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Strontium titanate

Strontium titanate (SrTiO₃), often abbreviated as STO, is a perovskite oxide that occurs naturally as the rare mineral tausonite but is primarily produced synthetically, with the chemical formula SrTiO₃, featuring strontium cations at the A-site and corner-sharing titanium-oxygen octahedra at the B-site in the prototypical ABO₃ framework.^[1]^[2] It crystallizes in a cubic structure (space group Pm-3m) above approximately 105 K, transitioning to a tetragonal phase below this temperature due to octahedral tilting, with a room-temperature lattice constant of 3.905 Å.^[1] As a wide-bandgap semiconductor, SrTiO₃ has an indirect bandgap of 3.20–3.25 eV and a direct bandgap of about 3.75 eV, rendering it transparent to visible light while absorbing ultraviolet radiation.^[1] First synthesized in the early 1950s through solid-state reactions of strontium carbonate and titanium dioxide, SrTiO₃ quickly became a subject of intensive study for its electrical properties, with early investigations into its dielectric behavior dating to 1953.^[3] Subsequent research in the 1960s revealed its photoconductivity and photochromic effects, while the 1980s marked a surge in interest for photocatalytic applications, particularly water splitting for hydrogen production.^[1] Today, various synthesis methods are employed, including hydrothermal, sol-gel, and microwave-assisted processes, which allow control over particle morphology—ranging from nanoparticles to nanorods—to tailor its performance in specific uses.^[4] SrTiO₃ exhibits exceptional chemical and thermal stability, low dielectric loss, and a high dielectric permittivity that peaks at low temperatures, making it a quantum paraelectric below 4 K.^[1] Its defect chemistry, dominated by oxygen vacancies and dopants like niobium or iron, enables tunable n-type or p-type conductivity, photoluminescence, and even room-temperature ferromagnetism in certain configurations.^[1] These attributes stem from its electronic structure, where photoexcitation generates charge carriers that interact with lattice defects, influencing properties like carrier mobility and recombination rates.^[1] Notable applications of SrTiO₃ include its role as a substrate for epitaxial growth of high-temperature superconductors and oxide thin films due to its lattice match with many perovskites.^[1] In energy technologies, it serves as a photocatalyst for environmental remediation, such as dye degradation and CO₂ reduction, and in solid oxide fuel cells for efficient ion conduction.^[1] Doped variants enhance visible-light activity for solar hydrogen production, while its memristive behavior supports neuromorphic computing devices.^[1] Ongoing research explores nanostructured forms for improved efficiency in photovoltaics and sensors.^[4]

Chemical and Structural Properties

Composition and Natural Occurrence

Strontium titanate is an oxide compound with the chemical formula SrTiO₃.^[5] It consists of strontium (Sr), titanium (Ti), and oxygen (O) in a 1:1:3 stoichiometric ratio, forming a perovskite-type structure.^[5] The molar mass of SrTiO₃ is 183.49 g/mol.^[5] In nature, strontium titanate occurs as the rare mineral tausonite (SrTiO₃), which was approved as a valid mineral species by the International Mineralogical Association in 1982 based on samples from the Murunsky massif in Siberia, Russia.^[6] Tausonite was first described in detail in a 1984 study as a new perovskite-group mineral found in metasomatic rocks of the massif.^[7] This mineral is extremely rare, appearing only in small, isolated cubic crystals or irregular masses within alkaline igneous environments, and natural specimens are often impure due to inclusions or substitutions.^[8] Despite its natural occurrence, all practical applications of strontium titanate rely on synthetic production methods, as natural sources are insufficient for industrial or commercial needs.^[5]

Crystal Structure

Strontium titanate (SrTiO₃) exhibits a cubic perovskite crystal structure at room temperature, characterized by the space group Pm3m (No. 221). This structure belongs to the ideal ABO₃ perovskite family, where Sr²⁺ occupies the A-site at the corners of the unit cell (0,0,0 position), Ti⁴⁺ resides at the body-centered B-site (0.5,0.5,0.5), and six O²⁻ ions form an octahedron around the Ti⁴⁺ cation, creating corner-sharing TiO₆ octahedra that define the framework. The lattice parameter is a = 3.905 Å, providing a highly symmetric arrangement that underpins many of its functional properties.^[9]^[10] Upon cooling below approximately 105 K, SrTiO₃ undergoes a structural phase transition from the high-temperature cubic phase to a tetragonal phase with space group I4/mcm, driven by an antiferrodistortive rotation of the TiO₆ octahedra.^[11] This transition involves a softening of phonon modes but does not lead to ferroelectricity, maintaining the material in a paraelectric state. The tetragonal distortion is subtle, with the c/a ratio close to 1.0006 at low temperatures, preserving much of the cubic symmetry's influence on overall behavior.^[11] Defect chemistry in SrTiO₃ significantly influences its crystal structure, particularly through oxygen vacancies, which are common in reduced or non-stoichiometric samples. These vacancies can induce local lattice expansion and symmetry breaking, leading to increased unit cell volume and the emergence of Raman-active modes that indicate deviation from perfect cubic symmetry. Doping with aliovalent ions, such as La³⁺ on the Sr²⁺ site or Nb⁵⁺ on the Ti⁴⁺ site, alters lattice parameters by charge compensation mechanisms, often resulting in slight expansions or contractions while generally retaining the perovskite framework, though heavy doping may stabilize alternative phases.^[12]^[13]^[14]

