KNbO3 Single Crystals Research Articles

Electromechanical properties of different phases in ferroelectric crystals regulated by variously oriented electric fields

Electric fields offer a convenient and tunable way to induce phase transitions for regulating the electromechanical properties of ferroelectrics. However, regulating the electromechanical properties by using electric fields in various directions for different ferroelectric phases has yet to be systematically investigated, especially for lead-free material KNbO3. Based on the nonlinear thermodynamics analysis, the electric field-temperature phase diagrams of KNbO3 single crystals under different electric field directions (E[001], E[011], E[111]) have been constructed, along with the electric-field-induced electromechanical responses. The results show that the phase diagrams are markedly different under different electric field directions. Specifically, the electric field-temperature phase diagram appears as a "line"-shaped phase boundary under E[001], while it appears as a "U"-shaped phase boundary under E[011], and an arrowhead-shaped phase boundary under E[111]. It is also found that there are excellent electromechanical responses near both "U"-shaped and arrowhead-shaped phase boundaries due to the significant alterations in polarization slopes near the phase boundaries, offering an alternative pathway to regulate and enhance the electromechanical properties in ferroelectrics.

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.

Large plastic deformability of bulk ferroelectric KNbO3 single crystals

Ferroelectrics have become indispensable electroceramic materials in modern electronic applications. While point-defect engineering is a common tool to modify the properties of ferroelectrics, dislocations have not been widely considered, although they were previously shown to alter electrical properties in other oxides. Here, we study the room temperature uniaxial deformation of the perovskite ferroelectric single crystalline KNbO3 in 010pc and 101pc orientations. We demonstrate a large plastic deformation in the range of 5%. Dislocation and domain structure were investigated across different length scales using optical microscopy, electron channeling contrast imaging, and transmission electron microscopy. Planar arrangement of dislocations in slip bands was observed, while the predominant appearance of edge dislocations indicates a dominant dislocation motion by the kink-pair mechanism. Increased dislocation density resulted in decreased ferroelectric domain size from 1.8 μm to 0.5 μm after deformation. Piezoresponse force microscopy highlights the effect of dislocations on domain nucleation and their mobility.

Domain imaging in Fe-doped KNbO3 single crystal via trinocular microscopy and scanning electron microscopy

Abstract Trinocular microscopy with a high spatial resolution is a promising technique for studying the surface morphology of materials. The cleavage face of ferroelectric Fe-doped K niobate (KNbO3) crystals was chemically etched by using HNO3. Trinocular microscopy was used to study the surface morphology of the grown ferroelectric KNbO3 single crystals. The grown surface of the crystal with a 60° domain wall revealed the same interaction and orientations. The results indicate that the type of domain walls that occur in ferroelectric crystals depend on the symmetry of both the nonferroelectric and ferroelectric phases of the crystal.

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.

A study of as-grown, poled and reduced Rh-doped KNbO3 single crystals by high-resolution X-ray diffraction, Raman scattering, photoluminescence and dielectric measurements

As-grown and chemically reduced Rh-doped (1500 p.p.m.) KnbO3 single crystals grown by the Czochralski method have been characterized. Reduction of the grown crystals at different levels was carried out under a mixture of CO and CO2 gases as the crystals were grown with excess oxygen. The effect of reduction and poling on crystalline perfection was studied by high-resolution X-ray diffraction (HRXRD) using a multi-crystal X-ray diffractometer developed in-house. The diffraction curves of the as-grown, electrically poled, moderately reduced and heavily reduced single-crystal specimens show remarkable differences. The studies by HRXRD reveal that poling has some influence on the improvement of crystalline perfection, while chemical reduction has a great influence on crystalline perfection; at moderate reduction the crystal becomes nearly perfect, but when the reduction is very heavy the crystal quality decreases slightly, although it is still better than for unreduced samples. Asymmetry of the diffraction curves with respect to the peak position reveals that the as-grown specimens contain a high concentration of both vacancies and self-interstitials. After poling, the concentration of self-interstitial defects is lowered to some extent. When the specimen is moderately chemically reduced, the scattered intensity on both sides of the peak is greatly reduced, showing that the concentrations of both vacancies and interstitials are reduced to a great extent owing to chemical reduction. This clearly indicates that, as a result of the chemical reduction of oxygen in the crystal, crystalline perfection is enhanced significantly. However, under heavy chemical reduction, the number of vacancy defects is increased to a significant extent. Raman scattering, dielectric and photoluminescence studies also show interesting features, with excellent correlation with the degree of crystalline perfection influenced by the processes of reduction and poling.

