Nanoscale potassium niobate crystal structure and phase transition
© Chen et al; licensee Springer. 2011
Received: 1 June 2011
Accepted: 23 September 2011
Published: 23 September 2011
Nanoscale potassium niobate (KNbO3) powders of orthorhombic structure were synthesized using the sol-gel method. The heat-treatment temperature of the gels had a pronounced effect on KNbO3 particle size and morphology. Field emission scanning electron microscopy and transmission electron microscopy were used to determine particle size and morphology. The average KNbO3 grain size was estimated to be less than 100 nm, and transmission electron microscopy images indicated that KNbO3 particles had a brick-like morphology. Synchrotron X-ray diffraction was used to identify the room-temperature structures using Rietveld refinement. The ferroelectric orthorhombic phase was retained even for particles smaller than 50 nm. The orthorhombic to tetragonal and tetragonal to cubic phase transitions of nanocrystalline KNbO3 were investigated using temperature-dependent powder X-ray diffraction. Differential scanning calorimetry was used to examine the temperature dependence of KNbO3 phase transition. The Curie temperature and phase transition were independent of particle size, and Rietveld analyses showed increasing distortions with decreasing particle size.
Keywordspotassium niobate crystal structure phase transition nanoscale powder.
Lead oxide-based perovskites are a commonly used piezoelectric material and are now widely used in transducers and other electromechanical devices [1–4]. However, the high toxicity and high processing vapor pressure of lead oxide cause serious environmental problems. A promising way to address this issue is to develop lead-free piezoelectric ceramics to minimize lead pollution. Recently, as demand has increased, many studies have focused on the development of high-quality lead-free piezoelectric materials [5–7].
Potassium niobate (KNbO3) is a ferroelectric compound with a perovskite-type structure and is a promising piezoelectric material owing to superior coupling in its single crystal form [8, 9]. KNbO3 materials have attracted considerable attention for applications in lead-free piezoelectric materials. KNbO3 has an orthorhombic structure and is a well-known ferroelectric material with extensive applications in electromechanical, nonlinear optical, and other technological fields [10–13].
KNbO3 phase transition temperatures have already been determined. KNbO3 can exist in orthorhombic, tetragonal, and cubic phases above room temperature, and at ambient pressure, it exhibits two structural transitions with decreasing temperature: cubic to tetragonal at 691 K and tetragonal to orthorhombic at 498 K . The cubic phase is paraelectric while the remaining two are ferroelectric; however, phase transitions of nanoscale KNbO3 have not yet been reported in detail.
The phase transition temperatures of ferroelectric ceramics are size dependent, with the ferroelectric phase becoming unstable at room temperature when the particle diameter decreases below a critical size [15–17]. However, this critical size usually encompasses a broad size range. Experimental discrepancies may arise because of intrinsic differences between ferroelectric samples, and several theoretical models based on Landau theory overestimate the critical sizes . Therefore, the phase structure of nanoscale KNbO3 at room temperature requires further investigations.
The current work is a systematic study of the crystal structure and phase transitions of nanoscale KNbO3, synthesized using the sol-gel method. The aim was to investigate the size dependence of the ferroelectric phase and the phase transition temperatures of nanoscale KNbO3 powders.
Results and discussion
Particle size dependence on gel heat-treatment temperature
Heat-treatment temperature (°C)
Particle size (nm)
40 ± 10
70 ± 15
80 ± 15
Rietveld refinement results of synchrotron XRD data collected at λ = 1.2348 Å
Heat-treatment temperature (°C)
Unit cell dimensions
Figure 4 shows that there was no obvious difference in transition temperature between the three samples. Temperature-dependent XRD showed that the actual transition temperature was nearly unchanged, and that the Curie temperature (T C) and phase transition were independent of particle size.
Transition temperature observed from DSC
Heat-treatment temperature (°C)
Transition temperature (°C)
Nanoscale KNbO3 powders were synthesized using the sol-gel method. The average KNbO3 grain size was estimated to be within 100 nm from FESEM and TEM images, and TEM images showed that nanoscale KNbO3 particles had a brick-like morphology.
Synchrotron XRD and Rietveld refinement showed that the ferroelectric orthorhombic phase was retained at room temperature, even for particles smaller than 50 nm. Temperature-dependent XRD confirmed that the actual transition temperature was nearly unchanged and that the T C and phase transition were independent of particle size. Rietveld analysis showed increasing distortions with decreasing particle size.
Precursor solutions were prepared using the sol-gel method reported in the literature . K-ethoxide, Nb-pentaethoxide, 2-methoxyethanol, K-ethoxide, and Nb-pentaethoxide were dissolved in 2-methoxyethanol and refluxed at 120°C for 90 min in dry N2. The concentrations of all precursor solutions were 0.32 mol/L. Weighed gel samples in Pt cells were calcined at 600°C to 800°C for 3 min in air to obtain crystalline powders, with a heating rate of 10°C/min.
Powder sizes and morphologies were examined using FESEM (JEOL JSM-7500F; JEOL Ltd., Tokyo, Japan) and TEM (JEOL JEM-2010; JEOL Ltd.). Crystal structures were determined using high-resolution synchrotron radiation diffractometry at the BL14B1 beam line of Shanghai Synchrotron Radiation Facility, using 1.2398 Å X-rays with a Huber 5021 6-axes diffractometer (energy = 3.5 GeV). Structural refinements were performed using the Rietveld analysis program X'Pert Highscore Plus (PANalytical X-ray Company, Almelo, The Netherlands). Phase transitions were investigated using non-ambient XRD (PANalytical X'pert Pro, Cu Kα, 40 kV, 40 mA) with a Pt strip stage from ambient temperature to 600°C. The differential scanning calorimetry (NETZSCH STA 449F3, Selb, Germany) was used to follow the phase transitions. Nitrogen was used in the DSC measurement at a flow rate of 50 ml/min with a heating rate of 5°C/min. The measurement was carried out in the temperature range of 50°C to 500°C.
differential scanning calorimetry
field emission scanning electron microscopy
- KNbO3 :
transmission electron microscopy
This work was supported by the Innovation Program of the Shanghai Municipal Education Commission in China (grant no. 11YZ128).
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