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We propose B-site Bi5+-doped ferroelectric perovskite materials as suitable candidates for the bulk photovoltaic effect and related solar applications. The low-lying 6s empty states of the electronegative Bi atom produce empty bands in the energy gap of the parent materials, effectively lowering the band gap by 1-2 eV, depending on the composition of the ferroelectric end member and the concentration of Bi5+ in the solid solution. The polarization decreases but survives upon doping, which enables the "shift-current" mechanism for photocurrent generation, while the decreased band gap allows absorption of much of the visible spectrum. The magnitude of shift-current response is calculated for 0.75Pb2InNbO6-0.25Ba2InBiO6 (PIN-BIB) and 0.75Pb2ScNbO6-0.25Ba2ScNbO6 (PSN-BSB) and is predicted to exceed the visible-light bulk photovoltaic response of all previously reported materials, including BiFeO3. Furthermore, the existence of their intermediate bands and multiple band gaps, combined with Fermi-level tuning by A-site co-doping, also allows for their potential application in traditional p - n junction-based solar cells as broad-spectrum photoabsorbers. [ABSTRACT FROM AUTHOR]

We demonstrate a new band engineering strategy for the design of semiconductor perovskite ferroelectrics for photovoltaic and other applications from first principles. We study six ferroelectric solid solutions created by partially substituting Zn2+ for Nb5+ into the parent KNbO3 material, combined with charge compensation at the A sites with different combinations of higher valence cations. Our first-principles calculations with the HSE06 functional yield a low band gap of only 2.1 eV for the 75%KNbO3-25%(Sr1/2La1/2)(Zn1/2Nb1/2)O3 solid solution, and this can be lowered further by 0.6 eV under strain through polarization rotation. The large polarization, especially under strain, of these materials provides a charge separation route by the bulk photovoltaic effect that could potentially allow power conversion efficiency beyond the Shockley-Queisser limit. This band engineering strategy is applicable to other perovskites and should be realizable by standard solid-state synthesis and thin film growth methods. [ABSTRACT FROM AUTHOR]

The piezoelectric properties of tetragonal perovskites PbTiO3, BaTiO3, and KNbO3 under uniaxial strain along the [001] direction are calculated from first principles. In order to study the piezoelectric response of individual atomic sublattices, the total polarization P and piezoelectric constant d33 are decomposed into the contributions of A-site and B-site atoms. These three materials show different behaviors at the atomic scale. In PbTiO3, the Pb and Ti contribute almost equally to the polarization and d33. In KNbO3, most of the polarization and d33 are contributed by the ferroelectrically active Nb. But in BaTiO3, as the c axis increases, the contribution of Ba to d33 rises rapidly, reaching 50% of the total d33 at c = 4.30 Å. This means that the covalent bonding between Ba and O is significant and has an important impact on the piezoelectric response of BaTiO3. [ABSTRACT FROM AUTHOR]

We present a modified bond-valence model of PbTiO3 based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials. [ABSTRACT FROM AUTHOR]

Stable ferroelectric (FE) phases in nanometer-thick films would enable ultra-high density and fast FE field effect transistors (FelTiTs), and the stability of ferroelectricity in ultrathin films has been under intense theoretical and experimental investigation. Here we predict, using density functional theory calculations, that the low-energy epitaxial PbTiO3 (001)/Pt interface strengthens the electrode-oxide bonds by breaking in-plane symmetry and stabilizes a ground state with enhanced polarization in subnanometer oxide films, with no critical-size limit. Additionally, we show that such enhancement is related to large work function differences between the P- and P+ PbTiO3 surfaces, which gives rise to a net polarizing field in the oxide. [ABSTRACT FROM AUTHOR]

We use density functional theory calculations to study the lattice vibrations and electronic properties of the correlated metal LaNiO3. To characterize the rhombohedral-to-cubic structural phase transition of perovskite LaNiO3, we examine the evolution of the Raman-active phonon modes with temperature. We find that the A1g Raman mode, whose frequency is sensitive to the electronic band structure, is a useful signature to characterize the octahedral rotations in rhombohedral LaNiO3. We also study the importance of electron-electron correlation effects on the electronic structure with two approaches that go beyond the conventional band theory [local spin density approximation (LSDA)]: the local spin density + Hubbard U method (LSDA + U) and hybrid exchangecorrelation density functionals that include portions of exact Fock exchange. We find that the conventional LSDA accurately reproduces the delocalized nature of the valence states in LaNiO3 and gives the best agreement with the available experimental data on the electronic structure of LaNiO3. Based on our calculations, we show that the electronic screening effect from the delocalized Ni 3d and O-2p states mitigates the electronic correlations of the d7 Ni cations, making LaNiO3 a weakly correlated metal. [ABSTRACT FROM AUTHOR]

We examine band-structure engineering in extremely tetragonal ferroelectric perovskites (ABO3) to make these materials suitable for photovoltaic applications. Using first-principles calculations, we study how B-site ordering, lattice strain, cation identity, and oxygen octahedral cage tilts affect the energies and the compositions of the valence and conduction bands. We find that extreme tetragonality makes the band gap highly sensitive to the B-cation ordering, with a layered B-site arrangement exhibiting a small band gap. It also leads to a strong sensitivity of the band gap to the oxygen octahedral tilting. These effects only occur for cations with filled d states located near the valence-band maximum or empty d states at the conduction-band minimum; this criterion is explained by crystal field theory. In addition to a smaller band gap, the layered B-site ordering has a strong impact on the carrier mobility. We find that the excited electron effective mass is similar to that found in Si and other classic semiconductors. Moreover, the hole effective mass is strongly anisotropic, indicating a two-dimensional hole gas in the layered B-cation ordering. [ABSTRACT FROM AUTHOR]

Using density functional theory (DFT) within the local density approximation (LDA), we calculate the physical and electronic properties of PbTiO3 (PTO) and a series of hypothetical compounds PbTiO3-xSx x = 0.2, 0.25, 0.33, 0.5, 1, 2, and 3 arranged in the comer-sharing cubic perovskite structure. We determine that replacing the apical oxygen atom in the PTO tetragonal unit cell with a sulfur atom reduces the x = 0 LDA calculated band gap of 1.47 eV to 0.43-0.67 eV for x = 0.2-1 and increases the polarization. PBE0 and GW methods predict that the compositions x = 0.2-2 will have band gaps in the visible range. For all values of x < 2, the oxysulfide perovskite retains the tetragonal phase of PbTiO3, and the a lattice parameter remains within 2.5% of the oxide. Thermodynamic analysis indicates that chemical routes using high-temperature gas, such as H2S and CS2, can be used to substitute ofor S in PTofor the compositions x = 0.2-0.5. [ABSTRACT FROM AUTHOR]

Oxygen vacancies in perovskite oxide solid solutions are fundamentally interesting and technologically important. However, experimental characterization of the vacancy locations and their impact on electronic structure is challenging. We have carried out first-principles calculations on two Zr-modified solid solutions, Pb(Zn1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3, in which vacancies are present. We find that the vacancies are more likely to reside between low-valent cation-cation pairs than high-valent cation-cation pairs. Based on the analysis of our results, we formulate guidelines that can be used to predict the location of oxygen vacancies in perovskite solid solutions. Our results show that vacancies can have a significant impact on both the conduction and valence band energies, in some cases lowering the band gap by ≈0.5 eV. The effects of vacancies on the electronic band structure can be understood within the framework of crystal field theory. [ABSTRACT FROM AUTHOR]