Orthorhombic KNbO3 Research Articles

Piezoelectric nanogenerators synthesized using KNbO3nanowires with various crystal structures

KNbO3 (KN) nanowires having a tetragonal structure or a polymorphic phase boundary (PPB) structure, which contains both tetragonal (P4mm) and orthorhombic (Amm2) structures, are formed at low temperatures. The presence of tetragonal and PPB KN nanowires is attributed to the existence of OH− and H2O defects. Further, the tetragonal and PPB KN nanowires change to orthorhombic KN nanowires in the temperature range between 300 and 400 °C owing to desorption of the lattice hydroxyl group. A composite consisting of polydimethylsiloxane (PDMS) and KN nanowires having a PPB structure shows large dielectric constant and low dielectric loss values of 9.2 and 0.5%, respectively, at 100 kHz. Moreover, a nanogenerator (NG) synthesized using the PPB KN nanowires exhibits the largest output voltage and current among NGs synthesized using the tetragonal or orthorhombic KN nanowires. In particular, the NG containing 0.7 g of PPB KN nanowires shows an output voltage of 10.5 V and an output current of 1.3 μA; these values are among the highest ever reported for NGs synthesized using a lead-free composite. In addition, this NG exhibited the maximum output power and energy conversion efficiency, which were 4.5 μW and 0.9%, respectively, for an external load of 1.0 MΩ.

Effect of potassium chloride amount on structures of hydrothermally grown KNbO3 nanostructure

KNbO3 nanocrystals were synthesised by hydrothermal process with KOH, Nb2O5, KCl and H2O mixture as principal raw materials. Pure phase orthorhombic and rhombohedral structure crystals were formed. The nanocrystals were good crystalline, and the morphology has been greatly changed when different amounts of KCl were added. The diameter of rods was ∼500 nm, which were formed when the molar ratios of KOH and KCl are 1∶0·030 and 1∶0·060; the nanorod yield was improved greatly. Two different structure crystals showed different peak positions at ∼250 cm−1 for rhombohedral KNbO3 and 279 cm−1 for orthorhombic KNbO3 in Raman spectrum.

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH− and H2O in the KN unit cell, tetragonal KNbO3 (KN) nanowires were formed when the synthesis was carried out at 120°C for 48h. However, when the fabrication was conducted at high temperatures (⩾150°C) or at 120°C for a long period of time (⩾72h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120°C for 60h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb5+-doped SrTiO3 substrate, and their d33 values were measured for the first time. A tetragonal KN nanowire exhibited a d33 value of 23.5pmV−1, which is larger than that of the orthorhombic KN nanowire (11.6pmV−1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d33 value of 40.0pmV−1. The d33 values of KN nanowires increased to 104.5, 137.1 and 146.0pmV−1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [100] direction by application of a DC voltage of 10V.

Crystalline phase-dependent photocatalytic water splitting for hydrogen generation on KNbO3 submicro-crystals

Potassium niobate (KNbO3) submicro-crystals with cubic and orthorhombic phases were hydrothermally prepared and characterized by powder X-ray diffraction, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, nitrogen adsorption–desorption, diffuse reflectance UV–visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The photocatalytic performance of the as-prepared KNbO3 samples was evaluated toward H2 generation from an aqueous methanol solution under UV. The surface area-normalized rate of H2 production over submicro-cubes with cubic phase is two times larger than that of submicro-rods with orthorhombic phase. In addition, both cubic and orthorhombic KNbO3 submicro-crystals showed much higher reactivity than commercial KNbO3 bulk-like powders. The underlying mechanism was discussed in terms of crystal structure and electronic structure. The results from this study are potentially applicable to a range of perovskites useful in water splitting as well as other areas of heterogeneous photocatalysis.

