We use molecular dynamics simulations for a first principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO3, the adiabatic temperature change ∆T and the isothermal entropy change ∆S, for different electric field strengths. We compare direct and indirect methods to obtain ∆T and ∆S, and we confirm that both methods indeed lead to identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating ∆T using the direct method within a molecular dynamics approach.
We study the electrocaloric (EC) effect in bulk BaTiO3 (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can in principle be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudo-cubic direction.
We address the question of how the electrocaloric effect in epitaxial thin films of the prototypical ferroelectric BaTiO 3 is affected by the clamping to the substrate and by substrate-induced misfit strain. We use molecular dynamics simulations and a first-principles-based effective Hamiltonian to calculate the adiabatic temperature change ∆T under different epitaxial constraints. Our results demonstrate that, consistent with phenomenological theory, clamping by the substrate reduces the maximum ∆T compared to bulk BaTiO 3 . On the other hand, compressive misfit-strain leads to a strong increase of ∆T and shifts the maximum of the electrocaloric effect to higher temperatures. A rather small compressive strain of −0.75 % is sufficient to obtain a ∆T that is larger than the corresponding bulk value.The electrocaloric (EC) effect manifests itself as temperature change of a dielectric material induced by an applied electric field. 1,2 The electric field increases the order of the electric dipoles, which decreases the entropy of the system and under adiabatic conditions leads to an increase in temperature. When the field is removed the dipoles disorder, which, depending on the process conditions, leads to an increase in entropy and/or a reduction in temperature.Even though the EC effect has been known for a long time (see e.g. Ref. 1), the observed temperature changes were considered too small to be suitable for applications. However, the recent report of a "giant electrocaloric effect" in Pb(Zr,Ti)O 3 thin films by Mischenko et al. 3 has stimulated extensive work in this area, and has established the EC effect as an attractive alternative for the design of future cooling devices. 4 The large EC temperature change that is observed in thin films is mostly due to the larger electric fields that can be applied compared to bulk ceramics. 2 However, it is also well known that the ferroelectric properties of thin film materials are strongly affected by substrateinduced clamping and misfit strain. 5,6 For example, thin films of BaTiO 3 (BTO), a typical textbook model ferroelectric, exhibit strongly enhanced ferroelectric properties. 7 Furthermore, epitaxial strain not only affects the electric polarization and ferroelectric transition temperatures, but also results in a different sequence of phase transitions compared to those observed in the unstrained bulk case. 8,9 Using phenomenological Landau-Devonshire theory, it has been shown that epitaxial strain also affects the EC temperature change in BTO and related materials. [10][11][12][13] Here, we use a first principles-based effective Hamiltonian [14][15][16] to study the effect of substrate-induced clamping and compressive epitaxial strain on the EC temperature change in BaTiO 3 . In the effective Hamiltonian, the structural degrees of freedom are described via a small number of collective modes: a polar soft mode related to the ferroelectric distortion in each unit cell, and several local and global strain variables. All parameters of the corresponding energy expression...
We revisit the phase diagram of BaTiO 3 under biaxial strain using a first principles-based effective Hamiltonian approach. We show that, in addition to the tetragonal (c), quasi-rhombohedral (r), and quasiorthorhombic (aa) ferroelectric phases, that have been discussed previously, there are temperature and strain regions, in particular under tensile strain, where the system decomposes into multi-domain structures. In such cases, the strained system, at least on a local level, recovers the same phase sequence as the unclamped bulk material. Furthermore, we extend these results from the case of "uniform" biaxial strain to the situation where the two in-plane lattice constants are strained differently and show that similar considerations apply in this case.The optimization of ferroelectric materials by epitaxial growth and interface-mediated strain is nowadays a wellestablished and highly successful method. 1 However, the experimental determination of strain-temperature phase diagrams is quite challenging, since only specific strain values, corresponding to the given lattice mismatch with a specific substrate, can be investigated. Therefore, the theoretical modeling of strain-dependent phase diagrams is highly relevant.An important case is the prototypical ferroelectric BaTiO 3 (BTO), which, in its free bulk form, exhibits one paraelectric and three different ferroelectric structures as function of temperature, 4 and thus gives rise to a rich strain dependence. Different levels of sophistication have been used to model/calculate the straindependent phase diagram of BTO, however, leading in part to conflicting results. First, various calculations based on Ginzburg-Landau-Devonshire theory have been performed, yielding qualitatively consistent phase diagrams as long as only mono-domain phases are taken into account. 5,9 Once multi-domain configurations are considered, different (meta-) stable domain patterns have been found, depending on the a priori assumptions of the models. 6,7 More recently, phase field simulations have been used in order to simulate different phases and domain structures without a priori assumptions. 12 Both approaches, however, require parameters that have to be obtained from experimental data, and different choices for these parameters can lead to significantly different results. 9,13 From this perspective, first principlesbased calculations are very attractive, since in principle they do not require fitting. Indeed, first principles-based strain-temperature phase diagrams for BTO have been calculated, 13-16 but only mono-domain states have been found for simulation cell sizes of about 12-16 unit cells along each cartesian direction.Apart from determining the stability of different phases, the mechanical boundary conditions can also modify the domain pattern of the material. For example, the formation of 90 • domain walls provides an efficient way for tetragonal perovskites to partially relax the elastic energy under clamping to a periodic substrate. 17 In PbTiO 3 , such domain configurations ...
Using scanning tunneling microscopy and a diffraction experiment, we have discovered a new ordered surface alloy made out of two bulk-immiscible components, Fe and Au, deposited on a Ru(0001) substrate. In such a system, substrate-mediated strain interactions are believed to provide the main driving force for mixing. However, spin-polarized ab initio calculations show that the most stable structures are always the ones with the highest magnetic moment per Fe atom and not the ones minimizing the surface stress, in remarkable agreement with the observations. This opens up novel possibilities for creating materials with unique properties of relevance to device applications.
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