Band gap narrowing in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics

The direct optical band gap in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics fabricated by the solid state reaction method varies from 3.2 eV for KNbO3 down to 2.2 eV for 0.95KNbO3-0.05BiYbO3, as revealed by optical spectroscopic ellipsometry. This narrowing of band gap is accompanied by an apparent increase of the room-temperature relative permittivity from 320 for KNbO3 to 900 for 0.95KNbO3-0.05BiYbO3. All compositions studied exhibit dielectric anomalies associated with structural phase transitions and their ferroelectric nature is corroborated by the presence of a sharp mixed mode (at ~190 cm-1) and by a Fano-type resonant dip in their Raman spectra

Processing-composition-structure effects on the optical band gap of KNbO3-based ceramics

This present work is focused on band-gap engineering of solid-solutions based on KNbO3, which was proposed as a promising photoferroelectric (Grinberg et al., 2013). The strategy to narrow the band-gap of the parent KNbO3 (3.22 eV), relies on replacing Nb5+ by lower valence transition metals (Me3+) and K+1 by cations which maintain the compositions stoichiometric. Ceramic processing of KNbO3 by conventional route was optimised in order to minimise K losses, which leads to the formation of a hygroscopic secondary phase, K4Nb6O17. This phase impairs the structural integrity of the samples. In addition, single-phase KNbO3 ceramics have the tendency to absorb moisture from the environment, increasing its conductivity near room temperature. Subsequently, all solid-solutions presented in this work, (1-x) KNbO3-x Ba0.5Bi0.5Nb0.5Zn0.5O3 and (1-x) KNbO3-x BiMeO3 (Me= Mn, Co and Ni) systems in a compositional range of 0≤x≤0.25, 0.90 KNbO3-0.1 BaNb0.5Ni0.5O3 and 0.98 K0.5Na0.5NbO3-0.02 BaNb0.5Ni0.5O3 compounds, were prepared by the same route as KNbO3. X-Ray Diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy (SEM) revealed compositionally inhomogeneities, suggesting difficulties in cation diffusion for low concentration of solutes by conventional routes. The systems evolve from orthorhombic (x=0) to pseudo-cubic symmetry with an increase of x, suggested by XRD, Raman spectroscopy, ferroelectric and dielectric response. Indeed, these two symmetries seem to coexist for intermediary concentrations. A solubility limit for orthorhombic KNbO3 phase is determined for each system. In addition, a continuous band-gap narrowing was observed in all systems. Nevertheless, (1-x) KNbO3-x BiFeO3 (0≤x≤0.25) system maintained the polar phase up to x=0.25 and its band-gap was narrowed down to 2.22 eV. Indeed, a photocurrent of 0.24 μA/cm2 was measured for 0.75 KNbO3- 0.25 BiFeO3 which is higher than reported for the controversial 0.90 KNbO3-0.1 BaNb0.5Ni0.5O3 compound (Grinberg et al., 2013). The literature does not agree about its band-gap value, which varies from 1.3 eV to 3 eV. Hypothetically, the impossibility of preparing chemically homogenised samples by solid-state reaction may lead to the occurrence of intraband states, which can be misinterpreted. Similar conclusions are reached for 0.98 K0.5Na0.5NbO3-0.02BaNb0.5Ni0.5O3

Site occupancy and electric-field induced strain response of Er-doped (Bi0.4Na0.4Sr0.2)TiO3 ceramics

© 2018 Elsevier B.V. Er-doped (Bi0.4Na0.4Sr0.2)TiO3 powders were prepared by solid state reactions according to A-site donor (Bi0.4-x/3Na0.4-x/3Sr0.2-x/3Erx)TiO3 (x = 0.0.015 and 0.02) and B-site acceptor (Bi0.4Na0.4Sr0.2)Ti1-yEryO3 (y = 0, 0.015 and 0.02) substitutional doping mechanisms. In both cases, room-temperature X-ray diffraction analyses revealed a decrease of the unit cell volume with increasing Er contents, suggesting A-site occupancy to be thermodynamically more favourable. Over the 25–175 °C temperature range, A-site doped ceramics, in particular x = 0.015, showed enhanced thermal stability of the maximum achievable electric-field induced strain. Importantly, this minor doping level also reduced dielectric loss at high temperature and led to a transition from non-ergodic to ergodic relaxor behaviour. These results may further motivate the study of the impact of other minor dopants in this family of Pb-free piezoceramics

Photoresponse of KNbO3–AFeO3 (A = Bi3+ or La3+) ceramics and its relationship with bandgap narrowing

The crystal structure of (1-x)KNbO3–xBiFeO3 (KNBF) and (1-x)KNbO3-LaFeO3 (KNLF) (where x=0.00; 0.01; 0.02; 0.04; 0.08; 0.16; 0.32) was evaluated by XRD and Raman spectroscopy. XRD data show the crystal symmetry to evolve from orthorhombic to tetragonal with increasing x. The optical bandgap was found to narrow systematically with increasing x. Raman spectroscopy analysis corroborated long-range polar order in all compositions. The photoresponse of x=0.32 shows a typical diode–like behaviour, with current and voltage of 0.115 µA and 0.075 V for KNBF and 0.19 µA and 0.035 V for KNLF, respectively. To our knowledge these represent the largest values among KNbO3–based ceramics, making them promising for photovoltaic applications

Band gap evolution and piezoelectric-to-electrostrictive crossover in (1-x)KNbO3-x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 ceramics

