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Solar-driven oxygen evolution is a critical technology for renewably synthesizing hydrogen- and carbon-containing fuels in solar fuel generators. New photoanode materials are needed to meet efficiency and stability requirements, motivating materials explorations for semiconductors with (i) band-gap energy in the visible spectrum and (ii) stable operation in aqueous electrolyte at the electrochemical potential needed to evolve oxygen from water. Motivated by the oxygen evolution competency of many Mn-based oxides, the existence of several Bi-containing ternary oxide photoanode materials, and the variety of known oxide materials combining these elements with Sm, we explore the Bi-Mn-Sm oxide system for new photoanodes. Through the use of a ferri/ferrocyanide redox couple in high-throughput screening, BiMn2O5 and its alloy with Sm are identified as photoanode materials with a near-ideal optical band gap of 1.8 eV. Using density functional theory-based calculations of the mullite Bi3+Mn3+Mn4+O5 phase, we identify electronic analogues to the well-known BiVO4 photoanode and demonstrate excellent Pourbaix stability above the oxygen evolution Nernstian potential from pH 4.5 to 15. Our suite of experimental and computational characterization indicates that BiMn2O5 is a complex oxide with the necessary optical and chemical properties to be an efficient, stable solar fuel photoanode. [ABSTRACT FROM AUTHOR]

Antiferroelectric PbZrO3 is being considered for a wide range of applications where the competition between centrosymmetric and noncentrosymmetric phases is important to the response. Here, we focus on the epitaxial growth of PbZrO3 thin films and understanding the chemistry-structure coupling in Pb1+δZrO3 (δ = 0, 0.1, 0.2). High-quality, single-phase Pb1+δZrO3 films are synthesized via pulsed-laser deposition. Although no significant lattice parameter change is observed in X-ray studies, electrical characterization reveals that while the PbZrO3 and Pb1.1ZrO3 heterostructures remain intrinsically antiferroelectric, the Pb1.2ZrO3 heterostructures exhibit a hysteresis loop indicative of ferroelectric response. Further X-ray scattering studies reveal strong quarter-order diffraction peaks in PbZrO3 and Pb1.1ZrO3 heterostructures indicative of antiferroelectricity, while no such peaks are observed for Pb1.2ZrO3 heterostructures. Density functional theory calculations suggest the large cation nonstoichiometry is accommodated by incorporation of antisite PbZr defects, which drive the Pb1.2ZrO3 heterostructures to a ferroelectric phase with R3c symmetry. In the end, stabilization of metastable phases in materials via chemical nonstoichiometry and defect engineering enables a novel route to manipulate the energy of the ground state of materials and the corresponding material properties. [ABSTRACT FROM AUTHOR]

Density-functional calculations are performed to investigate the experimentally reported field-induced phase transition in thin-film ZrO2 [J. Müller et al., Nano Lett. 12, 4318 (2012)]. We find a small energy difference of ∼1 meV/f.u. between the nonpolar tetragonal and polar orthorhombic structures, characteristic of antiferroelectricity. The requisite first-order transition between the two phases, which atypically for antiferroelectrics have a group-subgroup relation, results from coupling to other zone-boundary modes, as we show with a Landau-Devonshire model. Tetragonal ZrO2 is thus established as a lead-free antiferroelectric with excellent dielectric properties and compatibility with silicon. In addition, we demonstrate that a ferroelectric phase of ZrO2 can be stabilized through epitaxial strain, and suggest an alternative stabilization mechanism through continuous substitution of Zr by Hf. [ABSTRACT FROM AUTHOR]

Density-functional calculations are performed to study the effect of epitaxial strain on PbZrO3. We find a remarkably small energy difference between the epitaxially strained polar R3c and nonpolar Pbam structures over the full range of experimentally accessible epitaxial strains-3% ⩽ η ⩽ 4%. While ferroelectricity is favored for all compressive strains, for tensile strains the small energy difference between the nonpolar ground state and the alternative polar phase yields a robust antiferroelectric ground state. The coexistence of ferroelectricity and antiferroelectricity observed in thin films is attributed to a combination of strain and depolarization field effects. [ABSTRACT FROM AUTHOR]

The Ruddlesden-Popper (RP) homologous series Srn+1TinO3n+1 provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO3. We use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectra of Srn+1TinO3n+1 for n=1-5 and 8. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small n and reach a value of 330 meV for n=1, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr2TiO4 (n=1) localizes in the two-dimensional plane defined by the TiO2 layer, and we explain the origin of its localization. [ABSTRACT FROM AUTHOR]

Raman and photoluminescence (PL) spectroscopy are used to investigate dynamic structure–function relationships in methylammonium lead iodide (MAPbI3) perovskite. The intensity of the 150 cm–1 methylammonium (MA) librational Raman mode is found to be correlated with PL intensities in microstructures of MAPbI3. Because of the strong hydrogen bond between hydrogens in MA and iodine in the PbI6 perovskite octahedra, the Raman activity of MA is very sensitive to structural distortions of the inorganic framework. The structural distortions directly influence PL intensities, which in turn have been correlated with microstructure quality. Our measurements, supported with first-principles calculations, indicate how excited-state MA librational displacements mechanistically control PL efficiency and lifetime in MAPbI3material parameters that are likely important for efficient photovoltaic devices. [ABSTRACT FROM AUTHOR]

This study demonstrates that antiferroelectricity can be stabilized in NaNbO3 (NN) based ceramics by lowering the tolerance factor. Through consideration of the crystal chemistry via the Goldschmidt tolerance factor and polarizability, we show that simultaneous substitution of Zr4+ and Ca2+ ions in the Nb and Na sites, respectively, lowers the polarizability and tolerance factor of the (Na1−xCax)(Nb1−xZrx)O3 (CZNN100x) solid solution, while maintaining charge neutrality. Structural investigations using both X-ray diffraction and transmission electron microscopy (TEM) indicated an enhancement of antiferroelectric (AFE) superlattice peaks with CaZrO3 substitution. The TEM domain analysis revealed that only AFE domains existed in the CZNN4 and CZNN5 ceramics; in contrast, normal NN ceramics displayed coexistence of AFE and ferroelectric (FE) domains at room temperature. The CZNN100x (0.02 ≤x≤ 0.05) ceramics showed double polarization hysteresis loops, characteristic of reversible AFE↑FE phase transition switching. The field-induced polarization decreased drastically with increasing substitution, an effect of the decreases in tolerance factor. In addition, the AFE switching field was increased by the chemical substitution. First principles calculations are performed to obtain insights into the relative stability and coexistence of the AFE and FE phases in single domains. The large decrease of polarization in the CZNN system is explained by a modification of the relative stability of the relevant structures, which favours nonpolar-to-polar AFE transitions over polar-to-polar FE domain switching. [ABSTRACT FROM AUTHOR]