Bulk SrTiO3 Research Articles

Ab initio investigation of the role of vanadium impurity states in SrTiO3 for thermoelectricity

Despite their low cost, non-toxicity, thermal/chemical stability and relative abundance, oxides are not considered as promising materials for future thermoelectric devices. The main reason behind this lack of interest is their relatively high thermal conductivity. However, some oxides, such as n doped bulk SrTiO3 exhibit a rather good power factor. Keeping in mind that alloying and nanostructuring are two efficient procedures to suppress the thermal conductivity, it is worth exploring pathways towards an enhancement of the thermopower. Here, we report that vanadium doping of SrTiO3 gives rise to well defined resonant states allowing power factor of both n and p type over a relatively broad range of carrier concentrations. We investigate both the effects of a rigid band shift of the Fermi level and of realistic co-doping using DFT calculations.

Effect of Dy2O3 content on the dielectric, ferroelectric, and energy storage properties of lead-free 0.5Na0.5Bi0.5TiO3–0.5SrTiO3 bulk ceramics

In the past decades, lead-free ceramics with high polarization, low remnant polarization and high electric breakdown strength have drawn a lot of attention because of their potential applications in dielectric capacitors with excellent energy storage performance. In the current investigation, we develop a novel lead-free Dy doped 0.5Na0.5Bi0.5TiO3–0.5SrTiO3 ceramics, which were fabricated by the conventional electroceramic processing route. By doping Dy2O3 into 0.5Na0.5Bi0.5TiO3–0.5SrTiO3 with various contents from 0.1 at.% to 0.5 at.%, pure perovskite phases and relatively dense structures were obtained. The increase of Dy2O3 concentration not only shifts the Tm toward to lower temperature, but also reduces the permittivity and gain sizes, thus enhancing the temperature stability of dielectric properties. The enhanced dielectric energy storage properties were systematically investigated by testing the polarization–electric field hysteresis loops as a function of the applied electric field, temperature, and loading cycles. The dielectric energy storage density of in 0.1 at.% doped 0.5Na0.5Bi0.5TiO3–0.5SrTiO3 is 1.59 J/cm3, which is nearly twice higher than that of undoped NBT–ST. Hence, the Dy modified 0.5Na0.5Bi0.5TiO3–0.5SrTiO3 ceramics have exhibited potential application as dielectric capacitors with relatively good energy storage performance.

The ferroelectric, dielectric and energy storage properties of Pb-free 0.6Na0.5Bi0.5TiO3-0.4SrTiO3 bulk ceramics modified by Fe2O3

Lead-free 0.6Na0.5Bi0.5TiO3-0.4SrTiO3-xFe2O3 ceramics (x = 0, 0.25, 1 and 2, mol%) were fabricated by solid-state reaction processing. By means of XRD, SEM and Ferroelectric station, the effects of Fe2O3 on the phase, microstructure, dielectric properties and energy storage performances of NBT-ST ceramics were investigated systematically. The samples modified by Fe2O3 do not remarkable affect its perovskite crystal structure, but beneficial to its density and grain size. With introduction of Fe2O3, the dielectric properties enhanced, meanwhile, the P-E hysteresis loops were slim in appearance, presenting a typical relaxor ferroelectric characteristic. The sample with Fe2O3 doping amount of 0.25 mol% exhibits high η and the optimum energy storage performance with excellent stability of temperature, frequency and fatigue circle. These results of the Fe2O3 doped Pb-free NBT-BT based ceramics are attractive features for applications in energy storage capacitor and power electronics.

Reversible Control of the Mn Oxidation State in SrTiO3 Bulk Powders.

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO3) bulk powder. We employ NaBH4 as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn2+ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti3+ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn2+, Mn4+, and the Mn2+ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO3. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti3+ and O2. The ability to control the relative amounts of various paramagnetic defects in SrTiO3 provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis.

Metallicity in SrTiO3 substrates induced by pulsed laser deposition

Oxygen deficiency has been known to induce metallic conduction in bulk and thin film SrTiO3 (STO). Here, we report on the metallicity of STO substrates induced by the pulsed laser deposition (PLD) process of STO films under various oxygen-poor growth conditions. Depositions as short as 2 min result in conduction through the STO substrate. Films grown on other substrates are insulating, and STO substrates annealed under the same growth conditions without laser ablation remain insulating. By varying background gas composition during deposition, we find that the transport behavior transitions from metallic to insulating behavior at progressively higher ambient pressures for O2, 99% N2/1% O2, N2, and Ar. Metallic behavior persists to deposition pressures as high as 10−2 Torr in Ar. These results suggest that, during the PLD process, the deposition kinetics and plume energy are a dominant factor in the formation of oxygen vacancies which then diffuse into the substrate. Understanding these mechanisms is crucial to prevent STO substrate reduction during PLD of films which require low O2 partial pressures during growth.

