arXiv subjects

Claudio Cazorla

Publications and source records attributed to Claudio Cazorla.

At least 55 records · Page 3Linked to original sources

arXiv2020

Colossal barocaloric effects in the complex hydride Li$_{2}$B$_{12}$H$_{12}$

Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to external fields and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ($|ΔT| \sim 1$ K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict the existence of colossal barocaloric effects (isothermal entropy changes of $|ΔS| \sim 100$ JK$^{-1}$kg$^{-1}$) in the energy material Li$_{2}$B$_{12}$H$_{12}$ by means of molecular dynamics simulations. Specifically, we estimate $|ΔS| = 387$ JK$^{-1}$kg$^{-1}$ and $|ΔT| = 26$ K for an applied pressure of $P = 0.4$ GPa at $T = 475$ K. The disclosed colossal barocaloric effects are originated by an order-disorder phase transformation that exhibits a fair degree of reversibility and involves coexisting Li$^{+}$ diffusion and (BH)$_{12}^{-2}$ reorientational motion at high temperatures.

cond-mat.mtrl-sci↗

arXiv2020

Is the normal to superionic transformation occurring in type-II fast-ion conductors a real thermodynamic phase transition?

Fedorov has written a Comment on our recent paper J. Phys. Chem. C 112, 1267 (2018) in which he suggests that the transformation from a normal ($α$) to a superionic ($β$) state occurring in type-II fast-ion conductors (e.g., CaF$_{2}$ and Li-based superionic materials) cannot be regarded as a phase transition in a rigorous thermodynamic sense. Fedorov's arguments for such a criticism can be summarized as follows: (1) the available heat capacity data for the $α\to β$ transformation do not reproduce the expected behaviour for a thermodynamic phase transition, (2) there is not evidence for molar volume discontinuity during the superionic $α\to β$ transformation, and (3) the $α\to β$ transformation is symmetry preserving. Here, we demonstrate either by reproducing experimental data published by other authors or by explaining some simple solid-state arguments that the statements put forward by Fedorov are incorrect and do not correspond to reality. In fact, based on mounting experimental evidence, the normal to superionic phase transformation occurring in type-II fast-ion conductors can be rigorously considered, and should be referred to, as a phase transition. Notwithstanding, it remains less obvious to establish with generality which is the order of the $α\to β$ phase transition.

cond-mat.mtrl-sci↗

arXiv2020

Flexible complementary logic circuit built from two identical organic electrochemical transistors

The organic electrochemical transistor (OECT) with a conjugated polymer as the active material is the elementary unit of organic bioelectronic devices. Increased functionalities, such as low power consumption, can be achieved by building complementary circuits featuring two or more OECTs. Complementary circuits commonly combine both p- and n-type transistors to reduce power draw. While p-type OECTs are readily available, n-type OECTs are less common mainly due to poor stability of the n-type active channel material in aqueous electrolyte. Additionally, an OECT based complementary circuit requires well matched transport properties in the p- and n-type materials. Here, a complementary circuit is made using a pair of OECTs having polyaniline (PANI) as the channel material in both transistors. PANI is chosen due to its unique behaviour exhibiting a peak in current versus gate voltage when used as an active channel in an OECT. The PANI based circuit is shown to have excellent performance with gain of ~ 7 and could be transferred on a flexible biocompatible chitosan substrate with demonstrated operation in aqueous electrolyte. This study extends the capabilities of conjugated polymer based OECTs.

physics.app-ph↗

arXiv2020

Simulation of the electrocaloric effect based on first-principles methods

Due to critical environmental and technological issues, there is a pressing need to switch from current refrigeration methods based on compression of gases to novel solid-state cooling technologies. Solid-state cooling is based on the thermal response of materials to external magnetic, electric, or mechanic fields, the so-called caloric effect. The electrocaloric (EC) effect, which is caused by electric fields and typically occurs in polar materials, is particularly promising from a technological point of view owing to its good scalability and natural implementation in circuitry. Simulation of EC effects represents an efficient and physically insightful strategy for advancing the field of solid-state cooling by complementing, and in some cases guiding, experiments. Theoretical estimation of EC effects can be achieved with different approaches ranging from computationally inexpensive but physically insightful phenomenological free-energy models to computationally very demanding and quantitatively accurate first-principles methods. In this Chapter, we review EC simulation approaches that rely on first-principles methods. In this category, we include ab initio quasi-harmonic methods, bond-valence and classical interatomic potentials and effective Hamiltonians. In analogy to the experiments, these simulation approaches can be used to estimate EC effects either directly or indirectly and we review here well-established protocols that can be followed for each case. The Chapter finalises with a collection of representative examples in which first-principles based approaches have been used to predict and understand original EC effects.

