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In this paper, we report a density functional study of the structural, electronic and pressure-induced solid–solid phase transitions of SrTiO₃. These first-principles calculations have been performed using the full potential linearized augmented plane wave method (FP-LAPW) within the generalized gradient approximation (GGA) developed by Perdew–Burke–Ernzerhor for solids (PBEsol). The calculated structural parameters like the lattice parameters, the bulk modulus B and their pressure derivative B′ are used to analyze the relative stability and phase transitions under pressure of SrTiO₃. Calculations were done for the cubic (Pm-3m), tetragonal (I4/mcm, P4/mbm, P4mm) and orthorhombic (Cmcm, Pnma) structures where we found that the tetragonal I4/mcm phase is the most stable structure compared to the other structures at T = 0 K and P = 0 GPa. For the electronic properties calculations, the exchange and correlation effects were treated by the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential to prevent the shortcoming of the underestimation of the energy gaps in both LDA and GGA approximations. The obtained results are compared to available experimental data and to other theoretical calculations.

Suitably doped SrTiO₃ was found in 1964 to undergo a superconducting transition below 1 K with a dome-like T_c versus n (electron concentration) plot. The apex of the dome — a point of inflection — corresponds to the point (n≈9 ×10¹⁹cm^-3, T_c≈0.30 K). On either side of it, T_c goes down to ≈0.1 K for the extreme values between which n was varied. A single value of T_c is thus observed for two different values of n. The puzzle for the theory has been to explain this result. Treating the problem in all its generality, we present here three equations: the μ₁-incorporated BCS equation for T_c, the μ₀-incorporated equation for the T = 0 gap Δ₀, where μ₁ and μ₀ are the chemical potentials at T = T_c and T = 0 respectively, and an equation that relates the interaction parameters λ₁ and λ₀ at these temperatures. Because there are five unknowns in the problem, we tackle these equations via an approximation scheme that includes setting μ₁ = μ₀ and λ₁ = λ₀. The latter of these is factually a basic tenet of the BCS theory. Salient features of our findings are: (i) the solutions for T_c and Δ₀ on the RHS (LHS) of the dome correspond to μ> k_Bθ_D(μ < k_Bθ_D); k_B = Boltzmann constant, θ_D = Debye temperature, (ii) for solutions on the LHS the limits of the integrals in the equations need to be curtailed to obtain real solutions and (iii) the point μ = k_Bθ_D is a point of inflection in the T_c versus μ plot. Since the puzzle has remained unsolved for a long time, we also offer here a purely mathematical model for λ(μ) — sans physical justification — which leads to a T_c versus μ plot qualitatively in agreement with experiment.

Energy band engineering of semiconductors plays a crucial role in exploring high-efficiency visible-light response photocatalysts. Herein, we systematically study the electronic properties and optical response of Cr-, B-doped SrTiO₃, and (Cr, B)-codoped SrTiO₃ by using first-principles calculations to explore the mechanism for its superior photocatalytic activities in the visible light region. Special emphasis is placed on uncovering the synergy effects of nonmetal B dopant with metal Cr dopant at different cation sites. It is found that the electronic properties and optical absorption of SrTiO₃ can be dramatically engineered by mono- or co-doping. In particular, the intermediate levels lying in the bandgap of the codoped SrTiO₃ relay on the Cr impurity doped at Sr or Ti cation sites. Moreover, the (Cr@Sr, B@O)-SrTiO₃ retains the charge balancing without the generation of unexpected oxygen vacancies, and is more desirable for solar light harvesting due to its higher absorption than others in the entire visible light. The findings can rationalize the available experimental results and are helpful in designing SrTiO₃-based photocatalysts with high-efficiency performance.

Using strontium–titanium salts precursor, nanopowders (STO-based-NPs) were successfully synthesized by controlled gel-combustion method. Citric and nitric acids in an optimum ratio were used as the fuel and oxidizer agents, respectively. After heat treatment at 850${}^{\circ }$C, the crystalline structure of the products was investigated by X-ray diffraction. The effects of Ba and Ag dopants on particle size distribution were discussed by transmission electron microscopy (TEM). The optical and dielectric parameters such as energy band gap $(E_g)$, real and imaginary parts of refractive index, dielectric function and energy loss function of nanopowders have been investigated by UV–Vis and FTIR spectra. The band gap of SrTiO₃ increased with increasing Ba, Ag and Ba–Ag. Different atomic radii of dopants are responsible for changing optical and dielectric parameters due to the altered orbital configuration of the lattice structure.

Structural, elastic, mechanical and electronic properties of pure and zinc-doped SrTiO₃ at the concentration in the range (1–10%) are studied by first-principles calculations. The structural parameters of synthesized compounds agree well with the standard data depicting the growth of stable compounds. A slight obvious increase in the lattice constant of 3.9245Å is observed in Zn-doped SrTiO₃ due to the deviation of the atomic radii of Zn and Ti. Elastic constants and mechanical parameters of SrTiO₃ are closer to their available theoretical and experimental data. The investigated compounds exhibit brittle behavior for all Zn ratios. The doping zinc concentration reduces the indirect band gap value. The doping concentration 2%, gives a band gap value closer to the experimental one. The band gap of pure SrTiO₃ is 1.827eV and after doping with Zn for concentration from 1% to 10%, the optimized values are 1.970, 1.886, 1.802, 1.718, 1.635, 1.552, 1.470, 1.389, 1.310, 1.231 and 1.154eV.