The band gap of (1-x)KNbO3-x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 (0≤x≤0.25) ceramics narrows slightly from 3.22 eV for x=0 to 2.89 eV for x=0.25, in broad agreement with first-principle calculations [Phys. Rev. B 89, 235105 (2014)]. In addition, an unreported piezoelectric-to-electrostrictive crossover is observed in this compositional range, which is accompanied by a continuous decrease of the maximum electric field-induced strain due to the presence of a non-ferroelectric phase. An electrostriction coefficient of 0.023 m4/C2 is measured for x=0.05, whilst no electromechanical response is observed for non-ferroelectric x=0.25, even under an applied electric field of 80 kV/cm

Continuously controllable optical band gap in orthorhombic ferroelectric KNbO3-BiFeO3 ceramics

The optical band gap of orthorhombic ferroelectric KNbO3 is shown to be continuously controllable via Bi and Fe co-substitution according to a K1-xBixNb1-xFexO3 doping mechanism. Room temperature X-ray diffraction data combined with Raman spectroscopy analysis show the polar orthorhombic crystal structure to persist up to x=0.25, while the bandgap narrows monotonically by 1 eV (~33%). In-situ Raman spectroscopy corroborates the polar nature of all compositions in the temperature range -100 to 200 C. The ability to control the band gap while maintaining the spontaneous polarisation makes the K1-xBixNb1-xFexO3 system interesting for photoinduced processes in a wide temperature range

Yttrium Iron Garnet/Barium Titanate Multiferroic Composites

Dense multiferroic 0-3 type composites encompassing BaTiO3 and Y3Fe5O12 were fabricated by the solid state reaction method. X-ray diffraction data combined with Scanning Electron Microscopy imaging show virtual immiscibility between the two phases, with the Y3Fe5O12 ferrimagnetic phase well dispersed in the tetragonal BaTiO3 ferroelectric matrix. Raman spectroscopy analyses corroborate the polar nature of the BaTiO3 matrix in composites with a Y3Fe5O12 content as great as 40 wt%. Ferrimagnetism is detected in all composites and no additional magnetic phases are distinguished. Although these dense ceramics can be electrically poled, they exhibit a very weak magnetoelectric response, which slightly increases with Y3Fe5O12 content

Multiferroic and magnetoelectric properties of Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3–CoFe2O4 multilayer composites fabricated by tape casting

A 2-2 type multiferroic composite device encompassing three CoFe2O4 (CFO) layers confined between four Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3 (PZT) layers was fabricated by tape casting. X-ray diffraction data showed good chemical compatibility between the two phases, whereas Scanning Electron Microscopy imaging also revealed an intimate contact between CFO and PZT layers. Under an applied electric field of 65 kV/cm, this multilayer device shows a saturated polarisation of 7.5 C/cm2 and a strain of 0.12%, whereas under a magnetic field of 10 kOe it exhibits a typical ferromagnetic response and a magnetic moment of 33 emu/g. These devices can be electrically poled, after which they exhibit magnetoelectric coupling

Study of the temperature dependence of the giant electric field-induced strain in Nb-doped BNT-BT-BKT piezoceramics

Dense Bi0.487Na0.427K0.06Ba0.026TiO3 (BNKBT) and Nb-doped Bi0.487Na0.427K0.06Ba0.026Ti0.98Nb0.02O3 (Nb-BNKBT) ceramics were prepared by the solid state reaction route. BNKBT is a non-ergodic relaxor and exhibits a piezoelectric response typical for a ferroelectric, whereas Nb-BNKBT is an ergodic relaxor and exhibits an electromechanical response typical for an incipient ferroelectric. The incorporation of 2 mol% of Nb into the BNKBT lattice is accompanied by an enhancement of the room-temperature unipolar field-induced strain from 0.19% to 0.43% at 75 kV/cm. BNKBT shows a depolarisation temperature of 90C, above which an electrostrictive response is observed, whereas Nb-BNKBT shows an electrostrictive response in the entire temperature range studied. At 40 kV/cm, Nb-BNKBT exhibits a temperature stable electromechanical response in comparison with undoped BNKBT, but it worsens under higher electric fields. These results may motivate further investigations on the impact of minor doping and driving electric fields on the electromechanical response of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3-based ceramics

Crystal structure, dielectric properties, and optical bandgap control in KNbO 3 –BiMnO 3 ceramics

Abstract: (1 − x)KNbO3–xBiMnO3 (0 ≤ x ≤ 0.25) ceramics were prepared by the solid‐state reaction method. An X‐ray diffraction analysis combined with Raman spectroscopy showed the co‐solubility of Bi and Mn in the orthorhombic structure to be less than 5% BiMnO3. Orthorhombic and pseudocubic symmetries coexist in the 0.05 ≤ x ≤ 0.15 region, coinciding with a bimodal grain size distribution. This coexistence of crystal symmetries is further corroborated by several anomalies in the dielectric behavior, which can be ascribed to structural phase transitions. For x ≥ 0.20, only one dielectric anomaly is detected around 100°C, which is commensurate with in situ Raman spectroscopy analysis. This work also shows that Bi/Mn co‐doping can be employed to tailor the bandgap of KNbO3, which narrows continuously with increasing x, resulting in ∼1‐eV narrowing for single‐phase x = 0.25. This may offer the possibility to employ this ferroic material in photoresponsive technologies