Crystal growth and electric‐property change by rubidium or cesium doping on potassium‐sodium‐niobate

AbstractAlkali metals (Na, Rb or Cs) co‐doped with fiber‐ and bulk‐shaped KNbO3 single crystals were grown using two original methods by means of doping together of small ionic Na and large ionic Rb or Cs into KNbO3. Single‐phase crystals could be grown with an orthorhombic system at room temperature as well as pure KNbO3. Piezoelectric and ferroelectric property changes by the co‐doping of Rb or Cs with Na were estimated using d33 values and a polarization‐electric field hysteresis curve in fiber‐ and bulk‐shaped crystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal conductivity of perovskite ferroelectrics

Thermal conductivities of BaTiO3, PbTiO3, KNbO3, KTaO3, NaNbO3, and Pb(Mg1∕3Nb2∕3)O3 (PMN) single crystals have been measured from 2to390K. Pronounced jumps are found at structural transitions in BaTiO3 and KNbO3. A low-temperature anomaly from soft optical phonons is observed in KTaO3. For PMN and NaNbO3, glasslike behavior is observed in both the thermal conductivity and heat capacity measurements. We associate the glasslike behavior in NaNbO3 with the phase separation phenomena reported in recent studies.

Large and stable thickness coupling coefficients of [001]C-oriented KNbO3 and Li-modified (K,Na)NbO3 single crystals

Large thickness coupling factors kT⩽0.7 are measured in both [001]C-oriented (45° cut in the orthorhombic ac plane), monodomain and [001]C-poled, domain-engineered single crystals of KNbO3. kT is extremely stable over a large temperature range, remaining high (⩾0.65) even upon heating and cooling through the orthorhombic-tetragonal phase transition. This is likely related to another finding that spontaneously high kT values (⩽0.68) are observed in as-grown (unpoled), [001]C-cut, polydomain KNbO3 crystals. kT is also stable upon thinning to thicknesses below 60μm, as required for high-frequency applications. Finally, large kT values (⩽0.7) are reported in [001]C-poled, polydomain, Li-doped (K0.5Na0.5)NbO3.

Synthesis and Dielectric Characterization of Potassium Niobate Tantalate Ceramics

The preparation and dielectric properties of potassium niobate tantalate (KTN) have been investigated with the aim of exploring the material's potential for ferroelectric tunable applications. The samples were prepared both by conventional sintering in air and by uniaxial hot pressing. A relative average density greater than 92% was obtained with both methods. An inhomogeneous Nb/Ta distribution was found in the samples prepared by both methods, but the inhomogeneity extent was lower in the hot‐pressed samples. While both sintering processes resulted in ceramics of lower transition temperatures in comparison with the reported results on single crystals, a relative up shift of the temperature (Tmax) at which the dielectric constant is maximum was found for hot‐pressed samples compared with that of samples sintered in air. All the samples exhibited strong frequency dispersion in dielectric properties. The effect of DC bias at room temperature was measured and modeled using the Landau–Devonshire model. It was found that the nonlinear coefficient β of KTN, which is important for tunable applications, is comparable with the value reported on KNbO3 and SrTiO3 single crystals. A dielectric tunability of 16% and 42% at room temperature was demonstrated under 20 kV/cm for the sintered and hot‐pressed ceramics, respectively. The loss tangent, low at megahertz frequencies, was augmented to 9%–17% at low gigahertz frequencies. This is believed to be related to the frequency dispersion observed in the ceramics because of charged defects.

