Effect of Nb cationic substitution on the microstructure and Raman spectra of SrBi2(Ta,Nb)2O9 thin films

Micro-Raman spectroscopy and x-ray diffraction have been used to explore the lattice dynamics of Nb-substituted SrBi2(TaxNb1-x)2O9 (SBTN) thin crystalline films annealed at low temperature, 700oC. The substitution of Nb at Ta-site leads to the decrease in the SBT lattice parameters, as confirmed by x-ray diffraction data entire the concentration range of Nb varying from 0 to 50 wt. %. The relative intensity of the (115), (006) and (200) X-ray diffraction peaks increases with the increase of the Nb content. We observed nonmonotonic shift of the Raman band maximum from 810 cm-1 (for pure SBT) to 830 cm-1 (for SBTN with 20 % of Nb) and then back to 810 cm-1 with increasing Nb content from 40 to 50 wt. %. We assume that these changes are conditioned by the ferrodistortion occurring in ferroic perovskites, namely by the tilting distortion of (Ta,Nb)O6 octahedra at 20-40 wt. % of Nb. The octahedra tilting can change the coordination environment of the A-cite cation, as well as it lowers the SBTN symmetry below the cubic symmetry. The tilted state at 20-40 wt. % of Nb can explain the nonmonotonic changes of the perovskite phase fraction and remanent polarization with increasing Nb content from 0 to 50 wt.%, in particular their initial increase with Nb content increase up to 20 wt. % followed by a decrease at 30 wt. % of Nb, then increase at 40 wt.% and further decrease for 40 - 50 wt.% of Nb. Hence, the substitution by Nb of Ta-site in SBT has a pronounced impact on the O-Ta-O stretching modes by shifting and splitting the mode frequency at (810 - 830) cm-1; however, it does not influence the low frequency Raman modes of SBTN.Landau-Devonshire approach is used to explain the experimentally observed nonmonotonic dependence of the ferroelectric perovskite phase fraction and remanent polarization on Nb content in SBTN films.Comment: 24 pages, 12 figure

Photoactive Properties of Transport Sol-Gel Layers Based on Strontium Titanate for Perovskite Solar Cells

In this work, we have investigated the photocurrent and spectral sensitivity of the silicon/SrTiO3:xNb/perovskite structures. The sol–gel method carried out the deposition of undoped SrTiO3 layers as well as niobium-doped (SrTiO3:Nb) layers at atomic concentrations of 3 and 6% Nb. The perovskite layer, CH3NH3PbI3_xClx, has been deposited by the vacuum co-evaporation technique. The layers have been characterized by scanning electron microscopy and X-ray diffraction measurements. The volt–ampere characteristics and spectral sensitivity of the fabricated samples have been measured under illumination with selective wavelengths of 405, 450, 520, 660, 780, 808, 905, 980, and 1064 nm of laser diodes. We have shown that for different configurations of applied voltage between silicon, SrTiO3:xNb, and CH3NH3PbI3_xClx, the structures are photosensitive ones with a variation of photocurrent from microamperes to milliamperes depending on Nb concentration in SrTiO3, and the highest photocurrent and spectral sensitivity values are observed when a SrTiO3:Nb layer with 3 at.% of Nb is used. A possible application of the proposed structure with a SrTiO3:Nb layer for perovskite solar cells and photodetectors is being discussed

A combined theoretical and experimental study of the phase coexistence and morphotropic boundaries in ferroelectric-antiferroelectric-antiferrodistortive multiferroics

The physical nature of the ferroelectric (FE), ferrielectric (FEI) and antiferroelectric (AFE) phases, their coexistence and spatial distributions underpins the functionality of antiferrodistortive (AFD) multiferroics in the vicinity of morphotropic phase transitions. Using Landau-Ginzburg-Devonshire (LGD) phenomenology and a semi-microscopic four sublattice model (FSM), we explore the behavior of different AFE, FEI, and FE long-range orderings and their coexistence at the morphotropic phase boundaries in FE-AFE-AFD multiferroics. These theoretical predictions are compared with the experimental observations for dense Bi1-yRyFeO3 ceramics, where R is Sm or La atoms with the fraction 0 ≤ y ≤ 0.25, as confirmed by the X-ray diffraction (XRD) and Piezoresponse Force Microscopy (PFM). These complementary measurements were used to study the macroscopic and nanoscopic transformation of the crystal structure with doping. The comparison of the measured and calculated AFE/FE phase fractions demonstrate that the LGD-FSM approach well describes the experimental results obtained by XRD and PFM for Bi1-yRyFeO3. Hence, this combined theoretical and experimental approach provides further insight into the origin of the morphotropic boundaries and coexisting FE and AFE states in model rare-earth doped multiferroics. © 2021Authors acknowledge Dr. Bobby Sumpter (ORNL) and Reviewers for very useful suggestions and ideas. This material is based upon work (S.V.K.) supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and performed at the Center for Nanophase Materials Sciences, a US Department of Energy Office of Science User Facility. A portion of FEM was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility (CNMS Proposal ID: 257). A.N.M. work is supported by the National Academy of Sciences of Ukraine (the Target Program of Basic Research of the National Academy of Sciences of Ukraine "Prospective basic research and innovative development of nanomaterials and nanotechnologies for 2020 - 2024″, Project № 1/20-Н, state registration number: 0120U102306). A.N.M., D.V.K., A.D.Y., O.M.F., T.S., V.V.S. and A.L.K. received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778070. A.N.M. acknowledges the National Research Foundation of Ukraine. M.V.S. acknowledges financial support from the Ministry of Science and Higher Education of the Russian Federation within the framework of state support for the creation and development of World-Class Research Centers "Digital biodesign and personalized healthcare" №075–15–2020–926. Part of the work (A.L.K.) was supported by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST «MISiS» (No. K2–2019–015). V.V.S. and A.L.K. were additionally supported by RFBR and BRFBR, project numbers 20–58–0061 and T20R-359, respectively. Part of this work (A.L.K.) was developed within the scope of the project CICECO-Aveiro Institute of Materials, refs. UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MCTES