KNbO3 Crystals Research Articles

Growth of KNbO3 Single Crystals by the Flux Method Using KBO2 as a Flux

KNbO3 single crystals are grown by the self-flux method using K2CO3 as a flux, but often suffer from discolouration. In this work, KNbO3 single crystals were grown by the flux method using KBO2 as a flux. KNbO3 powder was prepared by the solid-state reaction of K2CO3 and Nb2O5. KBO2 was fabricated by the reaction of K2B4O7·4H2O and K2CO3. Single crystals of KNbO3 were grown in a Pt crucible and the structure and dielectric properties of the single crystals were investigated. X-ray diffraction showed the KNbO3 single crystals to have an orthorhombic Cmm2 perovskite unit cell at room temperature. The existence of ferroelastic domains was revealed by transmission electron microscopy. Electron probe microanalysis showed the single crystals to be stoichiometric and contain small amounts of B. Differential thermal analysis, Raman scattering and impedance spectroscopy were used to study the phase transitions. KBO2 may be a suitable flux for the growth of KNbO3 single crystals.

Semi-Experimental Determination of the Linear Clamped Electro-Optical Coefficients of Polar Crystals from Vibrational Spectroscopic Data

The present work highlights a new general method devoted to computations of the clamped linear electro-optical coefficients from the measured fundamental vibrational frequencies and the nonlinear dielectric susceptibility constants. The calculations are based on the formula analog to that of the Lyddane–Sachs–Teller relation, which is systematically used for the calculations of the clamped linear electro-optical coefficient of oxide ferroelectric crystals such as LiNbO3, LiTaO3, BaTiO3, PbTiO3, and KNbO3. The computed electro-optical coefficients are in good agreement with those obtained from direct measurements and the first-principles calculations or other semi-empirical models. In addition, the famous r51 or r42 coefficients of the tetragonal BaTiO3, PbTiO3, and KNbO3 crystals are finally calculated with high accuracy and discussed in connection with the soft mode behavior.

A critical field study of ferroelectric domain in Al-doped KNbO3 single crystal

-Ferroelectric domain engineering is important for studying the dipolar structure of these materials. This dipolar structure is formed because of defects and imperfections produced using various doping. The KNbO3 ferroelectric single crystal was prepared using flux methods. Al2O3 aluminum trioxide (Al) was used as a dopant in a KNbO3 crystal. The domain structure of the crystal was studied by applying electric fields of 50 V/cm, 60 V/cm, 70 V/cm, 80V/cm, and 100 V/cm in an Al-doped KNbO3 single crystal. Furthermore, the critical field on which domains were nucleated after applying the electric field, was also studied. This work confirmed that the critical field of nucleation of the new domain in an Al-doped KNbO3 single crystal was 70 V/cm, which is slightly less than that of pure KNbO3 single crystal.

Effect of Fe3+ doping on the ferrielectric and magnetic properties of KNbO3 ceramics

KNb1-xFexO3(x = 0–0.2) ceramics are prepared by the self-sacrificing method. The effects of Fe ion doping on the ferroelectricity, and magnetic properties of KNbO3 are studied. XRD analysis shows that all samples belong to the single-phase orthorhombic system with the space group of Amm2. The XPS test proves that Fe ions exist in the form of +2 and + 3 valence in KNbO3 sample, and the ratio of Fe2+/Fe3+ in the sample is calculated. SEM shows that Fe ions doping has a significant regulating effect on the microscopic morphology and grain size of KNbO3 sample. Fe ions partially replace Nb5+ in the B site of KNbO3 crystals, three kinds of defect dipoles of [FeNb``−VO··]*, [VO··−FeNb```−VO··]·and polarity[FeNb```−VO··]·are formed. The polarization of the defect dipole is one of the main factors that enhance the ferroelectricity of KNbO3 crystal. The interaction between Fe3+/Fe2+ and adjacent VO··to form Fe2+/3+-VO·· bound magnetic polarons is considered to be the main source of paramagnetism for KNb1-xFexO3(x = 0–0.2) samples. when x ≤ 0.1, Fe ions mainly exist in KNO sample in the form of Fe2+/3+-VO··bound magnetic polaron, x > 0.1, it mainly exists in the ceramic in the form of Fe–O–Fe super-exchange. M value of KNb1-xFexO3 (x = 0.05–0.2) samples has a linear relationship with the increase of temperatures (T) when T ≥ 100K, indicating that samples exhibits paramagnetism. The magnetic properties at different doping concentrations and different temperatures may be isolated Fe3+, Fe2+, Fe2+/3+-VO··, Fe3+-O2--Fe3+ and Fe2+-O2--Fe3+ is the result of the combined effect.