Control of specific domain structure in KNbO3 single crystals by differential vector poling method

The controllability of specific domain structures in KNbO3 single crystals was investigated by electric poling to several different orientations at room temperature. By applying electric field to the direction corresponding to the differetial direction between the original and intended spontaneous polarization directions (differential vector poling method), 60°-, 90°-, and 180°-domain pairs were successfully fabricated under control in KNbO3. The measured threshold electric field for 90°-domain pairs was the lowest (140V∕mm), which could be related to the difference between the piezoelectric coefficients of the poling direction. For 60°- and 90°-domain pairs, theoretical uncharged domain walls fabricated under control without the generation of other domain structures. For 120°-domain pairs, (011)pc uncharged domain wall was not observed, which suggests the difficulty in generating uncharged 120°-domain walls without passing through the phase transition.

Compact Integrated Folded Šolc Filter using Periodic 90°-Domain Structures in KNbO3 Single Crystals

A compact integrated folded Šolc filter using periodic 90°-domain structures in KNbO3 single crystals is proposed. The periodic 90°-domain structures with a period of 200 µm are fabricated on 0.7-mm-thick (101)-cut KNbO3 single-crystal plates by applying an electric field of 200 V/mm. Using the refractive index change between 90°-domains, we can construct the same arrangement for a folded Šolc filter. The measured filtering properties agree well with the calculated ones. This is the first demonstration of an optical filter using periodic domain structures other than 180°-domain structures. It is now possible to fabricate a compact folded Šolc filter without inconvenient alignment.

Effect of Poling Field on Piezoelectric Properties of KNbO3 Crystal Grown by Vertical Bridgman Method

Single crystals of KNbO3 were grown by the vertical Bridgman method. The crystals were cut into 6×3×0.5 mm3 plates, and dielectric constants and resonance characteristics were observed as a function of the electric field strength used in the poling treatment. For the [001]pc-poled crystal, which stands for [001] direction in pseudocubic crystal lattice, the dielectric constant increased once with increasing amount of poling field and then decreased again with higher fields. For [101]pc-poled crystal, both a reduction in dielectric constant and a higher phase angle shift in the resonant region were observed when the electric field exceeded 85 V/mm. The series of observations can be consistently interpreted using the domain structures estimated from the anisotropic dielectric constant. The vibrators having their longitudinal directions along the apc and bpc axes showed quite different piezoelectric responses, revealing that the polarization vector lies only in the [101]pc and [101]pc directions.

Fabrication of KNbO3 Epitaxial Thin Films on Sapphire Substrates by Pulsed Laser Deposition

KNbO3 epitaxial thin films were fabricated on sapphire r-plane substrates with an MgO buffer layer by pulsed laser deposition. X-ray diffraction measurements revealed that both MgO and KNbO3 were epitaxially grown on the Al2O3 (1-102) substrate with an orientation relationship as KNbO3 (100)pc/MgO (100)/Al2O3 (1-102) (out-of-plane) and KNbO3 [010]pc//MgO [010]//Al2O3 [11-20] (in-plane), although both MgO and KNbO3 were tilted 6–9° with respect to the substrate surface. Raman scattering measurements revealed that the KNbO3 film was composed of the orthorhombic ferroelectric phase, and that the (010)ortho (Y) domain, in which the polarization axis was parallel to the substrate surface, was dominant. The present study enables the production of KNbO3 epitaxial thin-film wafers, which correspond to Y-cut KNbO3 single crystals with a large electromechanical coupling coefficient (k2=0.53), for surface acoustic wave propagation.

An EPMA Study on KNbO3 and NaNbO3 Single Crystals – Potential Reference Materials for Quantitative Microanalysis

Single crystals of KNbO3 and NaNbO3 were selected from the limited number of suitable alkali compounds that are available and evaluated as possible reference materials for the electron-probe microanalysis (EPMA) of alkaline niobates with a composition described by the general formula K1−XNaXNbO3. The EPMA study verified that KNbO3 and NaNbO3 single crystals are stable under the electron beam and compositionally homogeneous. A quantitative microanalysis confirmed the composition of pure KNbO3, while the NaNbO3 crystal contained 0.3 mass fraction % of Ca. A significant improvement in the accuracy of the quantitative EPMA of polycrystalline potassium–sodium niobates was achieved using these single crystals as standards. The crystals can also be useful as reference materials for the analysis of sodium and potassium in other materials.