Physical Properties

Thermal and Mechanical Properties

Strontium titanate (SrTiO₃) exhibits a density of 5.11 g/cm³ for high-quality synthetic single crystals, which contributes to its suitability as a substrate material in thin-film applications.^[15] This value is consistent across Verneuil-grown crystals, reflecting the compact perovskite structure.^[16] The material has a high melting point of 2,080 °C, enabling its use in high-temperature processing environments without decomposition.^[17] Its thermal expansion coefficient is approximately 10.4 × 10⁻⁶ K⁻¹, indicating moderate dimensional stability under temperature variations, though slight anisotropy may occur along different crystallographic directions.^[16] Thermal conductivity at room temperature ranges from 10 to 12 W/m·K, which is relatively low for oxides and supports its role in insulating layers for electronic devices.^[17] Mechanically, strontium titanate behaves as a brittle ceramic, prone to cleavage along {100} planes under stress, with fracture toughness typically in the range of 1–2 MPa·m¹/² for single crystals.^[18] This low toughness limits its load-bearing applications but is adequate for substrate use where mechanical demands are minimal. Hardness is rated at 5.5–6 on the Mohs scale, comparable to feldspar, while Vickers hardness values for single crystals are around 450 HV (approximately 4.5 GPa), varying with surface preparation and dislocation density.^[17]^[19] These properties underscore the need for careful handling to avoid cracking during polishing or epitaxial growth.

Optical Properties

Strontium titanate (SrTiO₃) synthetic crystals typically appear white and range from opaque to translucent, depending on their purity and processing method. High-quality single crystals grown via flame fusion or other techniques can achieve greater transparency, enabling their use in optical components.^[17] The material exhibits a high refractive index of 2.41 at 589 nm, comparable to that of diamond (2.42), which contributes to its brilliance in gemological applications. This value arises from its perovskite crystal structure, which densely packs ions and enhances light bending.^[20]^[10] Strontium titanate displays strong dispersion of 0.190, significantly higher than diamond's 0.044, resulting in a pronounced "fire" effect where white light separates into vivid spectral colors. This optical property stems from the material's electronic transitions in the visible range and has made it a notable diamond simulant.^[8] In its cubic crystal form at room temperature, strontium titanate is isotropic and exhibits no birefringence, meaning light polarization remains unchanged regardless of propagation direction. This lack of double refraction simplifies its use in isotropic optical systems.^[21] The band gap of strontium titanate is indirect at 3.25 eV and direct at 3.75 eV, limiting its absorption to ultraviolet wavelengths below approximately 380 nm and rendering it transparent in the visible spectrum. These values, determined through spectroscopic ellipsometry and reflection measurements, indicate minimal electronic transitions in the visible range, with brief implications for photocarrier generation in optical contexts.^[22] Strontium titanate demonstrates high transparency in the infrared region, extending up to about 7 μm with low absorption coefficients below 20 cm⁻¹ for polished plates. This property supports its applications in infrared lenses and windows, particularly for detector immersion and thermal imaging components.^[21]^[23]

Electronic Properties

Dielectric and Ferroelectric Behavior

Strontium titanate (SrTiO₃) exhibits a high relative dielectric constant (ε_r) of approximately 300 at room temperature under low electric fields, which significantly increases to around 20,000 at liquid helium temperatures due to its proximity to a ferroelectric instability.^[24] This behavior arises from the polarizability of the TiO₆ octahedra in its perovskite structure, enabling substantial electric field-induced polarization without structural distortion at ambient conditions.^[25] As a prototypical quantum paraelectric, SrTiO₃ remains in a paraelectric state down to absolute zero, prevented from undergoing a ferroelectric phase transition by zero-point quantum fluctuations that suppress long-range polar order.^[26] The temperature dependence of ε_r follows the Curie-Weiss law above approximately 105 K, characterized by a Curie constant of about 8 × 10⁴ K and a Curie temperature near 35 K, reflecting incipient ferroelectricity that is quenched at lower temperatures.^[26] Below 4 K, ε_r becomes nearly temperature-independent, highlighting the dominance of quantum effects.^[27] Chemical doping can destabilize this quantum paraelectric state and induce ferroelectricity; for instance, partial substitution of Ca²⁺ for Sr²⁺ shifts the transition temperature above room temperature, enabling bulk ferroelectric and piezoelectric responses.^[28] The high ε_r and moderate field tunability of undoped SrTiO₃—up to 10% variation under applied fields at room temperature—make it suitable for capacitors in tunable microwave devices, where low losses and high capacitance density are essential.^[29]

Band Structure and Conductivity

Strontium titanate (SrTiO₃) is an indirect bandgap semiconductor, characterized by a valence band maximum at the R point and a conduction band minimum at the Γ point in the Brillouin zone.^[30] This band structure configuration results in a bandgap of approximately 3.2 eV, rendering it insulating under ambient conditions but responsive to doping or defect introduction for electronic applications.^[31] The material typically exhibits n-type electrical conductivity, arising from oxygen vacancies that act as shallow donors, donating electrons to the conduction band.^[1] In undoped single crystals, electron mobility reaches approximately 5–10 cm²/V·s at room temperature, enabling efficient charge transport in reduced atmospheres where vacancy concentrations are elevated.^[32] Defect chemistry plays a pivotal role in tuning this conductivity; oxygen vacancies form with an energy given by E_{\text{form}} = E_{\text{def}} - E_{\text{perfect}} + \mu_{\text{O}}, where E_{\text{def}} and E_{\text{perfect}} are the total energies of the defective and perfect structures, respectively, and \mu_{\text{O}} is the oxygen chemical potential (often approximated as half the energy of an O₂ molecule). Typical formation energies range from 5.9 eV at surfaces to 6.7 eV in the bulk, influencing vacancy stability and carrier density.^[33] Superconductivity in SrTiO₃ emerges at very low temperatures, with a critical temperature (T_c) below 0.35 K observed in undoped or lightly oxygen-deficient crystals due to low carrier densities near 10^{17}–10^{18} cm^{-3}.^[34] Doping with elements like Nb or La, or applying strain, can enhance T_c up to 1.5 K by increasing carrier density and modifying the electronic structure near a ferroelectric quantum critical point.^[35]^[36] Illumination with sub-bandgap light induces persistent photoconductivity in SrTiO₃, where conductivity increases by orders of magnitude and persists for days due to defect trapping of photoexcited carriers, particularly at oxygen vacancies that serve as electron traps.^[37] This photoinduced effect encompasses both electronic responses, such as enhanced charge carrier generation and separation, and ionic responses, including oxygen vacancy migration and stoichiometry adjustments under UV or visible light, especially at elevated temperatures above 200 °C.^[1] These phenomena highlight SrTiO₃'s quantum properties, with defects modulating band tail states and facilitating long-lived charge separation.

