We study the structural, ferroelectric, and magnetic properties of the potentially multiferroic Aurivillius phase material Bi5FeTi3O15 using first principles electronic structure calculations. Calculations are performed both with PBE and PBEsol exchange correlation functionals. We conclude that PBE systematically overestimates the lattice constants and the magnitude of the ferroelectric distortion, whereas PBEsol leads to good agreement with available experimental data. We then assess a potential site preference of the Fe 3+ cation by comparing 10 different distributions of the perovskite B-sites. We find a slight preference for the "inner" site, consistent with recent experimental observations. We obtain a large value of ∼55 µC/cm 2 for the spontaneous electric polarization, which is rather independent of the specific Fe distribution. Finally, we calculate the strength of the magnetic coupling constants and find strong antiferromagnetic coupling between Fe 3+ cations in nearest neighbor positions, whereas the coupling between further neighbors is rather weak. This poses the question whether magnetic long range order can occur in this system in spite of the low concentration of magnetic ions. arXiv:1407.3973v2 [cond-mat.mtrl-sci]

This work explores the impact of in-plane bi-axial (epitaxial) strain on the cation distribution and electric polarization of the Aurivillius-phase compound Bi5FeTi3O15 using first-principles electronic structure calculations. Our calculations indicate that the site preference of the Fe 3+ cation can be controlled via epitaxial strain. Tensile strain enhances the preference for the inner sites within the perovskite-like layers of the Aurivillius-phase structure, whereas compressive strain favors occupation of the outer sites, i.e., the sites close to the Bi2O2 layer. Controlling the distribution of the magnetic cations offers the possibility to control magnetic order in this magnetically dilute system. Furthermore, the magnitude of the electric polarization is strongly strain-dependent, increasing under tensile strain and decreasing under compressive strain. We find strongly anomalous Born effective charges, both of the Bi 3+ and the Ti 4+ cations.Controlling the properties of complex transition metal oxides by epitaxial strain, i.e., by growing thin films of a certain material on a substrate with specific lattice mismatch, has emerged as a very efficient way for designing optimized functionalities. 1 In particular, the effect of epitaxial strain on the ferroelectric properties of perovskite materials is well studied, 2 and dramatic enhancements of polarization and ferroelectric ordering temperatures, 3 as well as emergence of ferroelectricity in otherwise nonpolar materials have been reported. 4,5 Recently, layered perovskite-related systems have come into focus as being potentially more amenable to developing polar lattice distortions compared to bulk perovskites. 6 Examples include artificial perovskite superlattices and double perovskites, as well as several families of naturally-layered perovskite-derived crystal structures such as the Ruddlesden-Popper series, Aurivillius-phases, or Dion-Jacobson systems. Only few studies addressing the strain response of these naturally-layered materials are currently available. Such studies are, however, of great interest due to the different mechanism underlying the ferroelectricity in these systems, which could lead to a different strain response compared to bulk perovskite ferroelectrics.Here we study the case of Bi 5 FeTi 3 O 15 (BFTO), 7,8 a representative of the family of naturally-layered Aurivillius-phases, which is of particular interest due to its potential multiferroic properties. 9 The crystal structure of the Aurivillius-phases consists of m perovskite layers (A m−1 B m O 3m+1 ) 2− stacked periodically along the [001] direction, and separated by fluorite-like (Bi 2 O 2 ) 2+ layers (see Fig. 1). 10,11 BFTO corresponds to the case with m = 4.A previous first-principles study of polarization-strain coupling in the m = 3 Aurivillius compound Bi 4 Ti 3 O 12 has shown a large response of in-plane polarization under bi-axial strain. 13 Apart from the introduction of an additional perovskite layer and the presence of the nominally non-ferroelectric Fe 3+ cation, an ...

We determine the viability of 4-layered Aurivillius phases to exhibit long-range magnetic order above room temperature. We use Monte Carlo simulations to calculate transition temperatures for an effective Heisenberg model containing a minimal set of required couplings. The magnitude of the corresponding coupling constants has been determined previously from electronic structure calculations for Bi5FeTi3O15, for which we obtain a transition temperature far below room temperature. We analyze the role of further neighbor interactions within our Heisenberg model, in particular that of the second-nearest-neighbor coupling within the perovskite-like layers of the Aurivillius structure, as well as that of the weak inter-layer coupling, in order to identify the main bottleneck for achieving higher magnetic transition temperatures. Based on our findings, we show that the most promising strategy to obtain magnetic order at higher temperatures is to increase the concentration of magnetic cations within the perovskite-like layers, and we propose candidate compounds where magnetic order could be achieved above room temperature.

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO3 substrate exhibits a much higher superconducting critical temperature Tc than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO3 and the effect of an interfacial Ti1+xO2 layer on the increased Tc using a combination of scanning transmission electron microscopy and density functional theory. We find Ti1+xO2 forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer electron-dopes the FeSe monolayer in agreement with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that favor a Tc increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO3 and PbTiO3 (STO/PTO) is determined using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies Ev in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on the Ev are examined. The formation energies vary layer-by-layer in the superlattices, with Ev being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, Ev of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a 0 a 0 c -) in the FE superlattice have a significant effect on the Ev, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties via controlling the constituent materials of superlattices and interface engineering.