cond-mat.mtrl-sci↗

arXiv2020

Combining density functional theories to correctly describe the energy, lattice structure and electronic density of functional oxide perovskites

Functional oxide perovskites are the pillar of cutting-edge technological applications. Density functional theory (DFT) simulations are the theoretical methods of choice to understand and design perovskite materials. However, tests on the reliability of DFT to describe fundamental properties of oxide perovskites are scarce and mostly ill-defined due to a lack of rigorous theoretical benchmarks for solids. Here, we present a quantum Monte Carlo benchmark study of DFT on the archetypal perovskite BaTiO$_{3}$ (BTO). It shows that no DFT approximation can simultaneously reproduce the energy, structure, and electronic density of BTO. Traditional protocols to select DFT approximations are empirical and fail to detect this shortcoming. An approach combining two different non-empirical DFT schemes, "SCAN" and "HSE06", is able to holistically describe BTO with accuracy. Combined DFT approaches should thus be considered as a promising alternative to standard methods for simulating oxide perovskites.

cond-mat.mtrl-sci↗

arXiv2020

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe$_{1-x}$Co$_{x}$O$_{3}$ Solid Solutions Near Room Temperature

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe$_{1-x}$Co$_{x}$O$_{3}$ solid solutions with $0.25 \le x \le 0.50$ (BFCO) represent a noteworthy exception to this behaviour. In particular, we find that at room temperature and for moderate pressures of $0.1$-$1.0$ GPa, depending on the composition, the electric polarization of bulk BFCO increases by $\sim 200$%. The origin of such an exceptional behavior is a phase transformation involving a low-$T$ rhombohedral (${\cal R}$) phase and a high-$T$ super-tetragonal (${\cal T}$) phase. Both ${\cal R}$ and ${\cal T}$ phases are ferrimagnetic near room temperature with an approximate net magnetization of $0.13$$μ_{B}$ per formula unit. Contrarily to what occurs in either bulk BiFeO$_{3}$ or BiCoO$_{3}$, the ${\cal T}$ phase is stabilized over the ${\cal R}$ by increasing temperature due to its higher vibrational entropy. This extraordinary $T$-induced ${\cal R} \to {\cal T}$ phase transition is originated by polar phonon modes involving concerted displacements of transition-metal and oxygen ions.

cond-mat.mtrl-sci↗

arXiv2019

Prediction of large barocaloric effects in thermoelectric superionic materials

We predict the existence of large barocaloric effects above room temperature in the thermoelectric fast-ion conductor Cu$_{2}$Se by using classical molecular dynamics simulations and first-principles computational methods. A hydrostatic pressure of $1$ GPa induces large isothermal entropy changes of $|ΔS| \sim 15$-$45$ Jkg$^{-1}$K$^{-1}$ and adiabatic temperature shifts of $|ΔT| \sim 10$ K in the temperature interval $400 \le T \le 700$ K. Structural phase transitions are absent in the analysed thermodynamic range. The causes of such large barocaloric effects are significant $P$-induced variations on the ionic conductivity of Cu$_{2}$Se and the inherently high anharmonicity of the material. Uniaxial stresses of the same magnitude, either compressive or tensile, produce comparatively much smaller caloric effects, namely, $|ΔS| \sim 1$ Jkg$^{-1}$K$^{-1}$ and $|ΔT| \sim 0.1$ K, due to practically null influence on the ionic diffusivity of Cu$_{2}$Se. Our simulation work shows that thermoelectric compounds presenting high ionic disorder, like copper and silver-based chalcogenides, may render large mechanocaloric effects and thus are promising materials for engineering solid-state cooling applications that do not require the application of electric fields.

cond-mat.mtrl-sci↗

arXiv2019

Strain engineering of oxide thin films for photocatalytic applications

Photocatalytic materials are pivotal for the implementation of disruptive clean energy applications such as conversion of H$_{2}$O and CO$_{2}$ into fuels and chemicals driven by solar energy. However, efficient and cost-effective materials able to catalyze the chemical reactions of interest when exposed to visible light are scarce due to the stringent electronic conditions that they must satisfy. Chemical and nanostructuring approaches are capable of improving the catalytic performance of known photoactive compounds however the complexity of the synthesized nanomaterials and sophistication of the employed methods make systematic design of photocatalysts difficult. Here, we show by means of first-principles simulation methods that application of biaxial stress, $η$, on semiconductor oxide thin films can modify their optoelectronic and catalytic properties in a significant and predictable manner. In particular, we show that upon moderate tensile strains CeO$_{2}$ and TiO$_{2}$ thin films become suitable materials for photocatalytic conversion of H$_{2}$O into H$_{2}$ and CO$_{2}$ into CH$_{4}$ under sunlight. The band gap shifts induced by $η$ are reproduced qualitatively by a simple analytical model that depends only on structural and dielectric susceptibility changes. Thus, epitaxial strain represents a promising route for methodical screening and rational design of photocatalytic materials.