Synthesis and Preparation

Bulk Material Synthesis

Strontium titanate (SrTiO₃) bulk materials, including single crystals and powders, are primarily synthesized through scalable methods that ensure the formation of the cubic perovskite structure. These techniques prioritize high yield and purity while addressing challenges in maintaining precise stoichiometry, as deviations can lead to secondary phases like Sr₂TiO₄ or TiO₂.^[4] The modified Verneuil process, also known as flame-fusion, is a widely used technique for growing large single crystals of SrTiO₃ suitable for optical and substrate applications. In this method, a precursor of strontium titanyl oxalate, SrTiO(C₂O₄)₂·2H₂O, is first prepared by co-precipitation from solutions of strontium chloride and titanium tetrachloride with oxalic acid, followed by heating to approximately 1,000 °C to decompose the oxalate and form fine oxide powder.^[38] This powder is then fed into an oxy-hydrogen flame, where it melts at around 2,080 °C and crystallizes onto a seed boule, yielding transparent crystals up to several centimeters in diameter. The process operates under oxidizing conditions to minimize oxygen vacancies, achieving purities exceeding 99.9% with careful control of precursor stoichiometry and excess SrCO₃ addition to compensate for strontium volatilization.^[39] Solid-state reaction represents a straightforward, cost-effective route for producing polycrystalline SrTiO₃ powders in bulk quantities. It involves intimately mixing stoichiometric amounts of strontium carbonate (SrCO₃) and titanium dioxide (TiO₂), typically in a 1:1 molar ratio, via ball milling to ensure homogeneity, followed by calcination in air at 1,200–1,400 °C for several hours. During heating, SrCO₃ decomposes to SrO, which reacts with TiO₂ to form the perovskite phase, with CO₂ released as a byproduct. This method yields high-purity material (>99.9%) but requires multiple grinding and re-calcination steps to achieve complete reaction and eliminate unreacted precursors, as incomplete mixing can result in stoichiometric imbalances and impurity phases.^[40]^[41] Hydrothermal synthesis offers a milder, aqueous-based approach for bulk powder production, particularly suited for recent developments targeting finer particles. Strontium hydroxide (Sr(OH)₂) and TiO₂ (often anatase or rutile form) are suspended in water within a sealed autoclave, where the mixture reacts at 150–250 °C under autogenous pressure (typically 5–20 atm) for 12–48 hours. The high pH environment promotes dissolution of TiO₂ and subsequent precipitation of crystalline SrTiO₃ nanoparticles, with particle sizes controllable by reaction time and temperature. Yields are high and purity is excellent, though stoichiometry control remains challenging due to potential Sr/Ti imbalances from incomplete TiO₂ dissolution, often necessitating post-synthesis annealing for phase purity. This method's lower energy input compared to solid-state routes makes it advantageous for scalable nanoparticle production. Additional bulk methods, such as co-precipitation, provide alternatives for fine powders.^[42]^[43]

Nanostructured and Thin Film Methods

Strontium titanate (SrTiO₃) nanostructured materials and thin films are fabricated using advanced deposition and chemical synthesis techniques that enable precise control over morphology, crystallinity, and dimensionality, essential for integrating into nanoscale devices. These methods produce forms such as epitaxial films, nanoparticles, and nanowires, leveraging vacuum-based physical vapor deposition or solution-based processes to achieve high-quality structures with tailored properties. Pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) are prominent for epitaxial thin films, while sol-gel and solvothermal approaches excel in generating nanoparticles and one-dimensional nanostructures. Pulsed laser deposition involves ablating a stoichiometric SrTiO₃ target using a high-energy laser in an oxygen atmosphere, typically at substrate temperatures of 600–800 °C, to grow epitaxial films with excellent crystallinity. This technique ensures layer-by-layer growth on lattice-matched substrates, minimizing defects and enabling oxygen incorporation for stoichiometric films. For instance, PLD at 600 °C under low oxygen pressure (∼10⁻² mbar) yields highly oriented films on MgO substrates, as demonstrated in early seminal work on perovskite oxides. The process's versatility allows for in situ monitoring, resulting in films suitable for heterostructure integration. Molecular beam epitaxy (MBE) employs ultra-high vacuum conditions (∼10⁻¹⁰ Torr) to evaporate elemental sources of Sr and Ti, along with reactive oxygen, facilitating atomically smooth epitaxial layers of SrTiO₃. This method is particularly effective for homoepitaxial growth on SrTiO₃ substrates or heterostructures on silicon, achieving sub-nm surface roughness and enabling complex oxide interfaces. Hybrid MBE variants, using metal-organic sources, maintain stoichiometry within a narrow growth window (700–800 °C), as established in high-impact studies on oxide thin films. MBE's precision supports the creation of ultrathin layers for quantum confinement effects in electronic applications. Sol-gel and solvothermal methods provide cost-effective routes to SrTiO₃ nanoparticles and nanowires through solution chemistry. In sol-gel processing, precursors like titanium butoxide (Ti(OBu)₄) and strontium nitrate (Sr(NO₃)₂) are dissolved in ethanol, followed by hydrolysis and condensation to form a gel that is calcined to yield crystalline nanoparticles. Solvothermal variants, conducted in autoclaves at elevated temperatures (e.g., 180 °C for 24 h), promote nanowire formation via oriented attachment, using similar precursors in ethanol or mixed solvents with bases like NaOH. These wet-chemical approaches produce uniform nanostructures with sizes below 50 nm, as verified in comprehensive reviews of perovskite synthesis. Recent advances in nanostructuring SrTiO₃ emphasize in situ growth techniques to enhance thermoelectric performance. For example, 2025 studies demonstrate the integration of carbon nanotubes via in situ chemical vapor deposition on SrTiO₃ nanoparticles, reducing thermal conductivity while preserving electrical conductivity, leading to improved figure-of-merit (ZT) values.^[44] High-entropy alloying combined with nanostructuring under high pressure has also been reported to modulate band gaps and microstructures, boosting power factors in n-type variants.^[45] These developments build on earlier self-nanostructuring strategies, highlighting scalable routes for energy applications. Thickness control in SrTiO₃ thin films spans from 1 nm ultrathin layers to several microns, governed by deposition parameters like growth rate and substrate choice. Strain engineering exploits lattice mismatch (e.g., 1–7% with substrates like Si or MgO) to induce biaxial strain, altering ferroelectric and dielectric responses without altering composition. Critical thickness for strain relaxation, typically 10–50 nm, is tuned to stabilize desired phases, as explored in foundational work on epitaxial perovskites. This enables functional tuning, such as enhanced piezoelectricity in compressively strained films.

