scholarly journals Vibrational Spectroscopy of GaN:Mg Under Pressure

2001 ◽  
Vol 693 ◽  
Author(s):  
M.D. McCluskey ◽  
K.K. Zhuravlev ◽  
M. Kneissl ◽  
W. Wong ◽  
D. Treat ◽  
...  
Keyword(s):  
Ambient Pressure ◽  
Stable Configuration ◽  
Microscopic Structure ◽  
First Principles Calculations ◽  
Free Standing ◽  
Pressure Derivative ◽  
Local Vibrational Mode ◽  
Bond Center ◽  
Diamond Anvil ◽  
Effect Of Hydrostatic Pressure

AbstractThe microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration indeed gives rise to a frequency shift consistent with experiment. Based on the comparison between theory and experiment, we can exclude the bond-center configuration, which would result in a much larger pressure derivative than experimentally observed.

scholarly journals Can quasicrystals survive in planetary collisions?

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Vincenzo Stagno ◽  
Luca Bindi ◽  
Sota Takagi ◽  
Atsushi Kyono
Keyword(s):  
Ambient Pressure ◽  
Structural Phase ◽  
Volume Data ◽  
Pressure Derivative ◽  
Pressure Medium ◽  
History Of ◽  
Diamond Anvil ◽  
State Models ◽  
The Stability ◽  
Murnaghan Equation

AbstractWe investigated the compressional behavior of i-AlCuFe quasicrystal using diamond anvil cell under quasi-hydrostatic conditions by in situ angle-dispersive X-ray powder diffraction measurements (in both compression and decompression) up to 76 GPa at ambient temperature using neon as pressure medium. These data were compared with those collected up to 104 GPa using KCl as pressure medium available in literature. In general, both sets of data indicate that individual d-spacing shows a continuous decrease with pressure with no drastic changes associated to structural phase transformations or amorphization. The d/d0, where d0 is the d-spacing at ambient pressure, showed a general isotropic compression behavior. The zero-pressure bulk modulus and its pressure derivative were calculated fitting the volume data to both the Murnaghan- and Birch-Murnaghan equation of state models. Results from this study extend our knowledge on the stability of icosahedrite at very high pressure and reinforce the evidence that natural quasicrystals formed during a shock event in asteroidal collisions and survived for eons in the history of the Solar System.

First-Principles Investigation on Elastic Constants of TiN under High Pressure

2013 ◽  
Vol 802 ◽  
pp. 109-113
Author(s):  
Kittiya Prasert ◽  
Pitiporn Thanomngam ◽  
Kanoknan Sarasamak
Keyword(s):  
Elastic Constants ◽  
First Principles ◽  
Local Density ◽  
Ambient Pressure ◽  
Lattice Parameter ◽  
First Principles Calculations ◽  
Generalized Gradient ◽  
Generalized Gradient Approximation ◽  
Shear Distortion ◽  
Calculated Lattice Parameter

Elastic constants of NaCl-type TiN under pressure were investigated by first-principles calculations within both local density approximation (LDA) and Perdew-Burke-Ernzerhof generalized-gradient approximation (PBE-GGA). At ambient pressure, the calculated lattice parameter, bulk modulus, and elastic constants of NaCl-type TiN are in well agreement with other available values. Under pressure, all elastic constants,C11,C12, andC44, are found to increase with pressure.C11, which is related to the longitudinal distortion, increases rapidly with pressure whileC12andC44which are related to the transverse and shear distortion, respectively, are much less sensitive to pressure.

scholarly journals Lab in a DAC – high-pressure crystal chemistry in a diamond-anvil cell

2019 ◽  
Vol 75 (6) ◽  
pp. 918-926 ◽  
Author(s):  
Andrzej Katrusiak
Keyword(s):  
High Pressure ◽  
Diamond Anvil Cell ◽  
Ambient Pressure ◽  
Elevated Pressure ◽  
Sample Volume ◽  
Chemical Research ◽  
New Era ◽  
New Information ◽  
Intermolecular Contacts ◽  
Diamond Anvil

