Diversity of hydrogen configuration and its roles in SrTiO3−δ

Iwazaki, Yoshiki; Gohda, Yoshihiro; Tsuneyuki, Shinji

doi:10.1063/1.4854355

Cited by 33 publications

(37 citation statements)

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“…7,25 Thus, we assume that an electron spin and a Mu + donor are respectively located on a Ti site and an interstitial site near an oxygen ion to form an OMu − group in the same unit cell. Based on the atomic configuration around H + i reported from first-principles calculations, 11 Mu + i is expected to locate at θ = 29 • on a TiO 2 plane near an O-O edge of an oxygen octahedron ( Fig. 1(c)).…”

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Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Ito

Higemoto

Koda

et al. 2019

Applied Physics Letters

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The hyperfine features and thermal stability of a muonium (Mu)-related paramagnetic center were investigated in the SrTiO 3 perovskite titanate via muon spin rotation spectroscopy. The hyperfine coupling tensor of the paramagnetic center was found to have prominent dipolar characteristics, indicating that the electron spin density is dominantly distributed on a Ti site to form a small polaron near an ionized Mu + donor. Based on a hydrogen-Mu analogy, interstitial hydrogen is also expected to form such a polaronic center in the dilute doping limit. The small activation energy of 30(3) meV found for the thermal dissociation of the Mu + -polaron complex suggests that the strain energy required to distort the lattice is comparable to the electronic energy gained by localizing the electron.Electron doping into an archetypal perovskite titanate, SrTiO 3 , results in intriguing physical properties such as superconductivity 1 and ferromagnetism, 2 and can help form a two-dimensional (2D) electron gas system at the LaAlO 3 /SrTiO 3 heterointerface. 3,4 This makes electrondoped SrTiO 3 a promising material for emerging oxide electronics. For many of its applications, it is crucial to determine whether excess electrons form either free or trapped carriers. However, experimental studies on (La, Nb)-doped or oxygen-deficient SrTiO 3 seem contradictory on this point. Optical conductivity and dc transport measurements showed that excess electrons behave as free carriers or large polarons having bandlike characteristics. 5,6 On the other hand, photoemission spectroscopy (PES) studies revealed an incoherent in-gap state, which can be associated with small polarons, and its coexistence with a coherent delocalized state. 7 The Kondo effect observed in an electric field-effect-induced 2D electron system in SrTiO 3 also suggests the coexistence of free and trapped carriers. 8 Although many theoretical studies have been conducted to understand such puzzling phenomena, 9-11 the origin of the dual electron behavior in n-type SrTiO 3 remains unclear.Interstitial hydrogen (H i ) also serves as a shallow donor in SrTiO 3 . 12 PES measurements on a SrTiO 3 (001) surface exposed to hot atomic hydrogen showed both in-gap and delocalized states. 13 This implies that small polarons can also exist in H i -doped SrTiO 3 . However, little is known about the microscopic nature of hydrogen dopants and doped electrons in bulk SrTiO 3 except for some insight provided by infrared (IR) and polarized Raman scattering spectroscopies 14-16 and positive muon spin rotation (µ + SR) spectroscopy. 17,18 . Further experimental investigations are required to fully elucidate the role of H i in bulk SrTiO 3 and better understand the behavior of excess electrons in n-type SrTiO 3 . a) ito.takashi15@jaea.go.jp Since hydrogen and muonium (Mu: a µ + -e − bound state) have almost the same reduced mass, the µ + SR technique has been extensively used for the study of hydrogen-related defects in condensed matter. [17][18][19][20][21][22][23] In this letter, we report a d...

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Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Ito

Higemoto

Koda

et al. 2019

Applied Physics Letters

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“…Several theoretical works have been published to date on the stability of hydrogen ions in oxygen-deficient perovskite titanates ATiO 3−δ and their transport and exchange mechanisms [4][5][6][7] . These studies commonly concluded that the most stable single hydrogen configuration in an n-type carrier-rich environment is H − at a doubly charged oxygen vacancy V 2+ O , formally expressed as H + O .…”

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“…This scenario explains the hydrogen transport and exchange abilities of ATiO 3−x H x well, without involving migration of the other heavy elements. Some calculations dealt with an interaction between an H + O center and an incoming H + i , which should be taken into account when considering hydrogen transport and exchange in highly hydrogenated ATiO 3−x H x 5,6 . Three metastable configurations were theoretically proposed for two hydrogen atoms at the insights into the mechanisms of hydrogen transport and exchange in ATiO 3−x H x are still quite limited.…”