Applications

Optical and Gemological Uses

Strontium titanate emerged as a prominent diamond simulant in the mid-20th century, with commercial production beginning around 1955 under trade names like Fabulite. Developed by Union Carbide through flame-fusion techniques, it quickly gained favor in the jewelry industry for its exceptional optical properties, particularly its high dispersion of 0.190—over four times that of natural diamond (0.044)—which produces vivid spectral "fire" effects.^[46]^[47]^[48] This dispersion, combined with a refractive index of approximately 2.41, closely approximates diamond's sparkle in colorless, faceted forms.^[23] As a gem material, strontium titanate is typically cut into brilliant faceted shapes for use in costume and fashion jewelry, where its clarity and brilliance enhance affordable designs. However, its Mohs hardness of 5.5 to 6.5 renders it prone to scratches and abrasion, significantly limiting its durability compared to diamond's hardness of 10 and making it unsuitable for everyday wear in high-impact settings.^[48]^[8] Production of strontium titanate for gemological purposes peaked during the 1950s and 1960s but declined sharply in the late 1970s following the introduction of cubic zirconia, a harder (Mohs 8–8.5), more affordable alternative that could be produced in larger sizes and a broader color range.^[8] Beyond jewelry, strontium titanate finds applications in basic optical components owing to its broad transparency window from ultraviolet to mid-infrared wavelengths (0.35 to 6.5 μm). It is employed in prisms for dispersing light in spectroscopic instruments and as infrared windows in sensors and detectors, where its chemical stability and high refractive index support reliable transmission in the 5–6 μm range.^[23]^[49] Commercial suppliers offer polished strontium titanate elements for these purposes, capitalizing on the material's isotropic nature and resistance to thermal shock.^[49]

Substrates for Thin Films and Electronics

Strontium titanate (SrTiO₃, STO) serves as an ideal lattice-matched substrate for the epitaxial growth of perovskite-structured materials due to its cubic crystal structure and lattice constant of 3.905 Å, which closely matches that of many complex oxides.^[50] This compatibility enables high-quality, strain-controlled deposition of thin films such as high-temperature superconductors like YBa₂Cu₃O₇ (YBCO), where the minimal lattice mismatch (approximately 1.4%) promotes coherent interfaces and preserves superconducting properties. Such epitaxial growth is typically achieved via techniques like pulsed laser deposition or molecular beam epitaxy on TiO₂-terminated STO surfaces, ensuring atomically flat templates for subsequent layers. In microelectronics, STO substrates support the fabrication of oxide thin films for diverse applications, including sensors, varistors, and microwave devices. For instance, ferroelectric thin films grown on STO enable piezoelectric sensors with enhanced sensitivity due to the substrate's structural stability.^[51] In varistor devices, STO facilitates the deposition of multilayered SrTiO₃-based films exhibiting nonlinear current-voltage characteristics for overvoltage protection.^[52] Microwave components, such as tunable resonators and filters, benefit from oxide films on STO, leveraging the substrate's role in maintaining film orientation and reducing scattering losses.^[53] Key advantages of STO substrates include low dielectric loss (tan δ < 10⁻⁴ at microwave frequencies) and high electrical insulation (bandgap ~3.2 eV), which minimize signal attenuation and prevent unwanted conduction in heterostructures.^[54] Polished single-crystal STO wafers are commercially available up to 2-inch diameters, allowing scalable production of device prototypes with surface roughness below 0.2 nm RMS for optimal epitaxy. These properties, combined with chemical inertness, make STO preferable over alternatives like MgO for applications requiring precise lattice engineering. Recent advancements highlight STO's role in emerging technologies, such as interfaces for 2D materials and quantum computing. In LaAlO₃/STO heterostructures, a high-mobility two-dimensional electron gas (2DEG) forms at the interface, enabling spintronic devices and quantum dots for qubit manipulation. This 2DEG, with mobilities exceeding 10⁴ cm²/V·s, serves as a platform for oxide-based quantum interfaces, bridging conventional electronics with quantum systems.^[55] Additionally, STO exhibits memristive behavior due to its oxygen vacancy dynamics, which enables resistive switching in thin films. This property supports the development of neuromorphic computing devices, such as artificial synapses and memristor-based neural networks, where STO's tunable conductivity and low power consumption are advantageous. As of 2025, research has demonstrated STO-based memristors with endurance exceeding 10^6 cycles and on/off ratios over 10^3, facilitating energy-efficient hardware for machine learning applications.^[56]^[57]