cond-mat.mtrl-sci↗

arXiv2019

Novel Mechanocaloric Materials for Solid-State Cooling Applications

Solid-state cooling is an environmentally friendly and highly scalable technology that may solve most of the problems associated with current refrigerant methods. Solid-state cooling consists of applying external fields on caloric materials, which react thermally as a result of induced phase transformations. From an energy efficiency point of view, mechanocaloric compounds, in which the phase transitions of interest are driven by mechanical stresses, probably represent the most encouraging type of caloric materials. Conventional mechanocaloric materials like shape-memory alloys already display good cooling performances however in most cases they also present critical mechanical fatigue and hysteresis problems that limit their applicability. Finding new mechanocaloric materials and mechanisms able to overcome those problems while simultaneously rendering large temperature shifts, is necessary to further advance the field of solid-state cooling. In this article, we review novel families of mechanocaloric materials that in recent years have been shown to be specially promising in the aspects that conventional mechanocaloric materials are not, and which exhibit unconventional but significant caloric effects. We put an emphasis on elastocaloric materials, in which the targeted cooling spans are obtained through uniaxial stresses, since from an applied perspective these appear to be the most accomplished. Two different types of mechanocaloric materials emerge as particularly hopeful from our analysis, compounds that exhibit field-induced order disorder phase transitions involving either ions or molecules (fast-ion conductors and plastic crystals), and multiferroics in which the structural parameters are strongly coupled with polar and/or magnetic degrees of freedom (magnetic alloys and oxide perovskites).

cond-mat.mtrl-sci↗

arXiv2019

First-principles prediction of extraordinary thermoelectric efficiency in superionic Li2SnX3(X=S,Se)

Thermoelectric materials create an electric potential when subject to a temperature gradient and vice versa hence they can be used to harvest waste heat into electricity and in thermal management applications. However, finding highly efficient thermoelectrics with high figures of merit, zT$\geq$1, is very challenging because the combination of high power factor and low thermal conductivity is rare in materials. Here, we use first-principles methods to analyze the thermoelectric properties of Li$_2$Sn$X_3$ ($X$=S,Se), a recently synthesized class of lithium fast-ion conductors presenting high thermal stability. In p-type Li$_2$Sn$X_3$, we estimate highly flat electronic valence bands that render high Seebeck coefficients exceeding 400 $μ$VK$^{-1}$ at 700K. In n-type Li$_2$Sn$X_3$, the electronic conduction bands are slightly dispersive however the accompanying weak electron-acoustic phonon scattering induces high electrical conductivity. The combination of high Seebeck coefficient and electrical conductivity gives rise to high power factors, reaching a maximum of 4 mWm$^{-1}$K$^{-2}$ in p-type Li$_2$SnS$_3$ and 8 mWm$^{-1}$K$^{-2}$ in n-type Li$_2$SnSe$_3$ at 300 K. Likewise, the thermal conductivity in Li$_2$Sn$X_3$ is low as compared to conventional thermoelectric materials, 2-5 Wm$^{-1}$K$^{-1}$ at room temperature. As a result, we estimate a maximum zT = 1.05 in p-type Li$_2$SnS$_3$ at 700 K and an extraordinary 3.07 (1.5) in n-type Li$_2$SnSe$_3$ at the same temperature (300 K). Our findings of huge zT in Li$_2$Sn$X_3$ suggest that lithium fast-ion conductors, typically employed as electrolytes in solid-state batteries, hold exceptional promise as thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Oxygen-vacancy induced magnetic phase transitions in multiferroic thin films

Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and high temperatures involved in materials preparation. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with a significant net magnetization of 0.5 uB per formula unit. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics.

