1,721,009 research outputs found
Equation of state of α-Al2O3 (corundum) from quasi-harmonic atomistic simulations
No full textA two-body interatomic potential function, including fractional atomic charges and a shell model for oxygen, and supplemented by an O-Al-O bond-angle energy term, was fitted to the structural, elastic and vibrational properties of α-Al2O3, corundum, at ambient conditions. Full quasi-harmonic calculations were then carried out on a p,T grid of 54 points in the domain 0-40 GPa and 300-1700 K. The crystal structure was equilibrated at each point, taking into account the anisotropy of vibrational pressure and the thermal dependence of elastic constants, so as to obtain unit-cell edges, atomic coordinates, bulk modulus, thermal expansion coefficient and other thermodynamic properties. Polynomial approximations were developed to represent the p,T dependence of these quantities. Comparison with experimental results for the separate p (T = 300 K) and T (p = 0) behaviours shows very good agreement, with average deviations of 0.1% for the unit-cell volume and 6% for the thermal expansion coefficient. The coupled p,T dependence of the properties of corundum is predicted to be very small for the bulk modulus (∂2KT/∂p∂T = 8.4 × 10-5 K-1), but not at all negligible for the volume [(1/V)∂2V/∂p∂T in the range -1.2 to -7.5 × 10-7 GPa-1 K-1 over the p,T domain explored]
Theoretical structure factors and electron density of magnesite (MgCO3)
No full textTheoretical structure factors Fq of rhombohedral magnesite (MgCO3) have been computed by quantum-mechanical periodic Hartree-Fock methods and compared with observed |Fo| (literature data) and conventional Fc (independent-spherical-atom model) values. The corresponding Roq, Roc and Rqc agreement factors (overall values 2.03, 2.11 and 1.80%, respectively) are analysed versus sin(θ)/λ, showing the effects of chemical bonding and of experimental errors. Difference electron density maps Δρoq, ΔρOC and Δρqc have been calculated for the CO3 and the C - O - Mg planes. Features of the C - O covalent bonding and oxygen lone pair are compared and discussed
Powder neutron diffraction study of 2M1 muscovite at room pressure and at 2 GPa
No full textA powder sample of 2M1 muscovite, was studied by time-of-flight neutron diffraction at 1 bar and at 2(± 0.1) GPa. Rietveld refinements of the crystal structure were performed, varying all atomic coordinates in the room-pressure case and only the z coordinates of basal O atoms plus xyz of O and H of the OH group at 2 GPa. In both cases the lattice constants were refined. The unit-cell compression shows deviations from linear elastic behaviour. Inter-layer K-O bonds are compressed along z more than the c lattice constant. The O-H group forms an angle of 85(1)° with c* and establishes three very loose contacts with neighbouring O atoms. This environment does not change appreciably at 2 GPa. -Author
Synchrotron X-ray powder diffraction study of (Bi, Pb)(1.64)Sr1.43Ca1.57Mn2O9, Mn-analogue of BSCCO-2212 superconductor
No full textThe orthorhombic Mn-2212 phase, with nominal composition BiPbSr1.8CaMn2.1O9, was synthesized by solid state reaction. Synchrotron X-ray powder diffraction data (lambda = 0.9359 Angstrom) were collected at Brookhaven National Laboratory (U.S.A.) and used for a structure Rietveld refinement (a = 5.3314(3), b = 5.3697(2), c = 31.067(2) Angstrom). Convergence was obtained in space groups Fmmm, Amaa, and Abmm (Z= 4) with the same agreement indexes (wR(p) = 0.0216, R-p = 0.0158, R(/F/(2))= 0.0419). The two lower symmetry structure models correspond to different descriptions of partial ordering of oxygen atoms in the (Bi,Pb)-O layer. Refined site occupancies indicate a phase composition close to (Bi1-yPby)(1.64)(Sr0.33Ca0.67)(2)(Sr0.77Ca0.23)Mn2O9, with y between 0.4 and 0.5. This result, coupled with electroneutrality requirements, leads to an estimated Mn oxidation state of 3.8 /e/ Unlike in Cu-2212, an O atom is present in the (Sr,Ca) plane between adjacent Mn-O layers, producing a distorted octahedral (instead of pyramidal) coordination of Mn. (C) 1999 Academic Press