Energy Conversion Devices

Strontium titanate (SrTiO₃) serves as a critical encapsulant for strontium-90 (⁹⁰Sr) in radioisotope thermoelectric generators (RTGs), where it forms a stable ceramic matrix that contains the beta-emitting radioisotope while converting decay heat into electricity. The insolubility of SrTiO₃ in water, combined with its non-combustible nature and ability to withstand temperatures exceeding 2000 °C, makes it ideal for preventing radionuclide release during operation or potential accidents. In RTGs, ⁹⁰Sr is incorporated as strontium titanate pellets, which provide a reliable heat source without the need for mechanical moving parts, ensuring long-term power output in harsh environments. This design has been employed since the 1960s in Soviet-era RTGs, with over 1000 units manufactured primarily in Russia and Estonia for remote applications such as Arctic navigation beacons, where they delivered power levels from 30 W to 1 kW.^[58]^[59] In solid oxide fuel cells (SOFCs), doped variants of SrTiO₃, such as lanthanum-doped SrTiO₃ (LST, e.g., La₀.₂Sr₀.₈TiO₃), function as anode materials, leveraging their mixed ionic-electronic conductivity to facilitate fuel oxidation at intermediate temperatures of 600–800 °C. This doping introduces oxygen vacancies and enhances electronic mobility, enabling efficient charge transfer while maintaining structural integrity under anodic conditions. Research on LST-based anodes began in the 1990s, building on earlier explorations of donor-doped perovskites, and has progressed to demonstrate power densities up to 1 W/cm² in lab-scale cells when combined with electrolytes like yttria-stabilized zirconia (YSZ). The material's perovskite structure supports redox cycling without phase decomposition, addressing limitations of traditional nickel-based anodes.^[60]^[61] The suitability of SrTiO₃ for energy conversion devices stems from its exceptional stability, including chemical inertness in both oxidizing and reducing atmospheres, which prevents degradation during fuel cell operation or RTG exposure to environmental extremes. For instance, LST exhibits no reaction with YSZ electrolytes even after prolonged exposure at 1000 °C, ensuring reliable interface performance. Additionally, its thermal expansion coefficient, approximately 10 × 10⁻⁶ K⁻¹, closely matches that of common SOFC electrolytes like YSZ (around 10.5 × 10⁻⁶ K⁻¹), minimizing thermomechanical stresses during thermal cycling and enhancing device longevity. These properties have positioned doped SrTiO₃ as a promising component in next-generation SOFCs, with ongoing developments focusing on optimizing doping levels for balanced conductivity and durability.^[62]^[63]

Photocatalysis and Thermoelectric Applications

Strontium titanate (SrTiO₃) has been employed as a photocatalyst for overall water splitting since the 1980s, leveraging its chemical stability and perovskite structure to drive the evolution of hydrogen (H₂) and oxygen (O₂) from water under ultraviolet (UV) irradiation.^[64] Its wide band gap of approximately 3.2 eV positions the conduction band edge favorably for water reduction and the valence band edge for oxidation, facilitating photoinduced charge separation where photogenerated electrons and holes migrate to the surface to participate in redox reactions.^[64] Representative studies have demonstrated H₂ evolution rates on the order of 1200 μmol/g over five hours under simulated solar light with modified SrTiO₃ variants.^[65] Beyond water splitting, SrTiO₃ serves as an effective photocatalyst for environmental remediation, including the degradation of organic dyes and reduction of CO₂. In dye degradation, such as methylene blue or rhodamine B, SrTiO₃ nanoparticles achieve removal efficiencies exceeding 90% under UV or visible light (with doping), due to hydroxyl radical generation that mineralizes pollutants. For CO₂ reduction, doped SrTiO₃ (e.g., with Cu or Rh) promotes selective conversion to CH₄ or CO, with quantum yields up to 0.1% under UV irradiation, as demonstrated in lab-scale reactors as of 2023. These applications highlight SrTiO₃'s role in addressing water pollution and greenhouse gas mitigation.^[66]^[67] Doping strategies, such as incorporation of rhodium (Rh) and chromium (Cr), extend SrTiO₃'s responsiveness to visible light by introducing mid-gap states that narrow the effective band gap and enhance charge carrier lifetimes.^[64] For instance, Rh/Cr₂O₃-modified SrTiO₃ achieves visible-light-driven water splitting through improved separation of photogenerated carriers, with Rh acting as an electron trap to suppress recombination.^[68] Hydrothermal synthesis of SrTiO₃ nanoparticles further optimizes photocatalytic performance by yielding high-surface-area structures that promote efficient light absorption and mass transfer, as seen in single-step processes producing particles around 35–40 nm in size.^[43]^[69] Ongoing research as of 2025 explores nanostructured SrTiO₃ forms, such as nanowires and porous films, to enhance performance in photovoltaics and sensors. In dye-sensitized solar cells, Nb-doped STO scaffolds improve electron transport, achieving power conversion efficiencies up to 8% when integrated with perovskites. For gas sensors, STO-based nanostructures detect CO or NO₂ at ppm levels with response times under 10 s, benefiting from defect-mediated sensitivity at operating temperatures of 200–400 °C. These developments leverage STO's tunable bandgap and stability for next-generation energy and sensing technologies.^[70]^[71] In thermoelectric applications, SrTiO₃ exhibits a figure of merit (ZT) ranging from 0.2 to 0.5 across 300–1000 K, attributed to its n-type conductivity and ability to maintain performance at elevated temperatures.^[72] The Seebeck coefficient typically falls between 100 and 200 μV/K, reflecting the material's sensitivity to temperature gradients via electron diffusion and phonon drag effects.^[73] Recent advancements, including 2025 in situ growth of carbon nanotubes on Ni-doped SrTiO₃ via chemical vapor deposition, have boosted electrical conductivity by approximately 69% at room temperature while enhancing phonon scattering to reduce thermal conductivity, resulting in a ZT of 0.3 at 1000 K—a roughly 50% improvement over undoped baselines.^[44] Nanocomposites of SrTiO₃, such as those incorporating Nb or Ti₃AlC₂, further improve ZT to 0.52 at 1173 K by introducing interfaces that scatter phonons without severely impeding electrical transport, enabling applications in automotive waste heat recovery where modules convert exhaust heat to electricity at efficiencies suitable for high-temperature environments.^[72]^[74] These developments prioritize oxide stability and scalability for sustainable energy harvesting from low-grade heat sources.^[75]

