Defect separation in strontium titanate: Formation of a polar phase

Stoichiometric perovskite-type strontium titanate acts as an insulator because of its wide electronic band gap and has therefore great potential as high-k dielectric and storage material in memory applications. Degradation phenomena of insulating properties of transition metal oxides occur during long time voltage application. From the defect chemistry point of view the question arises how mobile species react on an external electric field and which impact the redistribution has on the stability of the crystal structure. Here, we discuss near-surface reversible structural changes in SrTiO3 single crystals caused by oxygen vacancy redistribution in an external electric field. We present in-situ X-ray diffraction during and after electroformation. Several reflections are monitored and show a tetragonal elongation of the cubic unit cell. Raman investigations were carried out to verify that the expansion involves a transition from the centrosymmetric to a less symmetric structure. Regarding a whole formation cycle, two different time scales occur: a slow one during the increase of the lattice constant and a fast one after switching off the electric field. Based on the experimental data we suggest a model containing the formation of a polar SrTiO3 unit cell stabilized by the electric field, which is referred to as migration-induced field-stabilized polar phase [1] at room temperature. As expected by a non-centrosymmetric crystal structure, pyroelectric properties will be presented in conjunction with temperature modulated electroformation cycles. Furthermore, we show that intrinsic defect separation establishes a non-equilibrium accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicates an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of a solid-state SrTiO3 battery is reported.

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Interactions of Ruddlesden-Popper Phases and Migration-Induced Field-Stabilized Polar Phase in Strontium Titanate

Crystals ◽

10.3390/cryst11060693 ◽

2021 ◽

Vol 11 (6) ◽

pp. 693

Author(s):

Christian Ludt ◽

Elena Ovchinnikova ◽

Anton Kulikov ◽

Dmitri Novikov ◽

Sibylle Gemming ◽

...

Keyword(s):

Strontium Titanate ◽

Density Functional ◽

Surface Region ◽

Point Of View ◽

Polar Phase ◽

Near Surface ◽

Atomic Displacements ◽

Induced Field ◽

And Migration ◽

Novel Concept

This work focuses on the validation of a possible connection of the known Ruddlesden-Popper (RP) phases and the novel concept of the migration-induced field-stabilized polar (MFP) phase. To study this subject, model structures of RP phases in bulk strontium titanate are analyzed by means of density functional theory (DFT). The obtained geometries are compared to experimental MFP data. Good agreement can be found concerning atomic displacements in the pm range and lattice strain inferred by the RP phases. Looking at the energy point of view, the defect structures are on the convex hull of the Gibb’s free energy. Although the dynamics to form the discussed defect models are not addressed in detail, the interplay and stability of the described defect model will add to the possible structure scenarios within the near-surface region of strontium titanate. As a result, it can be suggested that RP phases generally favor the MFP formation.

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Lateral deformations of a crystal of potassium acid phthalate in an external electric field

Journal of Applied Crystallography ◽

10.1107/s1600576721007366 ◽

2021 ◽

Vol 54 (5) ◽

pp. 1317-1326

Author(s):

Arsen Petrenko ◽

Nataliya Novikova ◽

Alexander Blagov ◽

Anton Kulikov ◽

Yury Pisarevskii ◽

...

Keyword(s):

Crystal Structure ◽

Electrical Conductivity ◽

Electric Field ◽

External Electric Field ◽

Applied Field ◽

X Ray Diffraction ◽

Time Resolved ◽

X Ray ◽

Potassium Acid Phthalate ◽

Acid Phthalate

The anisotropy of deformations in potassium acid phthalate crystals arising under the action of an external electric field up to 1 kV mm−1 applied along the [001] polar axis was studied using X-ray diffraction methods at room temperature. Electrical conductivity was measured and rocking curves for reflections 400, 070 and 004 were obtained by time-resolved X-ray diffractometry in Laue and Bragg geometries. Two saturation processes were observed from the time dependences of the electrical conductivity. A shift in the diffraction peaks and a change in their intensity were found, which indicated a deformation of the crystal structure. Rapid piezoelectric deformation and reversible relaxation-like deformation, kinetically similar to the electrical conductivity of a crystal, were revealed. The deformation depended on the polarity and strength of the applied field. The deformation was more noticeable in the [100] direction and was practically absent in the [001] direction of the applied field. X-ray diffraction analysis revealed a disordered arrangement of potassium atoms, i.e. additional positions and vacancies. The heights of potential barriers between the positions of K+ ions and the paths of their possible migration in the crystal structure of potassium acid phthalate were determined. The data obtained by time-resolved X-ray diffractometry and X-ray structure analysis, along with additional electrophysical measurements, allow the conclusion that the migration of charge carriers (potassium cations) leads to lateral deformation of the crystal structure of potassium phthalate in an external electric field.

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Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field

Nanoscale ◽

10.1039/c4nr00008k ◽

2014 ◽

Vol 6 (15) ◽

pp. 8624-8634 ◽

Cited By ~ 50

Author(s):

Qing Tang ◽

Jie Bao ◽

Yafei Li ◽

Zhen Zhou ◽

Zhongfang Chen

Keyword(s):

Electric Field ◽

External Electric Field ◽

Band Gaps ◽

Band Structures ◽

Electronic Band ◽

Dft Computations ◽

Electronic Band Structures

DFT computations with dispersion corrections were performed to investigate dihalogen interactions and their effect on the electronic band structures of halogenated BN bilayers and aligned halogen-passivated zigzag BN nanoribbons (BNNRs).

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