Domain wall displacements and piezoelectric activity of KNbO3 singlecrystals

The role of domain-orientation effects in forming the piezoelectric coefficientd33of polydomain KNbO3 single crystals in themm2 phaseis studied. Non-180° domain wall displacements in the electric field E ∥[001] are analysed, and their contributions tod33are evaluated. A link between these contributions and crystallographiccharacteristics of domain structures is revealed for the first time for different 60°,90° and 120° domain structures.

Poling Treatment and Piezoelectric Properties of Potassium Niobate Ferroelectric Single Crystals

Piezoelectric properties of potassium niobate (KNbO3) single crystals were investigated as a function of crystallographic orientations, i.e., [110]c of polar direction and [001]c of nonpolar direction. Prior to the piezoelectric measurements, the optimum conditions for a conventional poling method were investigated, and it was found that the optimum conditions for a conventional poling method were as follows, i.e., (a) high temperature above 100°C, (b) long soaking time over 1 h, (c) slow heating and cooling rates below 1°C/min, and (d) lower poling current below 1 µA. However, fully poled KNbO3 crystals were not obtained using the conventional poling method. Thus, to achieve fully poled KNbO3 single crystals, a new 2-step poling method was proposed. This method is composed of 2 poling stages, i.e., the 1st step for non-180° domain switchings at higher temperatures under a low DC bias field, and the 2nd step for 180° domain switchings at lower temperatures under a high DC bias field. Using the 2-step poling method, KNbO3 crystals were successfully poled, and then, their piezoelectric properties of k31 modes were measured using a resonance method. The [110]c poled KNbO3 crystals exhibited electromechanical coupling factor k31 of 28.9% and piezoelectric constant d31 of 18.4 pC/N while the [001]c poled KNbO3 crystals exhibited k31 of 31.2% and d31 of 51.7 pC/N. The piezoelectric constant d31 along nonpolar [001]c direction was 2.8 times higher than that along polar [110]c direction.

Quantitative study on the nonlinear piezoelectric effect in KNbO3 single crystals for a highly efficient surface acoustic wave elastic convolver

In this article we describe quantitative studies on the nonlinear piezoelectric effect in KNbO3 single crystals for a highly efficient surface acoustic wave elastic convolver. First, we demonstrate that a simple measurement of the nonlinear piezoelectric constant of e355 is very effective for quantitative estimation of the efficiency of an elastic convolver. Next, the third-order piezoelectric constants, e355, e333, and e344, in rotated Y-cut KNbO3 substrates are measured by varying the velocity under an applied electric field. Using the third-order piezoelectric constants measured, quantitative estimations of the convolution efficiency are discussed.

Planar Optical Waveguides in KNbO3 Formed by MeV B and He Ion Implantation

Planar optical waveguides in single crystals of KNbO3 were fabricated by MeV B and He ion implantation. The depth profiles of radiation damage in these waveguides were compared and analyzed by high resolution transmission electron microscope. Non leaky waveguiding modes (TE0 and TM0), which are due to implantation induced increase of nb and nc, were observed in 6.0 MeV B ion implantated waveguides with a dose of 1×1015> cm-2.

Pressure-induced phase transition in KNbO3 at 22.5 kbar

The effect of pressure on the low-frequency dielectric constant (E) of single crystals of KNbO3 is investigated by means of capacitance measurements. Measurements were carried out at pressure up to 35 kbar using a large volume cell. The dielectric constant increases with pressure up to 22.5 kbar, where it exhibits a large value (ϵ > 5000), and then decreases. This change in its behaviour is related to a phase transition induced by pressure. The character of the observed transition is also discussed. In addition, evidence indicating that the phase transition is not reversible has been also observed.