Editorial

3 pages, 181 KiB

Open AccessEditorial

Electronic Phenomena of Transition Metal Oxides

by Christian Rodenbücher and Kristof Szot

Crystals 2021, 11(3), 256; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11030256 - 05 Mar 2021

Cited by 2 | Viewed by 1379

Transition metal oxides with ABO₃ or BO₂ structures have become one of the major research fields in solid state science, as they exhibit an impressive variety of unusual and exotic phenomena with potential for their exploitation in real-world applications [...] Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

Research

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13 pages, 671 KiB

Open AccessArticle

Comparative Ab Initio Calculations of ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) Surfaces

by Roberts I. Eglitis, Juris Purans, Jevgenijs Gabrusenoks, Anatoli I. Popov and Ran Jia

Crystals 2020, 10(9), 745; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10090745 - 24 Aug 2020

Cited by 50 | Viewed by 3411

Abstract

We performed, for first time, ab initio calculations for the ReO₂-terminated ReO₃ (001) surface and analyzed systematic trends in the ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surfaces using first-principles calculations. According [...] Read more.

We performed, for first time, ab initio calculations for the ReO₂-terminated ReO₃ (001) surface and analyzed systematic trends in the ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surfaces using first-principles calculations. According to the ab initio calculation results, all ReO₃, SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surface upper-layer atoms relax inwards towards the crystal bulk, all second-layer atoms relax upwards and all third-layer atoms, again, relax inwards. The ReO₂-terminated ReO₃ and ZrO₂-terminated SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ (001) surface band gaps at the Γ–Γ point are always reduced in comparison to their bulk band gap values. The Zr–O chemical bond populations in the SrZrO₃, BaZrO₃, PbZrO₃ and CaZrO₃ perovskite bulk are always smaller than those near the ZrO₂-terminated (001) surfaces. In contrast, the Re–O chemical bond population in the ReO₃ bulk (0.212e) is larger than that near the ReO₂-terminated ReO₃ (001) surface (0.170e). Nevertheless, the Re–O chemical bond population between the Re atom located on the ReO₂-terminated ReO₃ (001) surface upper layer and the O atom located on the ReO₂-terminated ReO₃ (001) surface second layer (0.262e) is the largest. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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8 pages, 2533 KiB

Open AccessArticle

Electron-Beam-Induced Current and Cathodoluminescence Study of Dislocations in SrTiO₃

by Wei Yi, Jun Chen and Takashi Sekiguchi

Crystals 2020, 10(9), 736; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10090736 - 21 Aug 2020

Cited by 3 | Viewed by 2620

Abstract

Electron-beam-induced current (EBIC) and cathodoluminescence (CL) have been applied to investigate the electrical and optical behaviors of dislocations in SrTiO₃. The electrical recombination activity and defect energy levels of dislocations have been deduced from the temperature-dependent EBIC measurement. Dislocations contributed to [...] Read more.

Electron-beam-induced current (EBIC) and cathodoluminescence (CL) have been applied to investigate the electrical and optical behaviors of dislocations in SrTiO₃. The electrical recombination activity and defect energy levels of dislocations have been deduced from the temperature-dependent EBIC measurement. Dislocations contributed to resistive switching were clarified by bias-dependent EBIC. The distribution of oxygen vacancies around dislocations has been obtained by CL mapping. The correlation between switching, dislocation and oxygen vacancies was discussed. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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19 pages, 69349 KiB

Open AccessArticle

The Electronic Properties of Extended Defects in SrTiO₃—A Case Study of a Real Bicrystal Boundary

by Christian Rodenbücher, Dominik Wrana, Thomas Gensch, Franciszek Krok, Carsten Korte and Krzysztof Szot

Crystals 2020, 10(8), 665; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10080665 - 02 Aug 2020

Cited by 9 | Viewed by 3227

Abstract

This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO₃ by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has [...] Read more.

