First-principles Study of Unusual Behavior in PbTiO3/KTaO3 Superlattice

Abstract

Abstract. Ferroelectric superlattices have attracted wide attentions and become a research focus due to excellent performance. In order to investigate the effects of polar discontinuity on properties of superlattices. The polarization behavior of PbTiO3/KTaO3 (PKT) superlattices are studied by first-principles calculation in this paper. It is found that the polarization of local layer in PKT superlattices does not keep a constant value through each superlattice and the directions of polarizations of each local layer are all the same, because the residual plane charges in TaO2/PbO and TiO2/KO interfaces provide mask effect. Furthermore, polarization of (PbTiO3)m/(KTaO3)n superlattices decreases as m/n decreasing, (PbTiO3)n/(KTaO3)n superlattices show declining polarization and become more and more conductive with the increase of n. IntroductionRecently, ferroelectric superlattices have attracted lots of attentions due to their perfect properties and have more potential in the application of miniature devices. Superlattices, includingand LaAlO3/SrTiO3 [9] have been well researched in previous work. Firstprinciples calculations were extensively used to investigate interesting properties and design new ferroelectric material before experimental work is performed.Pentcheva and Pickett[10] revealed the compensation mechanisms in the LaAlO3/SrTiO3 system. The electron-doped n-type interface with an LaO layer next to a TiO2 layer and the hole-doped ptype interface where an SrO layer lies next to an AlO2 layer are two distinct types of interfaces. It reported that the unusually high carrier density is rather related to the formation of oxygen vacancies during the deposition process, and it is interesting that the n-type interface between these two band insulators is conducting, while the p-type interface was reported to be insulating. Polar discontinuity, as introduced by Wu and Vanderbilt[11], implied that head-to-head (HH) and tail-to-tail (TT) domain walls were induced to form a ferroelectric superlattice via the insertion of compensating "delta doping" layers. Generally a HH or TT domain wall is perpendicular to the polarization direction, and their formation would need an unacceptable Coulomb energy cost or induce the domain wall to become metallic so that the free carriers could compensate the domain wall. However, it was shown that the delta-doping layers could be arranged to compensate the polarization bound charge, allowing a structure where the ferroelectric domains are polarized in opposite directions and separated by HH and TT domain walls. This kind of superlattice with polar discontinuity was proved to be a promising new material with interesting properties. Das [12]found that systems containing polar discontinuities especially for ultra-thin superlattices have strongly broken inversion symmetry and possessed large nonswitchable polarization.In this study, the effect of polar discontinuity on the polarization of a series of period-5 superlattices PKT and cycle stack superlattice (PbTiO3)n/(KTaO3)n is invest...

Piezoelectric properties of II–IV/I–V and II–IV/III–III ferroelectric perovskite superlattices

“…The comparison of polarizations calculated in structures with and without octahedral rotations shows that in structures with the P c space group, the neglect of the AFD rotations can lead to an error in determining P z up to 30% and, in some cases, even to an error in the sign of this quantity. For the P 4mm phases, the obtained values agree well with the published values of P z = 0.532 C/m 2 for the PbTiO 3 /KNbO 3 SL and P z = 0.202 C/m 2 for the PbTiO 3 /LaAlO 3 SL [29], and with the P z = 0.38 C/m 2 value for the PbTiO 3 /KTaO 3 SL [40].…”

“…A possible reason for a stronger discrepancy here may be the neglect in [29] of the difference in displacements and effective charges of the oxygen atoms (their effective charge varies from −2.07 to −5.84). As for the PbTiO 3 /KTaO 3 superlattice studied in [40], the values of the e 33 and e 15 coefficients obtained there are two orders of magnitude lower than our data, and the e 31 coefficient is close to our result in magnitude, but differs in sign.…”

“…The reason for our interest to the ferroelectric instability in superlattices with the polar discontinuity is that in such SLs it is possible to obtain sufficiently high piezoelectric coefficients associated with the ferroelectric phase transitions occurring in them. The literature data on the piezoelectric properties of such superlattices are limited to calculations for the P 4mm phases of PbTiO 3 /LaAlO 3 and KNbO 3 /PbTiO 3 SLs [28,29] and of the PbTiO 3 /KTaO 3 one [40]. As the ground- In SLs with a tetragonal ground-state structure, in which the polarization is directed along the z axis, five components of the piezoelectric tensor are nonzero.…”

Piezoelectric properties of ferroelectric perovskite superlattices with polar discontinuity

“…A comparison of polarizations calculated for structures with and without octahedral rotations shows that in SLs with the P c ground-state structure, the neglect of these rotations can result in an error in determining P z up to 30% and, in some cases, even in an error in the sign of this quantity. For the P 4mm phase, our values reasonably agree with the published values of P z = 0.532 C/m 2 for PbTiO 3 /KNbO 3 [72], P z = 0.202 C/m 2 for PbTiO 3 /LaAlO 3 [72], and P z = 0.38 C/m 2 for PbTiO 3 /KTaO 3 [85] SLs.…”

“…The reason for our interest to ferroelectric instability in superlattices with the polar discontinuity is that in such SLs it is possible to obtain rather high piezoelectric coefficients resulting from the in-plane ferroelectric phase transitions appearing in them. The literature data on the piezoelectric properties of such superlattices are limited to calculations for the high-symmetry P 4mm phase of PbTiO 3 /LaAlO 3 and KNbO 3 /PbTiO 3 SLs [71,72] and of the PbTiO 3 /KTaO 3 one [85]. As the ground-state structure of the first two superlattices differs from P 4mm, more correct calculations for these SLs are needed.…”