References

[1]
Photoinduced electronic and ionic effects in strontium titanate - NIH
Here, we give an overview of the numerous photoinduced effects in SrTiO 3 and their inherent connection to electronic structure and defect chemistry.
[2]
SrTiO 3 : Thoroughly Investigated but Still Good for Surprises - MDPI
SrTiO3 (STO), a cubic perovskite oxide, was synthesized in the early 1950s [1] and has been intensively investigated since then. A breakthrough discovery was ...
[3]
A review of synthesis and morphology of SrTiO 3 for energy and ...
May 7, 2019 · SrTiO3 can be synthesised by hydrothermal, solvothermal, SSR, MSR, and sol-gel reaction. Hydrothermal and solvothermal can synthesise ...
[4]
Strontium titanium oxide (SrTiO3)
### Summary of Strontium Titanium Oxide (SrTiO3) from PubChem CID 82899
[5]
Tausonite: Mineral information, data and localities.
Sapozhnikov (1984) Tausonite, SrTiO3, a new mineral of the perovskite group. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva: 113(1): 86-89 ...
[6]
TAUSONITE, SrTiO3, A NEW MINERAL OF THE PEROVSKITE ...
Jun 29, 2010 · (1984). TAUSONITE, SrTiO3, A NEW MINERAL OF THE PEROVSKITE GROUP. International Geology Review: Vol. 26, No. 4, pp. 462-465.
[7]
Strontium Titanate: A diamond simulant with incredible fire
It is a very rare mineral, found in tiny cubic crystals, crystal clusters, and irregular masses. Natural specimens are typically so small and so rare that they ...Missing: primary | Show results with:primary
[8]
mp-5229: SrTiO3 (Cubic, Pm-3m, 221) - Materials Project
SrTiO₃ is (Cubic) Perovskite structured and crystallizes in the cubic Pm̅3m space group. Sr²⁺ is bonded to twelve equivalent O²⁻ atoms to form SrO₁₂ ...
[9]
[PDF] SrTiO (Strontium Titanate) Substrate
(Strontium Titanate) Substrate. SrTiO3 Structure. 23.21. 23.23. 23.25 ... Crystal structure. Perovskite. Space group. Pm3m. Lattice constant a = 0.3905 nm.Missing: parameter | Show results with:parameter
[10]
Abnormal cubic-tetragonal phase transition of barium strontium ...
A cubic to tetragonal phase transition will take place within the STO system, while either temperature is decreased to 105 K or a high pressure is applied.
[11]
(PDF) Effect of oxygen vacancies on intrinsic dielectric permittivity of ...
Aug 10, 2025 · In the presence of atomic vacancies in strontium titanate, the lattice volume increased and the dielectric permittivity decreased. The expansion ...
[12]
[PDF] Raman Study of Oxygen Reduced and Re-Oxidized Strontium Titanate
Raman studies of reduced SrTiO3 crystals demonstrated that oxygen vacancies do cause the symmetry breakdown and appearance of the first order Raman peaks. ...
[13]
A theoretical study of La-doping in strontium titanate - ScienceDirect
The aim of this work is to predict the most stable structures of SrTiO3:La for both phases and to study the influence of La impurity upon the band structure ...
[14]
https://www.sciencedirect.com/science/article/abs/pii/S0167577X00000185
[15]
Strontium Titanate Single Crystal (SrTiO3)
Density: 5.12 gm/cc. Melting Point: 2080 oC. Hardness: 6 - 6.5 Mohs. Thermal Expansion Coefficient: 10.4 x 10 -6 K -1. Thermal Coefficient at 100 oC: 120 Mw/cm ...
[16]
Strontium Titanate (SrTiO3) - Properties and Applications - AZoM
Feb 5, 2004 · Melting Point (°C), 2080. Mohs Hardness, 6. Dielectric Constant, 300. Thermal Conductivity (W/m.K), 12. Co-Efficient of Thermal Expansion (/°C) ...
[17]
Concurrent atomistic and continuum simulation of bi-crystal ...
STO is an extremely brittle crystalline material with a fracture toughness of 1.0 MPa m1/2. Cleavage is the common form of brittle fracture in crystalline ...
[18]
Self-Nanostructuring in SrTiO3: A Novel Strategy for Enhancement ...
Aug 16, 2019 · The Vickers hardness increased linearly from 0.62 to 0.79 GPa as 1.0 vol% SiC was added. The highest fracture toughness of 1.34 MPa m1/2 was ...
[19]
Refractive index of SrTiO3 (Strontium titanate, STO) - Dodge
SrTiO 3 is a crystalline solid that is often transparent or pale yellow. It possesses perovskite structure and has garnered significant attention in the field ...
[20]
https://refractiveindex.info/?shelf=main&book=SrTiO3&page=Dodge
[21]
Bulk electronic structure of SrTiO3: Experiment and theory
Dec 15, 2001 · The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in ...
[22]
Introduction to Strontium Titanate (SrTiO3) Substrates - Shalom EO
Dielectric constant (room temperature, 1 kHz). ε ≈ 310. Thermal conductivity. 12 W/(m·K). Coefficient of thermal expansion. 9.4 × 10⁻⁶ /K. Working principle ...
[23]
Epitaxial SrTiO3 films with dielectric constants exceeding ... - PNAS
Jun 2, 2022 · This property results in a very large increase in static dielectric constant from ∼300 at room temperature to ∼20,000 at liquid He temperature ...
[24]
Quantum paraelectricity and structural phase transitions in strontium ...
We investigate the cubic to tetragonal transition in strontium titanate and show that the paraelectric phase is stabilized by anharmonic quantum fluctuations.