cond-mat.mtrl-sci↗

arXiv2018

First-principles prediction of half-Heusler half-metals above room temperature

Half-metallicity (HM) offers great potential for engineering spintronic applications, yet only few magnetic materials present metallicity in just one spin channel. In addition, most HM systems become magnetically disordered at temperatures well below ambient conditions, which further hinders the development of spin-based electronic devices. Here, we use first-principles methods based on density functional theory (DFT) to investigate the electronic, magnetic, structural, mixing, and vibrational properties of $90$ $XYZ$ half-Heusler (HH) alloys ($X =$ Li, Na, K, Rb, Cs; $Y =$ V,Nb, Ta; $Z =$ Si, Ge, Sn, S, Se, Te). We disclose a total of $28$ new HH compounds that are ferromagnetic, vibrationally stable, and HM, with semiconductor band gaps in the range of $1$-$4$ eV and HM band gaps of $0.2$-$0.8$ eV. By performing Monte Carlo simulations of a spin Heisenberg model fitted to DFT energies, we estimate the Curie temperature, $T_{\rm C}$, of each HM compound. We find that $17$ HH HM remain magnetically ordered at and above room temperature, namely, $300 \le T_{\rm C} \le 450$ K, with total magnetic moments of $2$ and $4$ $μ_{\rm B}$. A further materials sieve based on zero-temperature mixing energies let us to conclude $5$ overall promising ferromagnetic HH HM at and above room temperature: NaVSi, RbVTe, CsVS, CsVSe, and RbNbTe. We also predict $2$ ferromagnetic materials that are semiconductor and magnetically ordered at ambient conditions: LiVSi and LiVGe.

cond-mat.mtrl-sci↗

arXiv2018

Giant Direct and Inverse Electrocaloric Effects in Multiferroic Thin Films

Refrigeration systems based on compression of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. In the vicinity of a phase transition caloric materials present large thermal responses to external fields, which makes them promising for developing alternative solid-state cooling devices. Electrocaloric effects are particularly well-suited for portable refrigeration applications; however, most electrocaloric materials operate best at non-ambient temperatures or require the application of large electric fields. Here, we predict that modest electric fields can yield giant room-temperature electrocaloric effects in multiferroic BiCoO$_{3}$ (BCO) thin films. Depending on the orientation of the applied field the resulting electrocaloric effect is either direct (heating) or inverse (cooling), which may enable the design of enhanced refrigeration cycles. We show that spin-phonon couplings and phase competition are the underlying causes of the disclosed caloric phenomena. The dual electrocaloric response of BCO thin films can be effectively tuned by means of epitaxial strain and we anticipate that other control strategies like chemical substitution are also possible.

cond-mat.mtrl-sci↗

arXiv2018

Comment on "High-pressure phases of group-II difluorides: Polymorphism and superionicity"

Nelson et al. [Phys. Rev. B 95, 054118 (2017)] recently have reported first-principles calculations on the behaviour of group-II difluorides (BeF$_{2}$, MgF$_{2}$, and CaF$_{2}$) under high-pressure and low- and high-temperature conditions. The calculations were based on ab initio random structure searching and the quasi-harmonic approximation (QHA). Here, we point out that, despite the of inestimable value of such calculations at high-pressure and low-temperature conditions, the high-$P$ high-$T$ phase diagram proposed by Nelson et al. for CaF$_{2}$ neither is in qualitative agreement with the results of previous ab initio molecular dynamics simulations nor with the existing corps of experimental data. Therefore, we conclude that the QHA-based approach employed by Nelson et al. cannot be applied reliably to the study of phase boundaries involving superionic phases. This conclusion is further corroborated by additional ab initio calculations performed in the superionic compounds SrF$_{2}$, BaF$_{2}$, Li$_{3}$OCl, and AgI.

cond-mat.mtrl-sci↗

arXiv2018

Polymorphism of Bulk Boron Nitride

Boron nitride (BN) is a material with outstanding technological promise because of its exceptional thermochemical stability, structural, electronic and thermal conductivity properties, and extreme hardness. Yet, the relative thermodynamic stability of its most common polymorphs (diamond-like cubic and graphite-like hexagonal) has not been resolved satisfactorily because of the crucial role played by kinetic factors in the formation of BN phases at high temperatures and pressures (experiments), and by competing bonding, electrostatic and many-body dispersion forces in BN cohesion (theory). This lack of understanding hampers the development of potential technological applications, and challenges the boundaries of fundamental science. Here, we use high-level first-principles theories that correctly reproduce all important electronic interactions (the adiabatic-connection fluctuation-dissipation theorem in the random phase approximation) to estimate with unprecedented accuracy the energy differences between BN polymorphs, and thus overcome the accuracy hurdle that hindered previous theoretical studies. We show that the ground-state phase of BN is cubic and that the frequently observed hexagonal polymorph becomes entropically stabilized over the cubic at temperatures slightly above ambient conditions ($T_{\rm c \to h} = 335 \pm 30$ K). We also reveal a low-symmetry monoclinic phase that is extremely competitive with the other low-energy polymorphs and which could explain the origins of the experimentally observed "compressed h-BN" phase. Our theoretical findings therefore should stimulate new experimental efforts in bulk BN as well as promote the use of high-level theories in modelling of technologically relevant van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2018

