Search results for "Bulk modulus"
showing 10 items of 54 documents
Characterization of theTiSiO4structure and its pressure-induced phase transformations: Density functional theory study
2009
Theoretical investigations concerning the possible titanium silicate polymorphs have been performed using density functional theory at B3LYP level. Total-energy calculations and geometry optimizations have been carried out for all phases involved. The following sequence of pressure-driven structural transitions has been found: ${\text{CrVO}}_{4}$-type, $Cmcm$ (in parenthesis the transition pressure), $\ensuremath{\rightarrow}$ zircon-type, $I{4}_{1}/amd$ (0.8 GPa), $\ensuremath{\rightarrow}$ scheelite-type, $I{4}_{1}/a$ (3.8 GPa). At higher pressure the last phase is found to be stable at least up to 25 GPa. The equation of state of the different polymorphs is also reported. We found that t…
High-pressure x-ray diffraction andab initiostudy ofNi2Mo3N,Pd2Mo3N,Pt2Mo3N,Co3Mo3N, andFe3Mo3N: Two families of ultra-incompressible bimetallic inte…
2010
We have studied by means of high-pressure x-ray diffraction the structural stability of ${\text{Ni}}_{2}{\text{Mo}}_{3}\text{N}$, ${\text{Co}}_{3}{\text{Mo}}_{3}\text{N}$, and ${\text{Fe}}_{3}{\text{Mo}}_{3}\text{N}$. We also report ab initio computing modeling of the high-pressure properties of these compounds, ${\text{Pd}}_{2}{\text{Mo}}_{3}\text{N}$ and ${\text{Pt}}_{2}{\text{Mo}}_{3}\text{N}$. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the i…
Ultrasonication as a Method of Investigation of the Mechanical Properties of Doped Hafnium Barium Titanate
2012
The purpose of this study was to fabricate dense ceramic specimens from the system of BaTiO3-BaHfO3 and then to perform mechanical tests. The effect of Hf content on a microstructure and mechanical properties of the BaTiO3-BaHfO3 solid solution was investigated at room temperature. To determine the elastic constants (the Young′s modulus E, the shear modulus G, bulk modulus K and the Poisson′s ratio v) of BaHfxTi1-xO3 a method of measurement of the longitudinal (VL ) and transverse (VT ) ultrasonic wave velocities for this type of material was developed.
Empirical molecular dynamics study of structural, elastic and thermodynamic properties of zinc-blende-like SiGe compound
2004
Abstract A three-body potential coupled with a molecular-dynamics method is used to calculate structural and thermodynamic properties of the hypothetical IV–IV compound SiGe in zinc-blende phase. A good agreement between the calculated and theoretical values of the lattice constant, the bulk modulus and its derivative, and the cohesive energy is obtained. We also compute the elastic constants, Debye temperature, lattice thermal expansion, and the specific heat. We investigate also, the structural properties of SiGe when rock-salt phase appears.
Lattice dynamics of wurtzite and rocksalt AlN under high pressure: Effect of compression on the crystal anisotropy of wurtzite-type semiconductors
2008
Raman spectra of aluminum nitride (AlN) under pressure have been measured up to $25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, i.e., beyond the onset of the wurtzite-to-rocksalt phase transition around $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The experimental pressure coefficients for all the Raman-active modes of the wurtzite phase are reported and compared to those obtained from ab initio lattice dynamical calculations, as well as to previous experimental and theoretical results. The pressure coefficients of all the Raman-active modes in wurtzite-type semiconductors (AlN, GaN, InN, ZnO, and BeO), as well as the relatively low bulk modulus and phase transition pressure in wurtzite AlN, a…
Structural Behavior of Natural Silicate–Carbonate Spurrite Mineral, Ca5(SiO4)2(CO3), under High-Pressure, High-Temperature Conditions
2017
We report on high-pressure and high-temperature angle-dispersive synchrotron X-ray diffraction and high-pressure Raman data up to 27 GPa and 700 K for natural silicate carbonate Ca5(SiO4)2(CO3) spurrite mineral. No phase transition was found in the studied P–T range. The room-temperature bulk modulus of spurrite using Ne as the pressure-transmitting medium is B0 = 77(1) GPa with a first-pressure derivative of B0′ = 5.9(2). The structure compression is highly anisotropic, the b axis being approximately 30% more compressible than the a and c axes. The volumetric thermal expansivity value around 8 GPa was estimated to be 4.1(3) × 10–5 K–1. A comparison with intimately related minerals CaCO3 ca…
High-pressure study of substrate material ScAlMgO4
2011
We report on the structural properties of ScAlMgO4 studied under quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at 300 K and 100 K. We found that the low-pressure phase remains stable up to 24 GPa. At 28 GPa, we detected a reversible phase transformation. The high-pressure phase is assigned to a monoclinic distortion of the low-pressure phase. No additional phase transition is observed up to 40 GPa. In addition, the equation of state, compressibility tensor, and thermal expansion coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found to be 143(8) GPa, with a str…
New Pressure-Induced Polymorphic Transitions of Anhydrous Magnesium Sulfate
2017
The effects of pressure on the crystal structure of the three known polymorphs of magnesium sulfate have been theoretically study by means of DFT calculations up to 45 GPa. We determined that at ambient conditions gamma MgSO4 is an unstable polymorph, which decompose into MgO and SO3, and that the response of the other two polymorphs to hydrostatic pressure is non isotropic. Additionally we found that at all pressures beta MgSO4 has a largest enthalpy than alpha MgSO4. This indicates that beta MgSO4 is thermodynamically unstable versus alpha MgSO4 and predicts the occurrence of a beta alpha phase transition under moderate compression. Our calculations also predict the existence under pressu…
LCAO calculation of neutral defects in GaN
2005
Four well known HF, LDA, GGA and B3LYP Hamiltonians in LCAO approximation have been used in band structure calculations to obtain the main properties of the perfect GaN crystal with hexagonal lattice (C space group). Calculated lattice parameters, elastic constants and the band gap have been compared with the experimental data and the results of other calculations. As a consequence, the GGA Hamiltonian has been chosen, giving the lattice parameters a = 3.20 A, c = 5.20 A, u = 0.377, the bulk modulus B = 206 GPa and the energy gap Eg = 2.7 eV. These results reasonably reproduce the experimental data. For the point defects calculation (VGa, VN, MgGa, ZnGa, CN, and SiN) the supercell model was…
Relaxation phenomena in mixed isomeric alcohols by Mandelstam-Brillouin scattering
1991
Mandelstam-Brillouin scattering data in mixed isomeric alcohols n-pentanol (nPe-OH) and 2-methyl-2-butanol (2Me-2BuOH) are presented. The hypersonic velocity and normalized absorption are measured as a function of the scattering angle, in the temperature range from - 15-degrees-C to + 45-degrees-C, and as a function of n-PeOH molar fraction going from the pure n-PeOH to the pure 2Me-2BuOH. The experimental results confirm the existence of a shear relaxation phenomenon in the GHz region, that has been previously detected in pure liquids. The temperature dependence of the relaxation time tau-s and of the shear modulus G-infinity evaluated within viscoelastic liquid models, support the existen…