This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO₃ by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has been proposed in the literature on the basis of ab initio simulations, as well as on experiments employing local-conductivity atomic force microscopy (LC-AFM), detailed investigations of the bicrystal boundary are conducted. In addition to LC-AFM, fluorescence lifetime imaging microscopy (FLIM) is applied herein as a complementary method for mapping the local electronic properties on the microscale. Both techniques confirm that the electronic structure and electronic transport in dislocation-rich regions significantly differ from those of undistorted SrTiO₃. Upon thermal reduction, a further confinement of conductivity to the bicrystal boundary region was found, indicating that extended defects can indeed be regarded as the origin of filament formation. This leads to the evolution of inhomogeneous properties of defective SrTiO₃ on the nano- and microscales. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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14 pages, 3584 KiB

Open AccessArticle

Investigation of Statistical Metal-Insulator Transition Properties of Electronic Domains in Spatially Confined VO₂ Nanostructure

by Azusa N. Hattori, Ai I. Osaka, Ken Hattori, Yasuhisa Naitoh, Hisashi Shima, Hiroyuki Akinaga and Hidekazu Tanaka

Crystals 2020, 10(8), 631; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10080631 - 22 Jul 2020

Cited by 14 | Viewed by 3632

Abstract

Functional oxides with strongly correlated electron systems, such as vanadium dioxide, manganite, and so on, show a metal-insulator transition and an insulator-metal transition (MIT and IMT) with a change in conductivity of several orders of magnitude. Since the discovery of phase separation during [...] Read more.

Functional oxides with strongly correlated electron systems, such as vanadium dioxide, manganite, and so on, show a metal-insulator transition and an insulator-metal transition (MIT and IMT) with a change in conductivity of several orders of magnitude. Since the discovery of phase separation during transition processes, many researchers have been trying to capture a nanoscale electronic domain and investigate its exotic properties. To understand the exotic properties of the nanoscale electronic domain, we studied the MIT and IMT properties for the VO₂ electronic domains confined into a 20 nm length scale. The confined domains in VO₂ exhibited an intrinsic first-order MIT and IMT with an unusually steep single-step change in the temperature dependent resistivity (R-T) curve. The investigation of the temperature-sweep-rate dependent MIT and IMT properties revealed the statistical transition behavior among the domains. These results are the first demonstration approaching the transition dynamics: the competition between the phase-transition kinetics and experimental temperature-sweep-rate in a nano scale. We proposed a statistical transition model to describe the correlation between the domain behavior and the observable R-T curve, which connect the progression of the MIT and IMT from the macroscopic to microscopic viewpoints. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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22 pages, 4310 KiB

Open AccessArticle

Defects and Lattice Instability in Doped Lead-Based Perovskite Antiferroelectrics: Revisited

by Dariusz Kajewski

Crystals 2020, 10(6), 501; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10060501 - 12 Jun 2020

Cited by 1 | Viewed by 2095

Abstract

This paper is a summary of earlier results that have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZrO₃ single crystals doped with niobium and Pb(Zr_0.70Ti_0.30)O₃ ceramics doped [...] Read more.

This paper is a summary of earlier results that have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZrO₃ single crystals doped with niobium and Pb(Zr_0.70Ti_0.30)O₃ ceramics doped with different concentration of Bi₂O₃. It was found that these crystals undergo new phase transitions never observed before. To investigate all phase transitions, different experimental methods were used to characterize the crystal properties. Temperature and time dependencies have been tentatively measured in a wide range, including a region above T_c, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices. Also, coexistence of antiferroelectric phase and one of the intermediate phases could be observed in a wide temperature range. The phase transition mechanism in PbZrO₃ is discussed, taking into account the local breaking of the crystal symmetry above T_c and the defects of crystal lattices, i.e., those generated during crystal growth, and intentionally introduced by preheating in a vacuum or doping with hetero-valent dopant. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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15 pages, 3488 KiB

Open AccessArticle

Unconventional Co-Existence of Insulating Nano-Regions and Conducting Filaments in Reduced SrTiO₃: Mode Softening, Local Piezoelectricity, and Metallicity

by Annette Bussmann-Holder, Hugo Keller, Arndt Simon, Gustav Bihlmayer, Krystian Roleder and Krzysztof Szot

Crystals 2020, 10(6), 437; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10060437 - 29 May 2020

Cited by 9 | Viewed by 2213

Abstract

Doped SrTiO₃ becomes a metal at extremely low doping concentrations n and is even superconducting at n < 10²⁰ cm⁻³, with the superconducting transition temperature adopting a dome-like shape with increasing carrier concentration. In this paper it is shown [...] Read more.