Quantum-mechanical Calculation of the Solid-state Equilibrium Mgo+alpha-al2o3-reversible-arrow-mgal2o4 (spinel) Versus Pressure
No full textThe ground-state crystal energies of cubic MgAl2O4 (spinel) and MgO (periclase) and of rhombohedral alpha-Al2O3 (corundum) have been calculated at different volumes, relaxing the corresponding structures, by all-electron periodic Hartree-Fock methods (CRYSTAL program). Basis sets of contracted Gaussian-type functions are employed for the 18 atomic (including d) orbitals representing each of the Mg, Al, and O atoms. Mulliken net atomic charges z(Mg)=1.86\e\ (MgO), z(Al)=2.30\e\ (alpha-Al2O3), z(Mg)=1.74\e\, and z(Al) = 2.24\e\ (spinel) are obtained. The elastic bulk modulus, the Murnaghan equation of state p(V) at the athermal limit, the Mg-O and Al-O bond compressibilities, and the binding energy have been derived for each phase (and the elastic constants C-11 and C-12 for spinel only). Comparison with existing experimental data is discussed. The enthalpy change for spinel decomposition into the simple oxides has been computed as a function of pressure, including a correction for the electron correlation energy based on local-density-functional theory. A decomposition pressure of 11 GPa at T=0 K is predicted, against values of 8 and 13 GPa derived from experimental thermodynamic data and from direct compression experiments, respectively
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No full textAb Inito Calculations on Possible Hard Materials Based on Interpenetrating Networks: SiO2.BeF2
No full textThe energetics and electronic behavior of SiO2·BeF2, based on interpenetrating diamondoid nets and recently proposed as a possible hard material, have been studied with ab initio periodic LCAO Hartree-Fock methods. The composite compound turns out to be unstable with respect to the precursors SiO2 and BeF2. The origin of the instability has been analyzed with the aid of crystal orbital displacement (COD) plots. A bulk modulus close to the sum of those of the two precursors is calculated for SiO2·BeF2. The additivity of the bulk moduli of subnets suggests that hard precursors should be required to obtain superhard materials
Elastic constants and electronic structure of fluorine (CaF2): an ab initio Hartree-Fock study
No full textThe ground state properties of fluorite (CaF2) have been studied using CRYSTAL, an ab initio periodic Hartree-Fock program. Twenty-two and thirteen atomic orbitals (represented as contracted Gaussian-type functions) are used for the calcium and fluorine atoms, respectively. The binding energy (BE), the equilibrium lattice parameter (a), the elastic constants (Cij) and the central zone phonon frequencies nu (IR) and nu (Raman) have been evaluated, and a good agreement obtained with experiment (for instance the error is +2.0, +1.7, -0.5% for BE, a and C11, respectively). The calculated C44 elastic constant reduces from 48 to 44 GPa (experimental value: 37 GPa) when the fluorine atoms are allowed to displace under strain, indicating the importance of inner deformation for shear elasticity. Electron density maps, density of states and band structure plots are reported which confirm the fully ionic nature of fluorite
Modelling of the thermal dependence of structural and elastic properties in calcite (CaCO3)
No full textA computational method, based on the quasiharmonic approximation, has been computer-coded to calculate the temperature dependence of elastic constants and structural features of crystals. The model is applied to calcite, CaCO3; an interatomic potential based on a C-O Morse function and Ca-O and O-O Born-type interactions, including a shell model for O, has been used. Equilibrations in the range 300-800 K reproduce the experimental unit-cell edges and bond lengths within 1%. The simulated thermal expansion coefficients are 22.3 (//c) and 2.6 (⊥c), against 25.5 and -3.7×10-6K-1 experimental values, respectively. The thermal coefficients of elastic constants tend to be underestimated; for the bulk modulus, -2.3 against -3.7×10-4K-1 is obtained
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