Optimizing thermal conduction in bulk polycrystalline SrTiO3−δ ceramics via oxygen non-stoichiometry

Abstract

Kondo effect with tunable spin–orbit interaction in LaTiO3/CeTiO3/SrTiO3 heterostructure

We have fabricated epitaxial films of CeTiO3 (CTO) on (0 0 1) oriented SrTiO3 (STO) substrates, which exhibit highly insulating and diamagnetic properties. X-ray photoelectron spectroscopy was used to establish the 3+ valence state of the Ce and Ti ions. Furthermore, we have also fabricated δ (CTO) doped LaTiO3 (LTO)/SrTiO3 thin films which exhibit variety of interesting properties including Kondo effect and spin–orbit interaction (SOI) at low temperatures. The SOI shows a non-monotonic behaviour as the thickness of the CTO layer is increased and is reflected in the value of characteristic SOI field () obtained from weak anti-localization fitting. The maximum value of is 1.00 T for δ layer thickness of 6 u.c. This non-monotonic behaviour of SOI is attributed to the strong screening of the confining potential at the interface. The screening effect is enhanced by the CTO layer thickness and the dielectric constant of STO which increases at low temperatures. Due to the strong screening, electrons confined at the interface are spread deeper into the STO bulk where it starts to populate the Ti subbands; consequently the Fermi level crosses over from to the subbands. At the crossover region of where there is orbital mixing, SOI goes through a maximum.

Enhancement of thermoelectric properties of SrTiO3/LaNb–SrTiO3 composite by different doping levels**Project supported by the National Natural Science Foundation of China (Grant Nos. 61751404, 51702168, and 51665042), the Fund from the State Key Laboratory of New Ceramic and Fine Processing (Tsinghua University), China (Grant No. KF201608), the Fund from the Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology,

Strontium titanate (STO) is an n-type oxide thermoelectric material, which has shown great prospects in recent years. The doping of La and Nb into STO can improve its power factor, whereas its thermal conductivity is still very high. Thus, in order to obtain a high thermoelectric figure-of-merit zT, it is very important to reduce its thermal conductivity. In this paper, using a combination of a hydrothermal method and a high-efficiency sintering method, we succeed in preparing a composite of pure STO and LaNb-doped STO, which simultaneously realizes lower thermal conductivity and higher Seebeck coefficient, therefore, the thermoelectric properties of STO are significantly improved. In the SrTiO3/LaNb–SrTiO3 bulk samples, the lowest thermal conductivity is 2.57 W·m−1·K−1 and the highest zT is 0.35 at 1000 K for the STO/La10Nb20–STO sample.

Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3

Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO3 and iron doped SrTiO3 were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO3 and compared it to DOS of iron-doped SrTiO3 with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO3 and iron-doped SrTiO3. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO3, are accessible only on TiO2 terminated SrTiO3 surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.

Oxygen vacancy formation in the SrTiO3 Σ5 [001] twist grain boundary from first‐principles

AbstractThe SrTiO3 5 [001] twist grain boundary (GB) is studied using first‐principles density functional theory calculations. Three types of GB structures, SrO/SrO (S/S), SrO/TiO2 (S/T), and TiO2/TiO2 (T/T), are modeled and their relative thermodynamic stabilities are examined. Our calculations show that the S/S and S/T structures can be formed within appropriate synthesis conditions, with the S/S structure thermodynamically favored over the S/T structure within a wide range of chemical potentials, while the T/T structure is unlikely to form. The segregation behavior of oxygen vacancies is also investigated by calculating oxygen vacancy formation energies with respect to the distance from GB plane. In the S/S system, oxygen vacancies tend to segregate to the layer adjacent to the GB layer, while in the S/T system, oxygen vacancies tend to segregate to the GB layer itself. In both S/S and S/T systems, oxygen vacancy formation energy is lower than that in bulk SrTiO3. To clearly show the experimental conditions necessary to promote oxygen vacancy formation in the 2 GB systems, we also generate grain boundary phase diagrams for oxygen vacancy with respect to synthesis temperature and oxygen partial pressure. Our calculations reveal different segregation behaviors and distributions of oxygen vacancies in the S/S and S/T systems, providing a possible avenue for GB engineering.

Photoconductivity of bulk SrTiO3 single crystals at room temperature

Room temperature photoconductivity measurements have been carried out on various as-grown SrTiO3 single crystals provided from different suppliers. We observed an increase in the conductivity of samples based on photon energy and photon intensity. While low energy photons decreased the conductivity, photons with energy close to the band gap of SrTiO3 enhanced the conductivity. No persistent photoconductivity was observed in these samples. Defects play the main role in inducing the photoconductivity. Positron annihilation lifetime spectroscopy and digital coincidence Doppler broadening spectroscopy were used to investigate the presence and nature of defects in the photoconductive samples. The measurements revealed high concentration of defects and the dependence of photoconductivity on defect concentration under 365 and 400 nm illumination.