Photocatalytic water splitting for hydrogen generation on cubic, orthorhombic, and tetragonal KNbO3 microcubes

Potassium niobate (KNbO3) microcubes with orthorhombic and tetragonal phases were hydrothermally prepared and characterized by powder X-ray diffraction, nitrogen adsorption-desorption, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The photoreactivity of the as-prepared KNbO3 samples was evaluated regarding the hydrogen evolution from aqueous methanol under UV, and the results were compared with that of cubic KNbO3 microcubes. The photocatalytic reactivity was shown to be phase-dependent, following the order cubic > orthorhombic > tetragonal. Insight into the phase-dependent photocatalytic properties was gained by first-principles density functional calculations. The best photocatalytic performance of cubic KNbO3 is ascribed to it having the highest symmetry in the bulk structure and associated unique electronic structure. Further, the surface electronic structure plays a key role leading to the discrepancy in photoreactivity between orthorhombic and tetragonal KNbO3. The results from this study are potentially applicable to a range of perovskite-type mixed metal oxides useful in water splitting as well as other areas of heterogeneous photocatalysis.

Structural variation of hydrothermally synthesized KNbO3 nanowires

KNbO3 (KN) nanowires were synthesized using various process conditions and their structures and morphologies were investigated. Homogeneous KN nanowires were formed in specimens synthesized at 130 °C for 24.0–48.0 h. These KN nanowires have a tetragonal structure that is known to be stable at high temperatures in the range of 225–435 °C. Tetragonal KN nanowires changed to orthorhombic KN nanoplates when the process time increased and homogeneous orthorhombic KN nanoplates existed for specimens synthesized for 144.0 h. In addition, tetragonal and orthorhombic structures coexisted in KN nanoplates synthesized at 130 °C for 72.0 h. For specimens synthesized at 100 °C, a long process time of 144.0 h was required to develop homogeneous KN nanowires that were also considered to have both tetragonal and orthorhombic structures. On the other hand, for specimens synthesized at 150 °C for 8.0 h, KN nanowires and a cube-shaped KN phase coexisted. Furthermore, a K4Nb6O17 second phase was formed in specimens synthesized for short periods of time (<8.0 h), indicating that the formation of homogeneous KN nanowires is difficult at 150 °C. Therefore, homogeneous KN nanowires with a tetragonal structure can be obtained at a low temperature of 130 °C with a short process time in the range of 24.0–48.0 h.

Growth of KNbO<sub>3</sub> Crystal by a Pulling down Method

In the present work, a pulling down method was introduced to prepare the KNbO3 crystals and the growth results were evaluated. It was found the Pt crucible with 60° cone angle and folds at the seed well was helpful to obtain KNbO3 crystal with nice integrity. The blue color occurred on the top of KNbO3 ingot was confirmed to be orthorhombic KNbO3 by XRD analysis. Some complex domain structures, including 60°, 90° and 180° domains, were happened in the crystal which were attributed to phase transitions when it was cooled down to room temperature.

An Efficient Approach for the Direct Synthesis of KNbO3 Powders

Pure potassium niobate (KNbO3) powders were prepared at low temperatures by an efficient method using K2CO3 and Nb2O5 as raw materials and urea as fuel. The crystal structure and morphology of the powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that cubic phase KNbO3 powders can be obtained at 550°C by the low-temperature combustion method and with the temperature increasing to 700°C they change to orthorhombic phase KNbO3. The synthesis parameters are studied to reveal the optimized synthesis conditions for preparing pure KNbO3 powders.

Fabrication and Evaluation of Highly (110)-Oriented Potassium Niobate Thin Films Prepared by RF-Magnetron Sputtering