Optical Properties of the Oxygen Vacancy in KNbO3 Crystal

The defect formation energy of an oxygen vacancy with different charge states (0, +1, +2) has been studied. The finite-size correction scheme (FNV) for the alignment of the defect formation energy is available. The corresponding correction energies are in the range of 0.32–1.29 eV. On the basis of the corrected defect formation energy, fairly accurate optical spectrum line shapes of F and F+ centers are obtained, taking account of electron–phonon coupling. The calculated absorption and luminescence peaks are located at 2.73 eV and 2.62 eV for the F center, and 2.44 eV and 2.25 eV for the F+ center, respectively, which agree well with experimental results. Finally, the result proves that the F+ center is the key point involved in the body color change of the KNbO3 crystal.

Effect of methyl alcohol and nitric acid as an etchant on the motions of ferroelectric domains in Al-doped KNbO3 single crystal

ABSTRACT The domain structure of the single crystal KNbO3 (KN) in the orthorhombic ferroelectrics phase at room temperature is investigated using optical and trinocular microscope. The cleavage face of ferroelectric Al-doped KN crystals was chemically etched using CH3OH (Methanol). At room temperature and before etching, the 60°, 90° domains were observed on the crystal surface. A dislocation model is proposed to explain the nucleation and evaporation of the domain wall under the influence of the electric field, which can be used to study the changes in motion of ferroelectric domains. After etching with HNO3 (Nitric acid) and applying an electric field, the 180° domains were observed.

MXene-Derived Ferroelectric Crystals.

This study demonstrates the first synthesis of MXene-derived ferroelectric crystals. Specifically, high-aspect-ratio potassium niobate (KNbO3 ) ferroelectric crystals is successfully synthesized using 2D Nb2 C, MXene, and potassium hydroxide (KOH) as the niobium and potassium source, respectively. Material analysis confirms that a KNbO3 orthorhombic phase with Amm2 symmetry is obtained. Additionally, ferroelectricity in KNbO3 is confirmed using standard ferroelectric, dielectric, and piezoresponse force microscopy measurements. The KNbO3 crystals exhibit a saturated polarization of ≈21 µC cm-2 , a remnant polarization of ≈17 µC cm-2 , and a coercive field of ≈50 kV cm-1 . This discovery illustrates that the 2D nature of MXenes can be exploited to grow ferroelectric crystals.

The influence of external stress/strain on the phase transitions and dielectric properties of potassium niobate crystal

We investigated the effects of one-dimensional shock wave compression on the structural, dielectric, and ferroelectric properties of a potassium niobate ferroelectric crystal using Landau–Ginsburg–Devonshire phenomenological theory. Our results show that the cubic to tetragonal and tetragonal to orthorhombic phase transition temperatures increase with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be 21.9 °C/10−3. Additionally, the orthorhombic to rhombohedral phase transition temperature decreased with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be −144.3 °C/10−3. This result represents a 15.3% increase in the remnant polarization of the tetragonal KNbO3 crystal when the stain induced by shock compression increased from 0 to 4 × 10−3 at 400 °C. The dielectric susceptibilities ε33 and ε22 were calculated to decrease by approximately 20% and 40%, respectively, when the stain induced by shock compression increased from 0 to 1.6 × 10−3 at 350 °C. The increase in the remnant polarization and the decrease in the dielectric properties induced by shock compression provide an effective way to improve the power output capacity of a ferroelectric-pulsed power supply.

