1,721,070 research outputs found
Comment on "Molybdenum at high pressure and temperature: Melting from another solid phase''
No full textA Comment on the Letter by A. B. Belonoshko et al., Phys. Rev. Lett. 100 135701 (2008). The authors of the Letter offer a Reply
Melting properties of a simple tight-binding model of transition metals. I. The region of half-filled d-band
No full textWe present calculations of the free energy, and hence the melting properties, of a simple tight-binding model for transition metals in the region of d-band filling near the middle of a d-series, the parameters of the model being designed to mimic molybdenum. The melting properties are calculated for pressures ranging from ambient to several megabars. The model is intended to be the simplest possible tight-binding representation of the two basic parts of the energy: first, the pairwise repulsion due to Fermi exclusion; and second, the d-band bonding energy described in terms of an electronic density of states that depends on structure. In addition to the number of d-electrons, the model contains four parameters, which are adjusted to fit the pressure dependent d-band-width and the zero-temperature pressure-volume relation of Mo. We show that the resulting model reproduces well the phonon dispersion relations of Mo in the body-centered-cubic structure, as well as the radial distribution function of the high-temperature solid and liquid given by earlier first-principles simulations. Our free energy calculations start from the free energy of the liquid and solid phases of the purely repulsive pair potential model, without d-band bonding. The free energy of the full tight-binding model is obtained from this by thermodynamic integration. The resulting melting properties of the model are quite close to those given by earlier first-principles work on Mo. An interpretation of these melting properties is provided by showing how they are related to those of the purely repulsive model
Zero-temperature generalized phase diagram of the 4d transition metals under pressure
No full textWe use an accurate implementation of density-functional theory to calculate the zero-temperature generalized phase diagram of the 4d series of transition metals from Y to Pd as a function of pressure P and atomic number Z. The implementation used is full-potential linearized augmented plane waves, and we employ the exchange-correlation functional recently developed by Wu and Cohen. For each element, we obtain the ground-state energy for several crystal structures over a range of volumes, the energy being converged with respect to all technical parameters to within similar to 1 meV/atom. The calculated transition pressures for all the elements and all transitions we have found are compared with experiment wherever possible, and we discuss the origin of the significant discrepancies. Agreement with experiment for the zero-temperature equation of state is generally excellent. The generalized phase diagram of the 4d series shows that the major boundaries slope toward lower Z with increasing P for the early elements, as expected from the pressure induced transfer of electrons from sp states to d states, but are almost independent of P for the later elements. Our results for Mo indicate a transition from body-centered cubic to face-centered cubic, rather than the bcc-hcp transition expected from sp-d transfer
A simple tight-binding model for the study of 4d transition metals under pressure
No full textRecently we have developed a simple tight-binding (TB) model of transition metals in the region near the middle of the 4d-series tuned to mimic molybdenum. The energetic, structural and melting properties deriving from this model are quite close to those obtained in previous first-principles work on Mo. TB approaches, reasonably accurate but computationally less demanding than first-principles calculations, therefore can be used to perform systematic analysis on the physical properties of transition metals across the 4d-series over wide thermodynamic ranges. Here we present a series of TB parametrizations designed to emulate the behavior of niobium, technetium, ruthenium, rhodium and palladium under extreme conditions of pressure and temperature. Our simple TB model is composed of two basic contributions to the energy: first, the pairwise repulsion due to Fermi exclusion, and second, the d-band bonding energy described in terms of an electronic density of states that depends on structure. The parameters of the model are adjusted to fit the dependence on pressure of the d-band width and the zero-temperature equation of state of the element in question. Calculated TB phonon spectra compare very well with ab initio results and experimental data, and the stable crystal structure in all transition metals at equilibrium is correctly predicted. (C) 2011 Elsevier B.V. All rights reserved
Constraints on the phase diagram of molybdenum from first-principles free-energy calculations
No full textWe use first-principles techniques to reexamine the suggestion that transitions seen in high-P experiments on Mo are solid-solid transitions from the bcc structure to either the fcc or hcp structures. We confirm that in the quasiharmonic approximation the free energies of fcc and hcp structures become lower than that of bcc at P > 325 GPa and T below the melting curve, as reported recently. However, we show that if anharmonic effects are fully included this is no longer true. We calculate fully anharmonic free energies of high-T crystal phases by integration of the thermal average stress with respect to strain as structures are deformed into each other, and also by thermodynamic integration from harmonic reference systems to the fully anharmonic system. Our finding that fcc is thermodynamically less stable than bcc in the relevant high-P/high-T region is supported by comparing the melting curves of the two structures calculated using the first-principles reference-coexistence technique. We present first-principles simulations based on the recently proposed Z method, which also support the stability of bcc over fcc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Melting curve and Hugoniot of molybdenum up to 400 GPa by ab initio simulations
No full textWe report ab initio calculations of the melting curve and Hugoniot of molybdenum for the pressure range 0 - 400 GPa, using density functional theory (DFT) in the projector augmented wave (PAW) implementation. We use the "reference coexistence" technique to overcome uncertainties inherent in earlier DFT calculations of the melting curve of Mo. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve from static compression experiments. Our calculated P (V) and T (P) Hugoniot relations agree well with shock measurements. We use calculations of phonon dispersion relations as a function of pressure to eliminate some possible interpretations of the solid-solid phase transition observed in shock experiments on Mo
Effect of the exchange-correlation energy and temperature on the generalized phase diagram of the 4d transition metals
No full textVery recently, we have used first-principles methods to calculate the zero-temperature phase diagram of all the 4d transition metals up to P 500GPa. Even though we used a very accurate implementation of density functional theory and the accurate exchange-correlation functional of Wu and Cohen, we found that solid-solid transition pressures were underestimated by 10GPa with respect to room-temperature measurements. Here, we report the dependence of the transition pressures on the choice of the exchange-correlation functional. We also perform first-principles calculations of the harmonic phonon frequencies of molybdenum and zirconium in different crystalline structures in order to extend the phase diagram to non-zero temperatures. We use the results to discuss the reasons for the earlier disagreements between calculated and experimental transition pressures
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