Epitaxial (110)-oriented KNbO3 thin films were deposited on a (100) MgO substrate by RF-magnetron sputtering. To clarify the sputtering conditions necessary for obtaining a highly oriented KNbO3 thin film, the relationships among the degree of orientation, the composition ratio of K/Nb, and the lattice plane spacing were evaluated for KNbO3 thin films deposited using several types of target. In the deposition method using dual targets of KNbO3 and K2CO3, a higher orientation and a larger K/Nb ratio were incompatible. In single-target sputtering with a mixture target of KNbO3:K2CO3, as the substrate temperature decreased, although the K/Nb ratio increased and the (220)-plane spacing approached that of orthorhombic bulk KNbO3, the orientation deteriorated. Using a single K-rich target, K5NbOx, containing no CO2, a highly (110)-oriented KNbO3 thin film with a K/Nb ratio of 0.40 and a (220)-plane spacing near that of orthorhombic bulk KNbO3 was obtained by increasing the RF power applied to the O2-radical source. Moreover, the displacement current in the oriented KNbO3 thin film with a K/Nb ratio of 0.40 was observed when an electric field was applied to the interdigital transducer formed on the thin film. This indicates the existence of spontaneous polarization in the oriented KNbO3 thin film.

Effect of Processing Parameters of KNbO3 Powder Prepared from Aqueous Solution of Layered Perovskite

Preparation of KNbO3 powder from aqueous solution of layered perovskite K2NbO3F has been investigated. The prepared powders showed a pseudo-cubic KNbO3 structure, and need heat treatment at 1000°C to convert in the crystal structure to orthorhombic ferroelectrics. In this paper, we report a modified processing for the preparation of a better organized orthorhombic KNbO3 powders by firing at lower temperature of 800°C by the introduction of Pechini method.

Synthesis of Potassium Niobiate (KNbO3) Thin Films by Low‐Temperature Hydrothermal Epitaxy.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of Potassium Niobiate (KNbO3) Thin Films by Low-Temperature Hydrothermal Epitaxy

Orthorhombic KNbO3 thin films were deposited on SrTiO3 (100) and LiTaO3 (001) substrates by low-temperature hydrothermal epitaxy at 180−210 °C using aqueous solutions containing Nb2O5 and KOH. The films were thoroughly characterized by X-ray diffraction, Rutherford backscattering, and scanning electron microscopy. The KNbO3 films were uniformly distributed on the entire substrate area, exhibited high stoichiometry, and did not show evidence of interdiffusion or chemical reaction at the film/substrate interface. Their microstructures were determined by the substrate used, the synthesis temperature, and the concentration of the reactants. The KNbO3 films on SrTiO3 (100) substrates with thickness of 45 nm to 1.28 μm were smooth and transparent, with microstructures strongly suggesting single crystal growth mechanism. They were pure orthorhombic phase with (011) orientation and probably single-domain. Conversely, the KNbO3 films with higher thickness (>3.5 μm) exhibited very rough microstructure with textured columnar grains. Such films were also orthorombic but with mixed (100)/(011) orientation. This work demonstrated for the first time deposition of heteroepitaxial KNbO3 films on single-crystal substrates using low-temperature hydrothermal epitaxy in a well-defined range of synthesis conditions. The low synthesis temperatures, which are below the 225 °C phase transition temperature of KNbO3, resulted in formation of films that appear to exhibit improved features as compared to the films prepared by high-temperature processing.

Growth of KNbO3 Films by Electron-Cyclotron-Resonance-Assisted Pulsed Laser Deposition

The (110) cut of KNbO3 piezoelectric crystal is suitable for generation of longitudinal waves, because of its high electromechanical coupling factor kt. Epitaxial (110) KNbO3 thin films have been grown on (100) MgO and (100) La-doped SrTiO3 substrates using an electron-cyclotron-resonance-assisted pulsed laser deposition (ECR-PLD) technique. In order to ascertain the effect of the ECR activation of oxygen, the X-ray diffraction pattern of a film grown by ECR-PLD is compared with that of a film grown in an oxygen atmosphere without ECR activation. As a consequence, the ECR-PLD technique is found to be notably effective for epitaxial growth of high-quality KNbO3 films. The in-plane orientation of the films is discussed and it is shown that the macroscopic in-plane structure of KNbO3 films should have a fourfold rotational symmetry. Some films grown on La-doped SrTiO3 have slightly longer (110) spacing than that of (110) orthorhombic KNbO3. This suggests that the films might be (001) tetragonal KNbO3.