Changes in cell parameters and improvement in photocatalytic activity of KNbO3 and NaNbO3 crystals via polarization

A representative ferroelectric tetragonal potassium niobate (TKN) and antiferroelectric orthorhombic sodium niobate (ONN) crystalline powder samples were polarized by using an external electric field. The changes in lattice distortion and crystal symmetry of these samples polarized were revealed. The photocatalytic activities of the samples before and after the polarization were evaluated by photocatalytic decomposition of water and photocatalytic reduction of Ag+. The results imply that the polarization of such kind of perovskite-type niobate samples using the external electric field is one of effective ways to improve the photocatalytic performance, whether ferroelectric TKN or antiferroelectric ONN. The exploration of mechanism suggests that the polarization can change the ferroelectricity of niobate powder crystals. Furthermore, the photocatalytic activity on different exposed facets of these niobate crystals was also evaluated by detecting Ag content on the surfaces of sample via photocatalytic reduction of Ag+. It is found that {001} facet is the highest active facet of TKN, and {101} facet of ONN crystals has the highest photocatalytic activity among the exposed facets. The photocatalytic activity on each exposed facet can be increased to a certain extent by polarization, and the influence of polarization on photocatalytic activity is greater than that of surface energy.

Isofrequency Opalescence in Ferroelectrics

Isofrequency dependences of the spectral intensities of inelastic scattering in LiTaO3, Pb5Ge3O11, BaTiO3, and KNbO3 crystals are registered at a fixed frequency of the spectrometer close to that of the exciting radiation. Results from soft mode analysis are presented.

First-principles investigation on the mechanism of photocatalytic properties for cubic and orthorhombic KNbO3

The geometric structures, band structures, density of states and optical absorption spectra are studied for cubic and orthorhombic KNbO3 (C- and O-KNO) crystals by using first-principles calculations. Based on the above calculation results, the mechanisms of photocatalytic properties for both crystals are further theoretically investigated to deepen the understandings of their photocatalytic activity from the electronic level. Calculations for the effective masses of electron and hole are carried out to make comparison in photocatalytic performance between cubic and orthorhombic phases. Optical absorption in cubic phase is found to be stronger than that in orthorhombic phase. C-KNO has smaller electron effective mass, higher mobility of photogenerated electrons, lower electron-hole recombination rate and better light absorption capacity than O-KNO. So, the photocatalytic activity of cubic phase can be higher than orthorhombic one. The present work may be beneficial to explore the series of perovskite photocatalysts.

Theoretical analysis of stimulated polariton scattering from the A1-symmetry modes of KNbO3 crystal

Stimulated polariton scattering (SPS) based on noncollinear phase matching scheme from the A1-symmetry modes of KNbO3 crystal is investigated for generating terahertz (THz) wave. Frequency tuning characteristics of THz wave by varying the phase matching angle and pump wavelength are analyzed. The expression of the effective parametric gain length under the noncollinear phase matching condition is deduced. Parametric gain and absorption characteristics of THz wave in KNbO3 are theoretically simulated. The characteristics of KNbO3 for parametric oscillator (TPO) are compared with those of MgO:LiNbO3. The analysis results indicate that KNbO3 is an excellent optical crystal for TPO to enhance the output of THz wave.

Features of the structural states of KNbO3 single crystals before and after fast-neutron irradiation

Neutron irradiation is a unique tool for forming new structural states of ferroelectrics, which cannot be obtained by conventional methods. The inf luence of the irradiation by two doses of fast neutrons (F = 1 × 1017 and 3 × 1017 cm–2) on the structure and properties of KNbO3 single crystals has been considered for the first time. The developed method for taking into account the experimental correction to the diffuse scattering has been used to analyze the structural changes occurring in KNbO3 samples at T = 295 K and their correlations with the behavior of dielectric and nonlinear optical characteristics. The irradiation to the aforementioned doses retains the KNbO3 polar structure, shifting Т С to lower temperatures and significantly affecting only the thermal parameters and microstructure of single crystals. Neutron irradiation with small atomic displacements provides a structure similar to the high-temperature modification of an unirradiated KNbO3 crystal.