Mixing thermodynamics and photocatalytic properties of GaP-ZnS solid solutions

Preparation of solid solutions represents an effective means to improve the photocatalytic properties of semiconductor-based materials. Nevertheless, the effects of site-occupancy disorder on the mixing stability and electronic properties of the resulting compounds are difficult to predict and consequently many experimental trials may be required before achieving enhanced photocatalytic activity. Here, we employ first-principles methods based on density functional theory to estimate the mixing free energy and the structural and electronic properties of (GaP)$_{x}$(ZnS)$_{1-x}$ solid solutions, a representative semiconductor-based optoelectronic material. Our method relies on a multi-configurational supercell approach that takes into account the configurational and vibrational contributions to the free energy. Phase competition among the zinc-blende and wurtzite polymorphs is also considered. We demonstrate overall excellent agreement with the available experimental data: (1)~zinc-blende emerges as the energetically most favorable phase, (2)~the solid solution energy band gap lies within the $2$--$3$~eV range for all compositions, and (3)~the energy band gap of the solid solution is direct for compositions $x \le 75$\%. We find that at ambient conditions most (GaP)$_{x}$(ZnS)$_{1-x}$ solid solutions are slightly unstable against decomposition into GaP- and ZnS-rich regions. Nevertheless, compositions $x \approx 25$, 50, and 75\% render robust metastable states that owing to their favorable energy band gaps and band levels relative to vacuum are promising hydrogen evolution photocatalysts for water splitting under visible light. The employed theoretical approach provides valuable insights into the physicochemical properties of potential solid-solution photocatalysts and offers useful guides for their experimental realization.

cond-mat.mtrl-sci↗

arXiv2018

Influence of lattice dynamics on lithium-ion conductivity: A first-principles study

In the context of novel solid electrolytes for solid-state batteries, first-principles calculations are becoming increasingly more popular due to their ability to reproduce and predict accurately the energy, structural, and dynamical properties of fast-ion conductors. In order to accelerate the discovery of new superionic conductors is convenient to establish meaningful relations between ionic transport and simple materials descriptors. Recently, several experimental studies on lithium fast-ion conductors have suggested a correlation between lattice softness and enhanced ionic conductivity due to a concomitant decrease in the activation energy for ion migration, $E_{a}$. In this article, we employ extensive \emph{ab initio} molecular dynamics simulations based on density functional theory to substantiate the links between ionic transport and lattice dynamics in a number of structurally and chemically distinct lithium superionic conductors. Our first-principles results show no evidence for a direct and general correlation between $E_{a}$, or the hopping attempt frequency, and lattice softness. However, we find that, in agreement with recent observations, the pre-exponential factor of lithium diffusivity, $D_{0}$, follows the Meyer-Neldel rule $\propto \exp{\left(E_{a}/\langle ω\rangle\right)}$, where $\langle ω\rangle$ represents an average phonon frequency. Hence, lattice softness can be identified with enhanced lithium diffusivity but only within families of superionic materials presenting very similar migration activation energies, due to larger $D_{0}$. On the technical side, we show that neglection of temperature effects in first-principles estimation of $E_{a}$ may lead to huge inaccuracies of $\sim 10$\%. The limitations of zero-temperature harmonic approaches in modeling of lithium-ion conductors are also illustrated.

cond-mat.mtrl-sci↗

arXiv2016

Lattice effects on the formation of oxygen vacancies in perovskite thin films

We use first-principles methods to investigate the effects of collective lattice excitations on the formation of oxygen vacancies in perovskite thin films. We find that phonons play a crucial role on the strain-mediated control of defect chemistry at finite temperatures. In particular, zero-temperature oxygen vacancy formation trends deduced as a function of epitaxial strain can be fully reversed near room temperature. Our first-principles calculations evidence a direct link between the lattice contribution to the oxygen vacancy free energy and the volume expansion that the system undergoes when is chemically reduced: The larger the resulting volume expansion, the more favorable thermal excitations are to point defect formation. However, the interplay between the vibrational vacancy entropy, or equivalently, chemical expansion, and epitaxial strain is difficult to generalise as this can be strongly influenced by underlying structural and magnetic transitions. In addition, we find that vacancy ordering can be largely hindered by the thermal lattice excitations.

cond-mat.mtrl-sci↗