[25]
[PDF] Quantum Paraelectric States in SrTiO3 and KTaO3 - CORE
where C is the Curie constant, T0 is the paraelectric Curie ... careful measurements of the dielectric constants of SrTiO3 and KTaO3 between 4 and 325.
[26]
[PDF] Permittivity of Strontium Titanate - Caltech Authors
The Curie-Weiss law is satisfied in the temperature range 60-. 300oK, giving a Curie temperature of 30± 2 oK for the three crystal orientations. The. Lyddane ...
[27]
Ferroelectricity in Strain-Free Thin Films | Phys. Rev. Lett.
Bulk piezoelectricity and ferroelectricity in the volume of SrTiO 3 can be induced by Ca doping [8] , electric field [3, 9] , mechanical stress [10] , and O ...
[28]
Dielectric tunability of SrTiO3 thin films in the terahertz range
Mar 6, 2006 · We have demonstrated up to 10% variation of the film permittivity at 300 GHz at room temperature induced by an applied electric field of 100 kV ...
[29]
Defect-induced photoluminescence of strontium titanate and its ...
In the cubic STO, the top of the valence band is located at the R point in the first Brillouin zone, whereas the lowest conduction band bottom lies at the Γ ...
[30]
Exploring the key features for enhanced SrTiO 3 functionality
SrTiO3's photocatalytic activity makes it ideal for energy and environmental uses. •. Doping SrTiO3 enhances its bandgap, optical, and electronic properties ...Missing: lenses | Show results with:lenses
[31]
Hall mobility of undoped n-type conducting strontium titanate single ...
The Hall mobility of undoped n-type conducting SrTiO3 single crystals was investigated in a temperature range between 19 and 1373 K. Field calculations were ...
[32]
The electronic properties of SrTiO 3-δ with oxygen vacancies or ...
Dec 2, 2021 · The defect formation energy is Evac = 5.9 eV. Again, we note that Eion and Evac do not depend on the spin state of the system. The calculations ...
[33]
[PDF] Superconductivity in diamond. - arXiv
Bulk superconductivity with Tc≈0.1–0.5 K was found20 in self-doped GeTe and SnTe at carrier densities n≈1021 cm−3 and in doped SrTiO3 (Tc≈0.05–0.25 K at ...<|separator|>
[34]
Enhancing superconductivity in SrTiO3 films with strain - PMC - NIH
Apr 26, 2019 · A doubling of the superconducting transition temperature occurs in strained SrTiO3 films, which are known to become ferroelectric.
[35]
Enhanced superconductivity close to a non-magnetic quantum ...
Feb 13, 2019 · We focus here on the non-magnetic counterpart, ie, the superconductivity of SrTiO 3 regarded as being close to a ferroelectric QCP.Missing: undoped | Show results with:undoped
[36]
Persistent Photoconductivity in Strontium Titanate | Phys. Rev. Lett.
Oct 30, 2013 · Persistent photoconductivity was observed in strontium titanate ( S r T i O 3 ) single crystals. When exposed to sub-bandgap light (2.9 eV or higher) at room ...
[37]
[PDF] Preparation of the Superconductor Substrate: Strontium Titanate
This report describes the synthesis of single crystals of strontium titanate with the flame-fusion technique and their fabrication to superconductor substrates.
[38]
Study on the Key Technology of High Purity Strontium Titanate ...
Aug 7, 2025 · consider to determine whether 60 degrees Celsius the best synthesis temperature centigrade. 4. 1234567890. International Conference on Materials ...
[39]
[PDF] Synthesis of SrTiO3 by the Calcination of SrCO3 and TiO2 Mixtures ...
The powder mixture resulting from the milling step was subjected to thermal treatments between 800 and 1300°C, in intervals of 100°C. This calcination was done ...
[40]
Formation of SrTiO 3 from Sr-oxalate and TiO 2 - ScienceDirect.com
The precursor completely transforms into SrTiO3 at 1100 °C. About 90 nm size SrTiO3 crystallites are produced at 1100 °C/1 h, due to the lower calcination ...
[41]
Hydrothermal Synthesis of SrTiO3: Role of Interfaces
The hydrothermal synthesis of SrTiO 3 in a Sr(OH) 2 /NaOH solution by reaction of four different single crystalline titanium precursors.
[42]
Single-step hydrothermal synthesis of strontium titanate ...
The traditional preparation method of SrTiO3 is the high temperature solid state reaction by employing strontium carbonate (SrCO3) or strontium oxide (SrO) to ...Missing: primary | Show results with:primary
[43]
[PDF] Special Price - American Watchmakers - Clockmakers Institute
Jun 19, 1979 · 1955-Strontium titanate (fabulite). 1955-General Electric produced ... UNION CARBIDE . .. ... ... ............... . 13 w. I. WIDESS ...
[44]
[PDF] THE JOURNAL OF GEMMOLOGY
branch of Union Carbide in the U.S.A. A large crystal and a cut stone ... rutile, strontium titanate ("fabulite"), yttrium aluminate (YAG) and some ...
[45]
Strontium titanate - CAMEO
Jan 4, 2023 · Dispersion = 0.190 (extreme fire). Composition, SrTiO3. Mohs Hardness, 6.0-6.5 (nat.) 5.5 (syn.) Melting Point, 2080 C. Density (g/ml), 4.81- ...Missing: value | Show results with:value
[46]