Novel n-SrTiO₃/p-BiOI heterojunction composites were successfully fabricated by loading SrTiO₃ particles onto the surface of BiOI nanoflakes via a two-step method. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), energy-disperse X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), diffuse reflectance spectroscopy (DRS) and electrochemical measurements. The results show that the n-SrTiO₃/p-BiOI heterojunction composites are composed of perovskite structure SrTiO₃ and tetragonal phase BiOI. The composites exhibit excellent photocatalytic performance for the degradation of crystal violet (CV) solution under simulated solar light irradiation, which is superior to that of pristine BiOI and SrTiO₃. The 30wt.%SrTiO₃/BiOI composite is found to be the optimal composite, over which the dye degradation reaches 92.5% for 30min of photocatalysis. The photocatalytic activity of the 30wt.%SrTiO₃/BiOI composite is found to be 3.94 times and 28.2 times higher than that of bare BiOI and SrTiO₃, respectively. The reactive species trapping experiments suggest that $•{\mathrm{\text{O}}}_2^{-}$ and holes are the main active species responsible for the CV degradation. In addition, the electrochemical measurements elucidate the effective separation of photoinduced electron–hole pairs. Moreover, on the basis of experimental and theoretical results, a possible mechanism for the enhanced photocatalytic performance of the SrTiO₃/BiOI heterojunction composites is also proposed.

A novel strontium titanate/binary metal sulfide (SrTiO₃/SnCoS₄) heterostructure was synthesized by a simple two-step hydrothermal method. The visible-light-driven photocatalytic performance of SrTiO₃/SnCoS₄ composites was evaluated in the degradation of methyl orange (MO) under visible light irradiation. The photocatalytic performance of SrTiO₃/SnCoS₄-5% is much higher than that of pure SrTiO₃, SnCoS₄, SrTiO₃/SnS₂ and SrTiO₃/CoS₂. The SrTiO₃/SnCoS₄ composite material with 5wt.% of SnCoS₄ showed the highest photocatalytic efficiency for MO degradation, and the degradation rate could reach 95% after 140min irradiation time. The enhanced photocatalytic activity was ascribed to not only the improvement of visible light absorption efficiency, but also the construction of a heterostructure which make it possible to effectively separate photoexcited electrons and holes in the two-phase interface.

Contrast with conventional dielectric tunable materials such as barium strontium titanate (BST), here, we report one new dielectric tunable behavior for Sr_1-xPr_xTiO₃ system at low temperature. Giant dielectric tunability is confirmed in this system. More importantly, the efficient dielectric tunability can be realized just using small bias field. In addition, critical threshold electric field is also confirmed. This phenomenon may be related with the competition interaction of polar state with quantum fluctuations.

${\mathrm{\text{BaTiO}}}_3$/${\mathrm{\text{SrTiO}}}_3$ (BTO/STO) multilayer films were successfully prepared on (La, Sr)${\mathrm{\text{CoO}}}_3$-coated (100) ${\mathrm{\text{SrTiO}}}_3$ substrates by using a radio-frequency (RF) magnetron sputtering process at $700^{\circ }$C. Benefiting from the flexible deposition configuration of a multi-target sputtering system, we were able to tune the dielectric properties of the multilayer film by varying the number of multilayer periods $N$ and the individual layer thickness $d$. It was found that, in a superlattice-like structure ($d_{\mathrm{\text{BTO}}}=d_{\mathrm{\text{STO}}}$=4 nm, $\sim$ 10 unit cells), the dielectric constant increased and the loss tangent decreased with an increasing $N$, especially in the high frequency range ($10^4–10^6$Hz). This can be attributed to a reduced volumetric contribution to the dielectric property from the leaky interface capacitor layer, which lies between the multilayer film and the (La, Sr)${\mathrm{\text{CoO}}}_3$ electrode. On the other hand, as the individual layer thickness $d$ exceeds the superlattice limit ($d_{\mathrm{\text{BTO}}}=d_{\mathrm{\text{STO}}}$=8 nm, $\sim$ 20 unit cells), the superlattice strain effect disappeared and the dielectric constant value dropped by $\sim$50%. However, owing to the reduced number of interfaces and associated defects, the dielectric loss of the multilayer film with a larger period was reduced significantly, as compared to its superlattice counterpart with the same thickness and more periods. The dielectric loss power density of the former was about one order of magnitude lower than that of the latter. These observations provide a solid foundation for using RF magnetron sputtering as an effective method to prepare various forms of multilayer film capacitors for integrated device applications.

A nonlinear thermodynamic formalism is developed to calculate the pyroelectric property of epitaxial single domain ${\mathrm{\text{SrTiO}}}_3∕\mathrm{\text{Si}}$ heterojunctions by taking into account the thermal expansion misfit strain at different temperatures. It has been demonstrated that the crucial role was played by the contribution associated with the structure order parameter arising from the rotations of oxygen octahedral on pyroelectricity. A dramatic decrease in the pyroelectric coefficient due to the strong coupling between the polarization and the structure order parameter is found at ferroelectric ${\mathbf{\text{T}}}_{\mathrm{\text{F1}}}$–${\mathbf{\text{T}}}_{\mathrm{\text{F2}}}$ phase transition. At the same time, the thermal expansion mismatch between film and substrate is also found to provide an additional weak decrease of pyroelectricity. The analytic relationship of the out-of-plane pyroelectric coefficient and dielectric constant of ferroelectric phases by considering the thermal expansion of thin films and substrates has been determined for the first time. Our research provides another avenue for the investigation of the pyroelectric effects of ferroic thin films, especially, such as antiferroelectric and multiferroic materials having two or more order parameters.