High pressure - high temperature synthesis and studies of nitride materials

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Distributed north-vergent shear and flattening through Greater and Tethyan Himalayan rocks: Insights from metamorphic and strain data from the Dang Chu region, central Bhutan

In several places in the Himalaya, there are debates over the location of and defining criteria for the South Tibetan detachment (STD) system. Here, we attempt to resolve this debate in central Bhutan by interpreting temperature, pressure, finite strain, and shear-sense data from an 11-km-thick structural transect through the Dang Chu region. Raman spectroscopy on carbonaceous material and garnet-biotite thermometry define a gradual, structurally upward decrease from 600-700 degrees C to 400-500 degrees C, and structural data indicate pure shear-dominant (W-m <= 0.4), layer-normal flattening strain and north-vergent shearing distributed through most of the section. Our data, when combined with published data from central Bhutan, define gradual, structurally upward cooling and an upright pressure gradient that is 1.2-2.4 times lithostatic distributed between 0 and 11 km above the Main Central thrust (MCT). Transport-parallel lengthening varies between similar to 20%-110% at 2-5 km above the MCT and between similar to 5%-55% at 5-11 km above the MCT, and north-vergent shearing is distributed between 2 and 11 km above the MCT. These data rule out the presence of a discrete, normal-sense shear zone and instead illustrate distributed structural thinning accommodated by north-vergent shearing. The strain data allow for similar to 85 km of distributed north-vergent displacement, which may be related to differential southward transport during MCT emplacement. Alternatively, distributed shear may have been translated northward into the STD system in northern Bhutan. Timing constraints for shearing on the MCT and STD allow for both possibilities. Central Bhutan provides a case study for large-scale, distributed structural thinning, and highlights the diverse range of processes that accommodate tectonic denudation during orogenesis

Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transport-parallel stretching and inverted metamorphism

In order to understand mass and heat transfer processes that operated during Himalayan orogenesis, we collected temperature, finite and incremental strain, and kinematic vorticity data through a 5 km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. This transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet-biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are separated by sections in which temperatures remain essentially constant. Temperatures increase from similar to 400-500 degrees C to similar to 700-750 degrees C between 675m below and 200m above the MCT. This defines a 269 +/- 44 degrees C/km inverted gradient, interpreted to have formed via high-magnitude (similar to 100-250 km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from similar to 300-400 degrees C to similar to 400-530 degrees C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. Strain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by layer-normal flattening. Transport-parallel lengthening and foliation-normal shortening increase from 38-71% to 36-49%, respectively, between 2.3 and 1.0 km below the MCT. The MCT acted as a "stretching fault," with translation on the order of hundreds of kilometers accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of tens of kilometers. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer

Bimodal phase percolation model for the structure of Ge-Se glasses and the existence of the intermediate phase

International audienceA detailed nuclear magnetic resonance and Raman study of GexSe1−x glasses indicate that the glass structure is composed of intertwined microdomains of GeSe2 and Sen. Static nuclear magnetic resonance spectra of glasses ranging from 0≤x≤1/3 reveal the absence of Ge-Se-Se fragments in the structure. High temperature nuclear magnetic resonance showing considerable line narrowing confirms this observation. More importantly, the fraction of Se-Se-Se obtained by integration of nuclear magnetic resonance lines matches closely the percentage predicted for a bimodal phase model and is not consistent with the existence of Ge-Se-Se fragments. Raman spectra collected on the same glass also confirm the existence of GeSe2 domains up to high selenium concentrations. The mobility of the Sen phase observed at high temperature while the GeSe2 phase remains rigid is consistent with their respective underconstrained and overconstrained structural nature. The proposed bimodal phase percolation model is consistent with the original Phillips and Thorpe theory however it is clearly at odds with the intermediate phase model which predicts large amounts of Ge-Se-Se fragments in the structure. A calorimetric study performed over a wide range of cooling/heating rates shows a narrow composition dependence centered at ⟨r⟩=2.4 in contrast with the wide reversibility window observed by Modulated Differential Scanning Calorimetry. This suggests that the observation of the reversibility window associated with the intermediate phase in Ge-Se glasses could be an experimental artifact resulting from the use of a single modulation frequency

Cell assemblies for reproducible multi-anvil experiments (the COMPRES assemblies)

The multi-anvil high-pressure technique is an important tool in high-pressure mineralogy and petrology, as well as in chemical synthesis, allowing the treatment of large (millimeter-size) samples of minerals, rocks, and other materials at pressures of a few GPa to over 25 GPa and simultaneous uniform temperatures up to 2500 °C and higher. A series of cell assemblies specially designed and implemented for interlaboratory use are described here. In terms of the size of the pressure medium and the anvil truncation size, the five sizes of assemblies developed here are an 8/3, 10/5, 14/8, 18/12, and 25/15 assembly. As of this writing, these assemblies are in widespread use at many laboratories. The details of design, construction, and materials developed or used for the assemblies are presented here

Strong dichroic emission in the pseudo one dimensional material ZrS3

Zirconium trisulphide (ZrS3), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS3 are highly anisotropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D (planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.NSF (DMR-1552220); Flemish Science Foundation (FWO-Vl); Methusalem foundation of the Flemish government; FW

Unusual lattice vibration characteristics in whiskers of the pseudo-one-dimensional titanium trisulfide TiS3

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS3) whiskers are made out of weakly interacting TiS3 layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS3 both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS3 have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S-S molecular oscillations. High-pressure Raman studies further reveal that the Ag S-S S-S molecular mode has an unconventional negative pressure dependence, whereas other peaks stiffen as anticipated. Various vibrational modes are doubly degenerate at ambient pressure, but the degeneracy is lifted at high pressures. These results establish the unusual vibrational properties of TiS3 with strong in-plane anisotropy, and may have relevance to understanding of vibrational properties in other anisotropic two-dimensional material systems. © The Author(s) 2016.National Science Foundation (DMR-1552220 - CMMI-1561839); Flemish Science Foundation (FWO-Vl); The Science Academy, Turke