = 2. The structure was determined by single-crystal X-ray and neutron powder diffraction on samples enriched in B-11. The compound is built up by an alternating stacking of boron and iridium layers with the sequence ABA'B'. Additionally, microcalorimetry, hardness, and compressibility measurements of the binary iridium borides alpha-Ir4B5, beta-Ir4B5, Ir5B4, hexagonal Ir4B3-x and orthorhombic Ir4B3-x were carried out and theoretical investigations based on density function theory (DFT) were employed to complement a comprehensive evaluation of structure-property relations. The incorporation of boron into the structures does not enhance the compressibility but leads to a significant reduction of the bulk moduli and elastic constants in comparison to elemental iridium.
Kinematische Beschreibung von Röntgeninterferenzen bei Anisotroper Anomaler Streuung (AAS)
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