Strontium Titanate SrTiO3 Polished Transmission Windows
In stock Free deliveryPrecision fabricated Strontium Titanate SrTiO3 optics with high quality optical properties for optimal spectral performance. Custom components are encouraged ...Missing: watch transparency
[47]
Structure, physical properties, and applications of S r R u O 3 thin films
Mar 8, 2012 · SrTiO 3 has a cubic perovskite unit cell with a lattice parameter of 3.905 Å, which provides good lattice matching with many complex oxides. A ...
[48]
Challenges and opportunities for multi-functional oxide thin films for ...
Nov 15, 2013 · RF MEMS varactors offer high Q at microwave and millimeter wave frequencies, but are complex in nature, and the slow speed of switching is ...
[49]
Processing of Novel Strontium Titanate‐Based Thin‐Film Varistors ...
Dec 20, 2004 · Strontium titanate (SrTiO3)-based multilayered film with varistor characteristics has successfully been fabricated by a chemical solution ...
[50]
SrTiO3 Thin Films on Dielectric Substrates for Microwave Applications
Dec 19, 2023 · Thin films of strontium titanate, which have nonlinear properties that are promising for microwave applications, were grown on a polycrystalline aluminum oxide ...Missing: varistors | Show results with:varistors
[51]
Nonlinearity and Microwave Losses in Cubic Strontium-Titanate
The complex dielectric constant of single-crystal strontium-titanate has been measured from 90° to 230°K at microwave frequencies.
[52]
LaAlO3/SrTiO3 Heterointerface: 20 Years and Beyond - Chen - 2024
Jan 4, 2024 · As a quantum leap towards oxide-based electronics, carrier mobility in oxide heterostructures such as LAO/STO could be significantly enhanced ...Mechanisms Governing the... · Beyond LAO/STO · Device Applications: LAO/STO...
[53]
[PDF] Critical design features of thermal-based radioisotope generators
Oct 28, 2019 · As a fuel source, strontium titanate (SrTiO3) is often used because it is in- soluble, non-combustible, and can withstand high temperatures.
[54]
https://link.aps.org/doi/10.1103/PhysRev.123.97
[55]
Progress in La-doped SrTiO3 (LST)-based anode materials for solid ...
In this review paper, we will describe the development of LST-based anode materials for SOFCs in recent years.
[56]
La-doped SrTiO3 as anode material for IT-SOFC - ScienceDirect.com
Jun 16, 2011 · A-site deficient La-doped SrTiO3 (La0.2Sr0.7TiO3) was evaluated as a potential anode component material for intermediate temperatures SOFCs.
[57]
The stability and mixed conductivity in La and Fe doped SrTiO3 in ...
Both physical properties and the level of mixed conduction obtained in La and Fe doped SrTiO3 are widely influenced by composition. In contrast to La free ...
[58]
Electrical properties and redox stability of tantalum-doped strontium ...
STT was found to be chemically stable under oxidizing and reducing conditions and chemically compatible with yttria-stabilized zirconia (YSZ).
[59]
Photocatalytic Overall Water Splitting by SrTiO3: Progress Report and Design Strategies
### Summary of SrTiO3 for Photocatalytic Water Splitting
[60]
Photocatalytic water splitting of improved strontium titanate for ...
May 5, 2023 · Nearly 1200 μmole/g of hydrogen was evolved in photocatalytic whole water splitting in five hours under simulated AM 1.5 G sunlight.
[61]
Photocatalytic Water Splitting of Al-Doped Rh x Cr 2–x O 3 /SrTiO 3 ...
May 19, 2023 · As an effective photocatalyst, RhxCr2–xO3/SrTiO3 has been widely studied for water splitting, mainly synthesized using SrCl2 molten salt as the ...
[62]
Sustainable synthesis of AgNPs/strontium-titanate-perovskite-like ...
Feb 1, 2020 · SrTiO 3 -cubic-like photocatalysts with average size of 35–40 nm were synthesized in 2 h without further calcination treatment.<|separator|>
[63]
High thermoelectric performance in donor-substituted strontium ...
Apr 20, 2025 · This study explores the design of oxide-based thermoelectric composites, involving the redistribution of a common substituting cation between composite phases.
[64]
Electronic transport and the thermoelectric properties of donor ...
Strontium titanate (SrTiO3) is widely recognised as an environmentally-benign perovskite material with potential for thermoelectric applications.
[65]
Enhancing Thermoelectric Properties of Strontium Titanate Through ...
Mar 19, 2025 · Theoretical calculations show that Ni/SrTiO3 compounds have an energy barrier of 0.41 eV for CH4 dissociation into carbon atoms, much lower than ...
[66]
Processing and thermal properties of SrTiO 3 /Ti 3 AlC 2 ceramic ...
Jul 1, 2022 · The thermal conductivity of SrTiO 3 based composites decreases substantially with the addition of nanostructured Ti 3 AlC 2 from the pristine SrTiO 3 bulk ...
[67]
Recent trends and future perspectives of thermoelectric materials ...
Jul 8, 2024 · In the automotive sector, thermoelectric generators offer the prospect of harnessing waste heat from exhaust systems to power vehicle ...<|control11|><|separator|>