The diamond-anvil cell (DAC) was invented 60 years ago, ushering in a new era for material sciences, extending research into the dimension of pressure. Most structural determinations and chemical research have been conducted at ambient pressure, i.e. the atmospheric pressure on Earth. However, modern experimental techniques are capable of generating pressure and temperature higher than those at the centre of Earth. Such extreme conditions can be used for obtaining unprecedented chemical compounds, but, most importantly, all fundamental phenomena can be viewed and understood from a broader perspective. This knowledge, in turn, is necessary for designing new generations of materials and applications, for example in the pharmaceutical industry or for obtaining super-hard materials. The high-pressure chambers in the DAC are already used for a considerable variety of experiments, such as chemical reactions, crystallizations, measurements of electric, dielectric and magnetic properties, transformations of biological materials as well as experiments on living tissue. Undoubtedly, more applications involving elevated pressure will follow. High-pressure methods become increasingly attractive, because they can reduce the sample volume and compress the intermolecular contacts to values unattainable by other methods, many times stronger than at low temperature. The compressed materials reveal new information about intermolecular interactions and new phases of single- and multi-component compounds can be obtained. At the same time, high-pressure techniques, and particularly those of X-ray diffraction using the DAC, have been considerably improved and many innovative developments implemented. Increasingly more equipment of in-house laboratories, as well as the instrumentation of beamlines at synchrotrons and thermal neutron sources are dedicated to high-pressure research.

scholarly journals A New Phase of GaN

2016 ◽  
Vol 2016 ◽  
pp. 1-9 ◽  
Author(s):  
Qingyang Fan ◽  
Changchun Chai ◽  
Qun Wei ◽  
Jionghao Yang ◽  
Peikun Zhou ◽  
...  
Keyword(s):  
Band Gap ◽  
First Principles ◽  
Elastic Anisotropy ◽  
Ambient Pressure ◽  
Young Modulus ◽  
Structure Study ◽  
First Principles Calculations ◽  
Hydrogen Storage Material ◽  
Storage Material ◽  
New Phase

The structural, mechanical, and electronic properties of the orthorhombic GaN (Pnma-GaN) are investigated at ambient pressure by using first-principles calculations method with the ultrasoft pseudopotential scheme. The elastic constants and phonon calculations reveal Pnma-GaN is mechanically and dynamically stable at ambient pressure. The calculated Young modulus of Pnma-GaN is 170 GPa, which is the three-fifths of wurtzite-GaN. Electronic structure study shows that Pnma-GaN is a direct semiconductor with band gap of 1.847 eV. The anisotropic calculation shows that wurtzite-GaN has a smaller elastic anisotropy than that of Pnma-GaN in Young’s modulus. In addition, when the composition of aluminum increases from 0 to 0.063 in the alloy, the band gap decreases initially and increases afterward for Pnma-Ga1−xAlxN, while, for wurtzite-Ga1−xAlxN, the band gap increases with the increasing compositionx. Due to the structural porous feature, Pnma-GaN can also be expected to be a good hydrogen storage material.

Structure–melting relations in isomeric dibromobenzenes

2014 ◽  
Vol 70 (3) ◽  
pp. 492-497 ◽  
Author(s):  
Kamil F. Dziubek ◽  
Andrzej Katrusiak
Keyword(s):  
Melting Point ◽  
Ambient Pressure ◽  
Monoclinic Space Group ◽  
X Ray Diffraction ◽  
Glass Capillary ◽  
Orthorhombic Space Group ◽  
Space Group ◽  
Diamond Anvil ◽  
Higher Temperature

1,4-Dibromobenzene melts at a considerably higher temperature than the 1,2- and 1,3-isomers. This melting-point difference is consistent with the molecular symmetry, as described by Carnelley's rule, and with the frequency of Br...Br halogen bonds. The lowest melting point of 1,3-dibromobenzene correlates with its two symmetry-independent molecules, indicating their inability to pack closely. Single crystals of 1,2- and 1,3-dibromobenzene have been grown under isochoric conditions in a diamond–anvil cell and at isobaric conditions in a glass capillary. Their structures have been determinedin situby X-ray diffraction. At 295 K 1,2-dibromobenzene crystallizes at 0.2 GPa as orthorhombic, space groupPbca,Z′ = 1, and 1,3-dibromobenzene at 0.3 GPa as orthorhombic, space groupP212121,Z′ = 2. The same crystal phases are formed at ambient pressure by freezing these liquids below 256.15 and 248.45 K, respectively. The third isomer, 1,4-dibromobenzene, is a solid at room temperature and crystallizes as monoclinic, space groupP21/a. Striking relations between the structures and melting points of the corresponding dibromobenzene and dichlorobenzene isomers have been discussed.

