Claudio Cazorla

Constraints on the phase diagram of molybdenum from first-principles free-energy calculations

We 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 quasiharmonicapproximationthefreeenergiesoffccandhcpstructuresbecomelowerthanthatofbccat P> 325 GPa and T below the melting curve, as reported recently. However, we show that if anharmonic effects are fully includedthisisnolongertrue.Wecalculatefullyanharmonicfreeenergiesofhigh-T crystalphasesbyintegration of the thermal average stress with respect to strain as structures are deformed into each other, and also by thermodynamicintegrationfromharmonicreferencesystemstothefullyanharmonicsystem.Ourfindingthatfccisthermodynamically less stable than bcc in the relevant high-P/high-T region is supported by comparing the melting curvesofthetwostructurescalculatedusingthefirst-principlesreference-coexistencetechnique.Wepresentfirstprinciples simulations based on the recently proposed Z method, which also support the stability of bcc over fcc.

Insights into the phase diagram of bismuth ferrite from quasiharmonic free-energy calculations

We have used first-principles methods to investigate the phase diagram of multiferroic bismuth ferrite (BiFeO3 or BFO), revealing the energetic and vibrational features that control the occurrence of various relevant structures. More precisely, we have studied the relative stability of four low-energy BFO polymorphs by computing their free energies within the quasi-harmonic approximation, introducing a practical scheme that allows us to account for the main effects of spin disorder. As expected, we find that the ferroelectric ground state of the material (with R3c space group) transforms into an orthorhombic paraelectric phase (Pnma) upon heating. We show that this transition is not significantly affected by magnetic disorder, and that the occurrence of the Pnma structure relies on its being vibrationally (although not elastically) softer than the R3c phase. We also investigate a representative member of the family of nano-twinned polymorphs recently predicted for BFO [Prosandeev et al., Adv. Funct. Mater. 23, 234 (2013)] and discuss their possible stabilization at the boundaries separating the R3c and Pnma regions in the corresponding pressure-temperature phase diagram. Finally, we elucidate the intriguing case of the so-called super-tetragonal phases of BFO: Our results explain why such structures have never been observed in the bulk material, despite their being stable polymorphs of very low energy. Quantitative comparison with experiment is provided whenever possible, and the relative importance of various physical effects (zero-point motion, spin fluctuations, thermal expansion) and technical features (employed exchange-correlation energy density functional) is discussed. Our work attests the validity and usefulness of the quasi-harmonic scheme to investigate the phase diagram of this complex oxide, and prospective applications are discussed.

Simulation and understanding of atomic and molecular quantum crystals

Quantum crystals abound in the whole range of solid-state species. Below a certain threshold temperature the physical behavior of rare gases (He and Ne), molecular solids (H2 and CH4), and some ionic (LiH), covalent (graphite), and metallic (Li) crystals can be only explained in terms of quantum nuclear effects (QNE). A detailed comprehension of the nature of quantum solids is critical for achieving progress in a number of fundamental and applied scientific fields like, for instance, planetary sciences, hydrogen storage, nuclear energy, quantum computing, and nanoelectronics. This review describes the current physical understanding of quantum crystals formed by atoms and small molecules, as well as the wide palette of simulation techniques that are used to investigate them. Relevant aspects in these materials such as phase transformations, structural properties, elasticity, crystalline defects and the effects of reduced dimensionality, are discussed thoroughly. An introduction to quantum Monte Carlo techniques, which in the present context are the simulation methods of choice, and other quantum simulation approaches (e. g., path-integral molecular dynamics and quantum thermal baths) is provided. The overarching objective of this article is twofold. First, to clarify in which crystals and physical situations the disregard of QNE may incur in important bias and erroneous interpretations. And second, to promote the study and appreciation of QNE, a topic that traditionally has been treated in the context of condensed matter physics, within the broad and interdisciplinary areas of materials science.

The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in the homogeneous melting of a superheated crystal. The interaction model used is an embedded-atom model for Fe developed in previous work, and the melting process is simulated in the microcanonical (N, V, E) ensemble. We study periodically repeated systems containing from 96 to 7776 atoms, and the initial system is always the perfect crystal without free surfaces or other defects. For each chosen total energy E and number of atoms N, we perform several hundred statistically independent simulations, with each simulation lasting for between 500 ps and 10 ns, in order to gather statistics for the waiting time τ(w) before melting occurs. We find that the probability distribution of τ(w) is roughly exponential, and that the mean value depends strongly on the excess of the initial steady temperature of the crystal above the superheating limit identified by other researchers. The mean also depends strongly on system size in a way that we have quantified. For very small systems of ~100 atoms, we observe a persistent alternation between the solid and liquid states, and we explain why this happens. Our results allow us to draw conclusions about the reliability of the recently proposed Z method for determining the melting properties of simulated materials and to suggest ways of correcting for the errors of the method.

Superionicity and polymorphism in calcium fluoride at high pressure.

We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF_{2} at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists of a decrease of the normal → superionic critical temperature with compression. This unexpected effect can be explained in terms of a P-induced softening of a zone-boundary X phonon that involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P2_{1}/c). We discuss the possibility of observing these intriguing phenomena in related isomorphic materials.

Comment on "Molybdenum at high pressure and temperature: melting from another solid phase".

A Comment on the Letter by A. B. Belonoshko et al., Phys. Rev. Lett. 100 135701 (2008). The authors of the Letter offer a Reply.

Calcium-based functionalization of carbon nanostructures for peptide immobilization in aqueous media

We predict a covalent functionalization strategy for precise immobilization of peptides on carbon nanostructures immersed in water, based on atomistic first-principles simulations. The proposed strategy consists of straightforward decoration of the carbon nanosurfaces (CNS, e.g. graphene and nanotubes) with calcium atoms. This approach presents a series of improvements with respect to customary covalent CNS functionalization techniques: (i) intense and highly selective biomolecule–CNS interactions are accomplished while preserving atomic CNS periodicity, (ii) under ambient conditions calcium-decorated CNS and their interactions with biomolecules remain strongly attractive both in vacuum and aqueous environment, and (iii) calcium coatings already deplete the intrinsic hydrophobicity of CNS thus additional functionalization for CNS water miscibility is not required. The observed biomolecule–CNS binding enhancement can be explained in terms of large electronic transfers from calcium to the oxygen atoms in the carboxyl and side-chain groups of the peptide. The kind of electronic, structural and thermodynamic properties revealed in this work strongly suggest the potential of Ca-decorated CNS for applications in drug delivery and biomaterials engineering.

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of 4He and H2 adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered √3 × √3 phase. The perfect solid phase of H2 shows no superfluid signal, whereas 4He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in 4He without destroying the spatial solid order.

First-principles modeling of Pt/LaAlO3/SrTiO3capacitors under an external bias potential

We study the electrical properties of Pt/LaAlO3/SrTiO3 capacitors under the action of an external bias potential, using first-principles simulations performed at constrained electric displacement field. A complete set of band diagrams, together with the relevant electrical characteristics (capacitance and built-in fields), are determined as a function of LaAlO3 thickness and the applied potential.We find that the internal field in LaAlO3 monotonically decreases with increasing thickness; hence, the occurrence of spontaneous Zener tunneling is ruled out in this system.We discuss the implications of our results in the light of recent experimental observations on biased LaAlO3/SrTiO3 junctions involving metallic top electrodes.

Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.