Hydrothermal synthesis of KNbO3 and NaNbO3 powders

Orthorhombic KNbO3 and NaNbO3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7–15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO3 powders as water molecules and hydroxyl ions.

A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations

The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO3 crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO3 resembles much more electron defects in the partly covalent SiO2 crystal (the so-called E1′ center) rather than usual F centers in ionic crystals like MgO and alkali halides. This covalency is confirmed by the analysis of the electronic density distribution. The absorption energies were calculated by means of the INDO method using the ΔSCF scheme after a relaxation of atoms surrounding the F center. For the orthorhombic phase three absoprtion bands are predicted, the first one is close to that observed experimentally under electron irradiaton.

Hydrothermal Synthesis and High Pressure Phase Transition of Impurity-stabilized Cubic KNbO3.

Cubic and orthorhombic KNbO3 have been synthesized under hydrothermal conditions respectively by tuning impurities and alkali concentration in initial reaction compositions, and characterized by X-ray diffraction, Laser Raman, Infrared and X-ray photoelectronic spectroscopies. Phase transitions of resulting niobates induced by pressure at high temperatures were investigated in comparison with hydrothermal pressure effect. Mechanisms of stabilization of cubic KNbO3 were discussed.

A study of KNbO3 in the pressure range to 12 GPa using synchrotron radiation

Orthorhombic KNbO3 has been studied by x-ray diffraction as a function of pressure. The lattice cell parameters, volume and stability range of this phase have been determined as a function of the applied pressure. No structural transformation has been observed up to 12 GPa. The resulting P-V data are fitted to a Murnaghan equation state of first-order.

Study of KTN Thin Films of Variable Composition grown by Pulsed Laser Deposition

ABSTRACTPotassium niobate tantalate (KTN) thin films of a range of compositions were grown by pulsed laser deposition onto (100) MgO substrates. Three-segment targets were used including KTaO3, KNbO3, and KNO3. Resulting films were well oriented with the substrates, with the “cubic” (h00) peaks positions shifting with Nb content to that of the (hh0) orthorhombic KNbO3 peak.

Structure‐Optical Property Correlation of Epitaxial Potassium Niobate Thin Films Deposited on Magnesium Oxide (100) Substrates Using a Strontium Titanate Transition Layer

Epitaxial thin films of orthorhombic KNbO3 (100)/(010) were deposited on MgO (100) single‐crystal substrates by pulsed laser ablation using an ε120 Å SrTiO3 (110) transition layer in between. From X‐ray diffraction, the orientation relationships were determined as (010) [100] or (100) [010] KNb03∥ (110) [110] SrTiO3∥ (100) [011] MgO and (010) [001] or (100) [001] KNbO3∥ (110) [001] SrTiO3∥ (100) [011] MgO. The measured film refractive indices at 632.8 nm were 2.213 ± 0.003, 2.278 ± 0.003, and 2.285 ± 0.003, respectively, along MgO [100], [011], and [011] directions. A model was developed to correlate the measured effective refractive indices of the film to area fractions of domain variants in the film. Using the model, the area fraction of domains with their polarization axis normal to the substrate was estimated to be 60.0 ± 2.7%.

Second harmonic generation in potassium niobate thin films

Green light produced by second harmonic generation has been observed in an epitaxial orthorhombic KNbO3 thin film planar waveguide produced by ion-beam sputter deposition on a (100)-oriented MgO single crystal substrate. A Nd:YLF laser beam, with a wavelength of 1.053 μm and ∼80 ps, 100 MHz pulses under mode-locked operation, was coupled into the waveguide using a rutile prism, and a green light streak 3–4 mm long was seen in the guide. The TM0 mode of the input beam was phase matched to the TE1 mode of the second harmonic for a film thickness of 2300 Å. Second harmonic generation was also observed in a nonwaveguided configuration on thicker (4600–6500 Å) films on both MgO and KTaO3 substrates.