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“…Our observation demonstrates that the uncommon (2H) ′0 O or (2H) 0 O center, where the hydrogen exchange can occur, is really allowed to form in the n-type perovskite lattice at least as a metastable state. This center is also important in carrier-control technology for transition-metal oxides because the V 2+ O donor can be completely passivated by forming this with hydrogen 5 .…”

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Excited configurations of hydrogen in the BaTiO3−xHx perovskite lattice associated with hydrogen exchange and transport

et al. 2017

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Excited configurations of hydrogen in the oxyhydride BaTiO3−xHx (x = 0.1 − 0.5), which are considered to be involved in its hydrogen transport and exchange processes, were investigated by positive muon spin relaxation (µ + SR) spectroscopy using muonium (Mu) as a pseudoisotope of hydrogen. Muons implanted into the BaTiO3−xHx perovskite lattice were mainly found in two qualitatively different metastable states. One was assigned to a highly mobile interstitial protonic state, which is commonly observed in perovskite oxides. The other was found to form an entangled two spin-1 2 system with the nuclear spin of an H − ion at the anion site. The structure of the (H,Mu) complex agrees well with that of a neutralized center containing two H − ions at a doubly charged oxygen vacancy, which was predicted to form in the SrTiO 3−δ perovskite lattice by a computational study [Y. Iwazaki et al., APL Materials 2, 012103 (2014)]. Above 100 K, interstitial Mu + diffusion and retrapping to a deep defect were observed, which could be a rate-limiting step of macroscopic Mu/H transport in the BaTiO3−xHx lattice.

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“…At the oxygen vacancy, hydrogen is expected to form a hydride ion (H − ). It has been suggested that a vacancy-hydrogen complex is formed with hydrogen negatively charged in SrTiO3 bulk experimentally [15,16] and theoretically [17]. Previous photoemission studies have shown that the in-gap state generated by ion bombardment or annealing on a SrTiO3(111) is decreased by H2 dosage [18,19].…”

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Electronic Structure of the SrTiO₃(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

Takeyasua¹,

Fukadaa²,

Ogura³

et al. 2014

Applied Science and Convergence Technology

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The influence of electron irradiation and hydrogen adsorption on the electronic structure of the SrTiO 3 (001) surface was investigated by ultraviolet photoemission spectroscopy (UPS).Upon electron irradiation of the surface, UPS revealed an electronic state within the band gap (in-gap state: IGS) with the surface kept at 1×1. This is considered to originate from oxygen vacancies at the topmost surface formed by electron-stimulated desorption of oxygen.Electron irradiation also caused a downward shift of the valence band maximum indicating downward band-bending and formation of a conductive layer on the surface. With oxygen dosage on the electron-irradiated surface, on the other hand, the IGS intensity was decreased along with upward band-bending, which points to disappearance of the conductive layer. The results indicate that electron irradiation and oxygen dosage allow us to control the surface electronic structure between semiconducting (nearly-vacancy free: NVF) and metallic (oxygen de cient: OD) regimes by changing the density of the oxygen vacancy. When the NVF surface was exposed to atomic hydrogen, in-gap states were induced along with downward band bending. The hydrogen saturation coverage was evaluated to be 3.1±0.8×10 14 cm −2 with nuclear reaction analysis. From the IGS intensity and H coverage, we argue that H is positively charged as H~0 :3+ on the NVF surface. On the OD surface, on the other hand, the IGS intensity due to oxygen vacancies was found to decrease to half the initial value with molecular hydrogen dosage. H is expected to be negatively charged as H − on the OD surface by occupying the oxygen vacancy site.

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Diversity of hydrogen configuration and its roles in SrTiO3−δ

Cited by 33 publications

References 32 publications

Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Excited configurations of hydrogen in the BaTiO3−xHx perovskite lattice associated with hydrogen exchange and transport

Electronic Structure of the SrTiO₃(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

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Diversity of hydrogen configuration and its roles in SrTiO3−δ

Cited by 33 publications

References 32 publications

Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Polaronic nature of a muonium-related paramagnetic center in SrTiO3

Excited configurations of hydrogen in the BaTiO3−xHx perovskite lattice associated with hydrogen exchange and transport

Electronic Structure of the SrTiO3(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

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Electronic Structure of the SrTiO₃(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption