Spectroscopic mapping of local structural distortions in ferroelectric PbTiO3/SrTiO3superlattices at the unit-cell scale

Abstract

The local structural distortions in polydomain ferroelectric PbTiO 3 /SrTiO 3 superlattices are investigated by means of high spatial and energy resolution electron-energy-loss spectroscopy combined with high-angle annular dark field imaging. Local structural variations across the interfaces have been identified with unit-cell resolution through the analysis of the energy-loss near-edge structure of the Ti L 2,3 and O K edges. Ab initio and multiplet calculations of the Ti L 2,3 edges provide unambiguous evidence for an inhomogeneous polarization profile associated with the observed structural distortions across the superlattice. Complex oxide heterostructures offer a vast playground for exploring and combining the many functional properties of these interesting materials arising from the subtle interplay between their charge, spin, orbital, and lattice degrees of freedom. Bilayers, multilayers, and superlattices composed of ultrathin oxide layers not only shed light on our fundamental understanding of the constituent materials, but frequently reveal unexpected phases at their interfaces.1 Superlattices composed of ferroelectric and paraelectric oxides have been the subject of numerous studies, motivated by fundamental questions about ferroelectric size effects, by the possibilities these artificially layered materials offer for tailoring their functional properties, and by the interesting interface physics they display. Ultrafine period superlattices, composed of ferroelectric PbTiO 3 (PTO) and paraelectric SrTiO 3 (STO), for example, have been shown to exhibit improper ferroelectricity driven by the coupling of the polar and nonpolar distortions at the interface.2 More recently, regular ferroelectric nanodomains have been observed in PTO/STO superlattices with larger periodicities and were shown to be responsible for large enhancements in the effective dielectric constant.3 Such domains are expected to give rise to complex inhomogeneous structural distortions and polarization profiles, 4,5 departing from uniform polarization models, frequently used to describe the properties of ferroelectric/paraelectric superlattices in the absence of domains. 6 Thus a microscopic insight into the local structure is key to understanding the behavior of these artificially layered materials, and here transmission electron microscopy (TEM), with recent advances in spatial resolution, provides an invaluable tool. Individual ionic displacements within a single perovskite unit cell can now be identified from phase contrast images. The spatial resolution is high enough to determine the direction and even the magnitude of the local dipole moments in ferroelectric materials, 7-9 and has recently enabled the direct verification of the existence of polarization rotation at domain walls in Pb(Zr,Ti)O 3 ferroelectric thin films. 10In this Rapid Communication, we focus on an alternative spectroscopic technique to study local ferroelectric distortions at the single unit-cell scale. Atomically resolved electronenergy-loss spectra (EELS) and ...

Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

“…The largest shift of this peak occurs on approaching the grain boundary, leading to a spectrum shape which has a remarkable resemblance to the O-K edge of bulk PTO. 31 This shift confirms how the bonding nature of oxygen is altered as a function of the change in the A-site chemistry between the more BTO-like bulk-of-grain regions compared to the Pb-rich grain boundary regions. To rule out any orientation effects influencing the ELNES the same area was measured twice, once with the bottom grain tilted onto zone axis, and once with the top grain tilted onto zone axis, similar features were seen either side of the grain boundary (Fig.…”

“…30 The most striking feature observed here is a gradual shift in the peak marked with an asterisk. This peak is recognized to arise from the hybridization of the O-2p orbitals with states attributed to the A-site cation, 31 making it possible to distinguish a chemical transition in the A-site (Ba, Pb, and Ca). The largest shift of this peak occurs on approaching the grain boundary, leading to a spectrum shape which has a remarkable resemblance to the O-K edge of bulk PTO.…”

“…Due to the energy resolution in experiment, the splitting of the e g peak is not easily resolved; however, it is well established that this splitting effect is observed as an e g peak broadening. 31,36,38 As the e g peak broadens, the CFS is reduced and an inverse relationship between CFS and the average Ti-O bond length is established. 37,39 Due to the clear chemical inhomogeneity in this ceramic and the intrinsic change in symmetry between BTO, PTO, and CTO, the CFS is expected to change.…”

“…37,39 Due to the clear chemical inhomogeneity in this ceramic and the intrinsic change in symmetry between BTO, PTO, and CTO, the CFS is expected to change. 30,31,37 It has been shown that bulk BTO has a higher CFS compared to bulk PTO, 36,38 and therefore a quantitative analysis of the CFS across the grain boundary was carried out here. The contribution from CTO can be disregarded since the Ca content is constant.…”

Structural and energetic properties of domains in PbTiO3/SrTiO3superlattices from first principles

“…While the results of the superlattices asymmetrically sandwiched between Nb-doped SrTiO 3 (bottom) and gold (top) electrodes were consistent with a monodomain configuration, 28 those corresponding to the use of symmetrically coated SrRuO 3 electrodes (both top and bottom) suggested a polydomain phase with DW motion, which dynamics might be quite different than the conventional one. 29 Furthermore, through a combination of x-ray diffraction, transmission electron microscopy, and ultrahigh resolution electron energy loss spectroscopy (EELS), Zubko et al 30,31 have also recently showed the progressive transition between two different regimes, controlled by the thickness of the individual SrTiO 3 and PbTiO 3 layers. In the first regime, present for sufficiently thin paraelectric layers, SrTiO 3 and PbTiO 3 are strongly electrostatically coupled: a uniform monodomain polarization is adopted throughout the thickness of the superlattice to minimize the depolarizing field.…”