Doped SrTiO₃ becomes a metal at extremely low doping concentrations n and is even superconducting at n < 10²⁰ cm⁻³, with the superconducting transition temperature adopting a dome-like shape with increasing carrier concentration. In this paper it is shown within the polarizability model and from first principles calculations that up to a well-defined carrier concentration n_c transverse optic mode softening takes place together with polar nano-domain formation, which provides evidence of inhomogeneity and a two-component type behavior with metallicity coexisting with polarity. Beyond this region, a conventional metal is formed where superconductivity as well as mode softening is absent. For n ≤ n_c the effective electron-phonon coupling follows the superconducting transition temperature. Effusion measurements, as well as macroscopic and nanoscopic conductivity measurements, indicate that the distribution of oxygen vacancies is local and inhomogeneous, from which it is concluded that metallicity stems from filaments which are embedded in a polar matrix as long as n ≤ n_c. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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12 pages, 5885 KiB

Open AccessArticle

Inhomogeneity and Segregation Effect in the Surface Layer of Fe-Doped SrTiO₃ Single Crystals

by Marcin Wojtyniak, Katarzyna Balin, Jacek Szade and Krzysztof Szot

Crystals 2020, 10(1), 33; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10010033 - 10 Jan 2020

Cited by 8 | Viewed by 2702

Abstract

The effect of Fe doping on SrTiO₃ single crystals was investigated in terms of crystal and electronic structure over a wide temperature range in both oxidizing and reducing conditions. The electrical properties were thoroughly studied with a special focus on the resistive [...] Read more.

The effect of Fe doping on SrTiO₃ single crystals was investigated in terms of crystal and electronic structure over a wide temperature range in both oxidizing and reducing conditions. The electrical properties were thoroughly studied with a special focus on the resistive switching phenomenon. Contrary to the undoped SrTiO₃ crystals, where isolated filaments are responsible for resistive switching, the iron-doped crystals showed stripe-like conducting regions at the nanoscale. The results showed a non-uniform Fe distribution of as-received crystals and the formation of new phases in the surface layer of reduced/oxidized samples. The oxidation procedure led to a separation of Ti(Fe) and Sr, while the reduction resulted in the tendency of Fe to agglomerate and migrate away from the surface as seen from the time of flight mass spectroscopy measurements. Moreover, a clear presence of Fe-rich nano-filament in the reduced sample was found. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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10 pages, 2418 KiB

Open AccessArticle

Electronic Structure of Oxygen-Deficient SrTiO₃ and Sr₂TiO₄

by Ali Al-Zubi, Gustav Bihlmayer and Stefan Blügel

Crystals 2019, 9(11), 580; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9110580 - 07 Nov 2019

Cited by 16 | Viewed by 3495

Abstract

The conductive behavior of the perovskite SrTiO

_{3}

is strongly influenced by the presence of oxygen vacancies in this material, therefore the identification of such defects with spectroscopic methods is of high importance. We use density functional theory to characterize the defect-induced states [...] Read more.