High-pressure transformations of NbO2F

2000 ◽  
Vol 56 (2) ◽  
pp. 189-196 ◽  
Author(s):  
Stefan Carlson ◽  
Ann-Kristin Larsson ◽  
Franziska E. Rohrer
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
High Pressures ◽  
Ambient Pressure ◽  
Close Packing ◽  
Hexagonal Close Packing ◽  
Rietveld Refinements ◽  
X Ray ◽  
Anion Coordination ◽  
Diamond Anvil

The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at high pressures using diamond anvil cells and synchrotron X-ray radiation. High-pressure powder diffraction measurements have been performed up to 40.1 GPa. A phase transition from the cubic (Pm3¯m) ambient pressure structure to a rhombohedral (R3¯c) structure at 0.47 GPa has been observed. Rietveld refinements at 1.38, 1.96, 3.20, 6.23, 9.00 and 10.5 GPa showed that the transition involves an a − a − a − tilting of the cation–anion coordination octahedra and a change of the anion–anion arrangement to approach hexagonal close packing. Compression and distortion of the Nb(O/F)6 octahedra is also revealed by the Rietveld refinements. At 17–18 GPa, the diffraction pattern disappears and the structure becomes X-ray amorphous.

On the use of diamond as a pressure calibrant for near-infrared FT–Raman microspectroscopy at high pressures

1995 ◽  
Vol 73 (7) ◽  
pp. 1019-1022 ◽  
Author(s):  
Ross D. Markwell ◽  
Ian S. Butler
Keyword(s):  
Near Infrared ◽  
Phonon Mode ◽  
High Pressures ◽  
Ambient Pressure ◽  
Raman Microspectroscopy ◽  
Near Ir ◽  
Ft Raman Spectroscopy ◽  
Diamond Anvil ◽  
Ft Raman

The pressure-induced shift of the Raman-active t2g phonon mode of the diamond windows in a commercial diamond-anvil cell, located at l332.5 cm−1 at ambient pressure, can be used as an in situ calibrant for near-IR FT–Raman microspectroscopy at high pressures. The measured pressures are considered to be accurate to within ±2 kbar throughout the 4–45 kbar range for which the associated pressure (P, kbar) vs. wavenumber (v, cm−1) relationship is P = 6.66v − 1335.9. Keywords: FT–Raman spectroscopy, diamond, high pressures.

A free-standing platinum monolayer as an efficient and selective catalyst for the oxygen reduction reaction

2017 ◽  
Vol 5 (11) ◽  
pp. 5303-5313 ◽  
Author(s):  
Arup Mahata ◽  
Priyanka Garg ◽  
Kuber Singh Rawat ◽  
Preeti Bhauriyal ◽  
Biswarup Pathak
Keyword(s):  
Oxygen Reduction Reaction ◽  
Oxygen Reduction ◽  
First Principles ◽  
Reduction Reaction ◽  
Two Dimensional ◽  
First Principles Calculations ◽  
Free Standing ◽  
Selective Catalyst ◽  
Platinum Monolayer ◽  
Orr Activity

We report a two-dimensional platinum monolayer (Pt-ML) sheet for oxygen reduction reaction (ORR) activity using first-principles calculations.

scholarly journals Experimental and Computational Studies on Superhard Material Rhenium Diboride under Ultrahigh Pressures

Materials ◽  
2020 ◽  
Vol 13 (7) ◽  
pp. 1657 ◽  
Author(s):  
Kaleb C. Burrage ◽  
Chia-Min Lin ◽  
Wei-Chih Chen ◽  
Cheng-Chien Chen ◽  
Yogesh K. Vohra
Keyword(s):  
Density Functional ◽  
Hexagonal Phase ◽  
Extreme Environment ◽  
Ultrahigh Pressure ◽  
Maximum Pressure ◽  
Pressure Derivative ◽  
Crystal Anisotropy ◽  
Axis Compression ◽  
Diamond Anvil ◽  
Rhenium Diboride

An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 ± 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures.

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