Continuum thermodynamics of unusual domain evolution-induced toughening effect in nanocracked strontium titanate

This work is concerned with the analysis of both unusual mechanical and electric behaviors of nanoscale pre-cracked incipient ferroelectric of SrTiO3 at room temperature. A nonlinear thermodynamic model based on the Ginzburg-Landau theory is thus constructed and employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the mechanical strain, and the self-strains of the ferroelastic and ferroelectric phase transformations. A large toughening effect is explored as a consequence of a softening nonlinear mechanical behavior, which characterizes for nanoscale SrTiO3, despite of the brittleness of bulk SrTiO3 ceramic. Depending upon applied strain, the large toughening and nonlinear mechanical behavior are attributed to unusual domain evolution of strain-induced polarization from the crack tip, in which a local-to-global transition of ferroelectric phase takes place from polarization vortex to hybrid structure of vortex and stripe domains, and finally to stripe domain structure with Néel type domain walls. Here, we also show the great importance of cross-coupling between the ferroelectric polarization and mechanical strain as it intrinsically leads to the local-to-global transition of ferroelectric phase. More interestingly, such cross-coupling additionally scatters the concentrated stress near the crack tip to Néel type domain walls. The present findings shed light on the crucial role of mechanical strain in controlling both the size and topology of polarization structures in nanoscale SrTiO3. Our numerical results also provide an insight into the vital role of strain-induced polarization for the toughness and strength of SrTiO3 material. Based on our study, the applicability of the present results to other incipient ferroelectrics such as CaTiO3, EuTiO3, and KTaO3 with the same mechanism would be interesting and possible.

Towards atomically resolved EELS elemental and fine structure mapping via multi-frame and energy-offset correction spectroscopy

Electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy are two of the most common means for chemical analysis in the scanning transmission electron microscope. The marked progress of the instrumentation hardware has made chemical analysis at atomic resolution readily possible nowadays. However, the acquisition and interpretation of atomically resolved spectra can still be problematic due to image distortions and poor signal-to-noise ratio of the spectra, especially for investigation of energy-loss near-edge fine structures. By combining multi-frame spectrum imaging and automatic energy-offset correction, we developed a spectrum imaging technique implemented into customized DigitalMicrograph scripts for suppressing image distortions and improving the signal-to-noise ratio. With practical examples, i.e. SrTiO3 bulk material and Sr-doped La2CuO4 superlattices, we demonstrate the improvement of elemental mapping and the EELS spectrum quality, which opens up new possibilities for atomically resolved EELS fine structure mapping.

Study of Trapping Phenomena in SrTiO3 by Thermally Stimulated Techniques

Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO3 such as transient and persistent photoconductivity.

Universality of electron mobility in LaAlO3/SrTiO3 and bulk SrTiO3

Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N≃5×1018 cm−3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D&amp;lt;N, background impurities determine the electron scattering. Thus, when n3D&amp;lt;N, it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D&amp;gt;N, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.

Published Tunneling Results of Binnig et al Interpreted as Related to Surface Superconductivity in SrTiO3

In 1980, Binnig et al. reported tunneling measurements on Nb-doped SrTiO3, and interpreted their results as indicating two-band superconductivity in the bulk of SrTiO3. However, (1) effective masses determined from tunneling results in the normal state by Sroubek in 1969 and 1970 are much smaller than those determined by most other methods. The much smaller masses were attributed to properties of a surface layer by the present author in 1971. (2) The only other reports of two-band superconductivity in bulk SrTiO3 can be used to infer much smaller values for the band separations than found by Binnig et al. In this paper, we give an alternative explanation of the results of Binnig et al. in terms of superconductivity in a surface layer. We obtain fair fits to the band gaps versus Fermi energy for the two bands in the three samples where two surface subbands are occupied and to the temperature dependence of the gaps in one crystal, using a model with three adjustable interaction parameters, an adjustable energy for the phonons which dominate the pairing, and an adjustable ratio of the mean-field T c to the actual T c . We show results for a combined fit to the low-temperature band gaps and to the T-dependence in one crystal. The phonon energy which gives the best fit is 21 meV. This is probably an appropriate average over the three longitudinal polar modes and acoustic modes in the material. A large value of about two is found for the ratio T c m f /T c , and we conjecture that this arises because a band with a small Fermi energy, not seen in the tunneling results, plays a part in increasing T c m f /T c .