The conductive behavior of the perovskite SrTiO

_{3}

is strongly influenced by the presence of oxygen vacancies in this material, therefore the identification of such defects with spectroscopic methods is of high importance. We use density functional theory to characterize the defect-induced states in SrTiO

_{3}

and Sr

_{2}

TiO

_{4}

. Their signatures at the surface, the visibility for scanning tunneling spectroscopy and locally conductive atomic force microscopy, and the core-level shifts observed on Ti atoms in the vicinity of the defect are studied. In particular, we find that the exact location of the defect state (e.g., in SrO or TiO

_{2}

planes relative to the surface) are decisive for their visibility for scanning-probe methods. Moreover, the usual distinction between Ti

^{3 +}

and Ti

^{2 +}

species, which can occur near defects or their aggregates, cannot be directly translated in characteristic shifts of the core levels. The width of the defect-induced in-gap states is found to depend critically on the arrangement of the defects. This also has consequences for the spectroscopic signatures observed in so-called resistive switching phenomena. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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12 pages, 801 KiB

Open AccessArticle

Intrinsic Ferroelectricity in Charge-Ordered Magnetite

by Manuel Angst, Shilpa Adiga, Semen Gorfman, Michael Ziolkowski, Jörg Strempfer, Christoph Grams, Manuel Pietsch and Joachim Hemberger

Crystals 2019, 9(11), 546; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9110546 - 23 Oct 2019

Cited by 6 | Viewed by 2957

Abstract

Single crystalline magnetite Fe₃O₄ was investigated at low temperatures in the charge ordered state by electric measurements and time-resolved diffraction with voltage applied in-situ. Dielectric spectroscopy indicates relaxor ferroelectric characteristics, with polarization switching observably only at sufficiently low temperatures and [...] Read more.

Single crystalline magnetite Fe₃O₄ was investigated at low temperatures in the charge ordered state by electric measurements and time-resolved diffraction with voltage applied in-situ. Dielectric spectroscopy indicates relaxor ferroelectric characteristics, with polarization switching observably only at sufficiently low temperatures and in a suitably chosen time-window. PUND measurements with a ms time scale indicate a switchable polarization of about 0.6 µC/cm². Significant switching occurs only above a threshold field of about 3 kV/mm, and it occurs with a time delay of about 20 µs. The time-resolved diffraction experiment yields, for sufficiently high voltage pulses, a systematic variation by about 0.1% of the intensity of the (

2, \bar{2}, \bar{10}

) Bragg reflection, which is attributed to structural switching of domains of the non-centrosymmetric

C c

structure to its inversion twins, providing proof of intrinsic ferroelectricity in charge ordered magnetite. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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Review

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23 pages, 10591 KiB

Open AccessEditor’s ChoiceReview

Polaronic Emergent Phases in Manganite-based Heterostructures

by Vasily Moshnyaga and Konrad Samwer

Crystals 2019, 9(10), 489; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9100489 - 22 Sep 2019

Cited by 4 | Viewed by 3695

Abstract

Transition metal functional oxides, e.g., perovskite manganites, with strong electron, spin and lattice correlations, are well-known for different phase transitions and field-induced colossal effects at the phase transition. Recently, the interfaces between dissimilar perovskites were shown to be a promising concept for the [...] Read more.

Transition metal functional oxides, e.g., perovskite manganites, with strong electron, spin and lattice correlations, are well-known for different phase transitions and field-induced colossal effects at the phase transition. Recently, the interfaces between dissimilar perovskites were shown to be a promising concept for the search of emerging phases with novel functionalities. We demonstrate that the properties of manganite films are effectively controlled by low dimensional emerging phases at intrinsic and extrinsic interfaces and appeared as a result of symmetry breaking. The examples include correlated Jahn–Teller polarons in the phase-separated (La_1−yPr_y)_0.7Ca_0.3MnO₃, electron-rich Jahn–Teller-distorted surface or “dead” layer in La_0.7Sr_0.3MnO₃, electric-field-induced healing of “dead” layer as an origin of resistance switching effect, and high-T_C ferromagnetic emerging phase at the SrMnO₃/LaMnO₃ interface in superlattices. These 2D polaronic phases with short-range electron, spin, and lattice reconstructions could be extremely sensitive to external fields, thus, providing a rational explanation of colossal effects in perovskite manganites. Full article

(This article belongs to the Special Issue Electronic Phenomena of Transition Metal Oxides)

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