First‐principles study of structural, electronic, elastic, and optical properties of cubic KNbO3 and KTaO3 crystals

Cubic KNbO3 and KTaO3 crystals are studied using the first‐principles VASP code. By employing local density approximation (LDA), GGA‐PBE, and HSE06, the lattice parameters are optimized and compared with available experimental data, and the best agreement is achieved with HSE06. Electronic structures such as density of states, band structures, and charge‐density distribution are discussed in detail for both crystals. The single‐crystal elastic constants, polycrystalline elastic modulus, compressibility, Poisson's ratio, and anisotropy factors are obtained from the Voigt–Reuss–Hill approximation. The elastic anisotropy is modeled and visualized in the light of the elastic properties of both systems. The bandgaps of the two crystals are calculated with LDA, PBE, HSE06, and GW approximations (GW0), and the results of HSE06 and GW0 agree well with experimental data. Then the more reliable optical properties of KNbO3 and KTaO3 are acquired based on the bandgaps with HSE06, in which the previous scissors operator correction is avoided. Both crystals are brittle, and KTaO3 exhibits higher hardness and stiffness than KNbO3. Some novel results, such as Debye temperatures, sound velocities, and the extreme values of Young's modulus are obtained.

Preparation of Ferroelectric KNbO3 Based Borate Glass System.

The incorporation method was employed to produce ferroelectric glass ceramics from the K2O-Nb2O5-B2O3 glass system. The nanocrystalline potassium niobate (KNbO3) was first prepared using a simple mixed oxide method, where the B2O3 was initially mixed and then melted to form glass. The successfully produced optically transparent glass was then subjected to a heat treatment schedule for further crystallization at temperatures ranging from 500 to 650 degrees C, which resulted in the precipitation of the KNbO3 phase, together with the K3B2Nb3O12 phase. Scanning electron microscopy (SEM) showed the presence of randomly oriented KNbO3 crystals dispersed in a continuous glass matrix. It was found that the glass ceramics subjected to the heat treatment at temperatures higher than 545 degrees C were opaque, while the lower gave a highly transparent glass ceramics. The crystal size and crystallinity were found to increase with increasing heat treatment temperature, which in turn plays an important role in controlling the properties of the glass ceramics, including physical, optical, and dielectric properties.

Role of structure gradient region on dielectric properties in Ba(Zr,Ti)O3–KNbO3 nanocomposite ceramics

Crystal structures of KNbO3 (KN)/BaZrxTi1−xO3 [BZT, (0.1 ≤ x ≤ 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.

Propagation characteristics of SH wave in an mm2 piezoelectric layer on an elastic substrate

We investigate the propagation characteristics of shear horizontal (SH) waves in a structure consisting of an elastic substrate and an mm2 piezoelectric layer with different cut orientations. The dispersion equations are derived for electrically open and shorted conditions on the free surface of the piezoelectric layer. The phase velocity and electromechanical coupling coefficient are calculated for a layered structure with a KNbO3 layer perfectly bonded to a diamond substrate. The dispersion curves for the electrically shorted boundary condition indicate that for a given cut orientation, the phase velocity of the first mode approaches the B-G wave velocity of the KNbO3 layer, while the phase velocities of the higher modes tend towards the limit velocity of the KNbO3 layer. For the electrically open boundary condition, the asymptotic phase velocities of all modes are the limit velocity of the KNbO3 layer. In addition, it is found that the electromechanical coupling coefficient strongly depends on the cut orientation of the KNbO3 crystal. The obtained results are useful in device applications.

Effect of epitaxy on interband transitions in ferroelectric KNbO3

Very large lattice strain and strain-induced polarization are achieved in KNbO3 using epitaxial growth of a thin KNbO3 film onto a (001)-oriented SrTiO3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.

Potential Advantage of Multiple Alkali Metal Doped KNbO3 Single Crystals

Potassium niobate crystal KNbO3 (KN) is a well-known crystal for lead free piezoelectric or nonlinear optical applications. The KN crystal has been studied in both single crystal form and in thin film form which has resulted in many review articles being published. In order to exceed the KN crystal, it is important to study KN phase forming and doping effects on the K site. This article summarizes the authors’ study towards a multiple alkali metal doped KN crystal and related single crystals briefly from the viewpoint of crystal growth.

Calculation of the dispersion of the electro-optical and second harmonic coefficients from the refractive index dispersion

The relationships of the wavelength dependence of both the electro-optic and second harmonic generation coefficients are established within the same model applied to nonlinear optical materials. It is demonstrated that the dispersion of these coefficients can be obtained from the own dependence of the refractive indices only, without any fitted parameter. Solely the measurement of the coefficient at one wavelength is required within this approach. A very good agreement in the λ-dependence of electrooptic and second harmonic generation coefficients is achieved between calculated values and experimental data in SBN, DAST, KTP and KNbO3 crystals.