Enhancing Charge Separation and Photocatalytic Activity of Cubic SrTiO3 with Perovskite‐Type Materials MTaO3 (M=Na, K) for Environmental Remediation: A First‐Principles Study

AbstractDue to increasingly global environmental and energy crises, visible light semiconductor photocatalyst with a tunable bandgap and optical properties have received attention. This study aid in the design of bifunctional MTaO3/SrTiO3(010) (M=Na, K) heterostructure photocatalyst material for environmental remediation. In this study, the stability, electronic and optical properties of coupled MTaO3 and SrTiO3 are systematically studied using the hybrid HSE06 method. The MTaO3/SrTiO3(010) heterostructure show high photocatalytic activity under visible light irradiation with good stability and reduced bandgap compared to the bulk SrTiO3. The heterostructures formed a type‐II band alignment to accelerates the interfacial charge transfer process and the photocatalytic activity. By comparing the relative ratio of effective mass, we could conclude that MTaO3/SrTiO3(010) heterostructure has not only superior mobility of charge carriers but also higher separation of photoinduced electrons and holes. The band alignment results showed that the MTaO3/SrTiO3(010) heterostructure are highly efficient for pollutants degradation and energy conversion. Significantly, the origin of the enhanced photocatalytic activity is observed from the O 2p state of SrTiO3 to the Ta 5d state of MTaO3. In summary, this study shows a key role of SrTiO3 as an electron donor to enhance the optical properties and stability of MTaO3/SrTiO3(010) heterostructure.

Improved energy-storage performance and breakdown enhancement mechanism of Mg-doped SrTiO3 bulk ceramics for high energy density capacitor applications

We investigated the structure, dielectric properties and energy density performances of cubic perovskite-structured Mg-doped SrTiO3 ceramics that were prepared by the solid-state reaction method. SrTiO3 ceramic exhibited a relatively stable permittivity about 265–290 and enhanced dielectric breakdown strength (DBS) by Mg isovalent doping. Doping effects on the energy-storage properties in SrTiO3 ceramics was performed by complex impedance analysis and polarization–electric field hysteresis loops. The energy storage density was dependent on DBS while energy efficiency was closely related to the remnant polarization. The possible physical mechanisms, including grain, gain boundary and interfacial polarization effects, were discussed to explain the improvement of dielectric breakdown strength. The bulk Mg-doped SrTiO3 materials have shown interesting energy densities (1.86 J/cm3) with good energy storage efficiency (about 89.3%) indicating that they can be a promising candidate for high energy density capacitor applications.

Record high thermoelectric performance in bulk SrTiO3 via nano-scale modulation doping

Strontium titanite (SrTiO3), which is an experimentally-friendly thermoelectric material, could be a promising candidate for thermoelectric power generation applications. The theorectical study indicates the co-doping of La and Nb could enhance the thermoelectric performance, however, the thermoelectric figure of merits (ZTs) of SrTiO3 are still low because the co-doping process at nano-scale is experimentally difficult to control. Here we report a high performance SrTiO3 with La-Nb co-doping, which are prepared by a combination of hydrothermal method and high-efficiency sintering. Nano-scale co-doping is successfully modulated by hydrothermal method, and nano-inclusions precipitate during sintering process, to form complex microstructures. In this case, the electrical and thermal transport properties are optimized simultaneously by doping concentration and dopants type, resulting in a record-high ZT >0.6 at 1000−1100K in the 10mol% La and 10mol% Nb doped SrTiO3 bulk materials. The nano-scale modulation doping and microstructure controlling approach validated in the present study should be also applicable for other thermoelectric materials.

Anisotropic electrical resistance in mesoscopic LaAlO3/SrTiO3 devices with individual domain walls

The crystal structure of bulk SrTiO3(STO) transitions from cubic to tetragonal at around 105 K. Recent local scanning probe measurements of LaAlO3/SrTiO3 (LAO/STO) interfaces indicated the existence of spatially inhomogeneous electrical current paths and electrostatic potential associated with the structural domain formation in the tetragonal phase of STO. Here we report a study of temperature dependent electronic transport in combination with the polarized light microscopy of structural domains in mesoscopic LAO/STO devices. By reducing the size of the conductive interface to be comparable to that of a single tetragonal domain of STO, the anisotropy of interfacial electron conduction in relationship to the domain wall and its direction was characterized between T = 10–300 K. It was found that the four-point resistance measured with current parallel to the domain wall is larger than the resistance measured perpendicular to the domain wall. This observation is qualitatively consistent with the current diverting effect from a more conductive domain wall within the sample. Among all the samples studied, the maximum resistance ratio found is at least 10 and could be as large as 105 at T = 10 K. This electronic anisotropy may have implications on other oxide hetero-interfaces and the further understanding of electronic/magnetic phenomena found in LAO/STO.