Materials Research Bulletin 45 (2010) 1899–1902

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Preparation of nanocrystalline perovskite KNbO3 by peroxo-precursor
decomposition method
Isao Tsuyumoto *, Tomohiro Kato, Tatsuya Arai
Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, JapanA R T I C L E I N F O

A B S T R A C TArticle history:
Received 5 June 2010
Received in revised form 30 August 2010Accepted 1 September 2010
Available online 17 September 2010

Nanocrystalline perovskite KNbO3 is prepared by a peroxo-precursor decomposition method atmoderate temperatures of 650–900 8C. Peroxo-heteropoly-niobic acid is prepared by direct reaction
between NbC powder and H2O2 aq, and mixing the peroxo-heteropoly-niobic acid with KOH aq yields anamorphous precursor salt. Perovskite KNbO3 is obtained by heating the precursor in air for 1 h at 650–900 8C. The X-ray diffraction patterns were well ﬁtted with the space group Pm3m in the Rietveldanalysis. The X-ray diffraction peak proﬁles and ﬁeld emission scanning electron microscope images
indicate the crystallite size is in the range of 25–35 nm.
ß 2010 Elsevier Ltd. All rights reserved.

Keywords:
A. Oxides
A. Inorganic compounds
B. Sol–gel chemistry
B. Chemical synthesis
C. X-ray diffraction

1. Introduction
Potassium niobate (KNbO3) is an important ferroelectricmaterial for acousto-optic, electro-optic, nonlinear optical, andphotorefractive applications in frequency doublers, optical waveguides, and holographic storage devices [1]. It is also of interest for
replacing the piezoelectric lead zirconate titanate (PZT) materials
because of the toxicity of lead [2]. Potassium niobate isconventionally prepared by solid state reactions between potassium carbonate and niobium oxide [3]. However, the heterogeneous
mixing of the starting materials and the evaporation of potassiumoxide make it difﬁcult to obtain a homogeneous product in a single
phase. Many investigations have been reported on soft-chemical
processes such as the hydrothermal method [4–13], the sol–gel
method [14–18], and the precursor decomposition method [19,20]
to overcome the disadvantages in preparation of KNbO3. In thehydrothermal method, KNbO3 was synthesized by reacting Nb2O5
with KOH at elevated temperatures 150–250 8C, and the differencein KOH concentrations was reported to bring about large variationsin the amount of the product [10]. Kim and Matijevic prepared
KNbO3 by calcination of amorphous potassium niobate precipitated from the solution of potassium salt and polymeric niobium oxohydroxide ion, and discussed the chemical mechanism for theformation and transformation [20]. Self-assembly technique using* Corresponding author. Tel.: +81 76 294 6712; fax: +81 76 248 9417.
E-mail address: tsuyu@neptune.kanazawa-it.ac.jp (I. Tsuyumoto).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.09.009

layered perovskite K2NbO3F was also reported for preparation ofKNbO3 thin ﬁlm [21].
We have developed soft-chemical synthetic routes for preparing
metal oxides from peroxo-polymetallic acid obtained by direct
reaction of metallic or metallic carbide powder with hydrogen
peroxide solution. Our synthetic methods have advantages in
preparation of homogeneous and ﬁne powders, because nucleationand subsequent crystalline growth in amorphous solid matrix
proceed at lower temperatures. Generally, it is essential to mix raw
materials equally at microscopic scale to ensure the quality of theproducts. In our synthetic routes, raw materials are mixed equally at
nanometer scale, and they are appropriate for the preparation ofordinary particles as well as nano-sized particles. For example,
peroxo-polytungstic acid is obtained by vigorous reaction between
metallic tungsten powder and hydrogen peroxide solution, and an
amorphous precursor is precipitated by mixing an alkali solution
with the peroxo-polytungstic acid [22–25]. Heating the precursor at
500–700 8C yields alkali tungstates or tungsten bronzes. Similarly,
molybdenum, niobium, vanadium, titanium and their carbide
powders react with hydrogen peroxide, and yield the corresponding
peroxo-polymetallic acids [26–32]. Such peroxo-polyacids based ontungsten and niobium were investigated as negative inorganic resist
materials for microlithography and as lithium intercalation materials[23,28]. Preparation of nano-sized tetragonal BaTiO3 was achieved byheating the barium salt of peroxo-polytitanic acid at 650 8C [33]. In
the present paper, we describe the synthetic details of nanocrystalline KNbO3 powders through a new route and its characterizations
using the Rietveld reﬁnement of X-ray diffraction (XRD).

[()TD$FIG]

I. Tsuyumoto et al. / Materials Research Bulletin 45 (2010) 1899–1902

1900

2. Experimental
2.1. Synthesis
Niobium carbide powder (NbC, 1.9 g, 1–3 mm particle size, as
received from Rare Metallic Co., Ltd., Tokyo, Japan) was dissolved in
100 mL of 30 wt% H2O2 solution (Wako Pure Chemical Industries,Ltd., Osaka, Japan) at 40 8C. A severe reaction involving gas
generation occurred and yielded a yellow transparent aqueous
solution of peroxo-heteropoly-niobic acid (denoted by NbC-HPA).
The heteroatom is carbon, and the solute is a kind of peroxooxalate complex of niobium. The solution was ﬁltered using
0.45 mm ﬁlter to remove insoluble after reaction completion. The
insoluble mostly consisted of carbon derived from NbC. Reducedpressure distillation of the solution using rotary evaporator yielded
4.8 g of yellow amorphous solid. Because this yellow solid wasinsoluble in water, we used the as-prepared NbC-HPA solution forthe synthesis.
A white gelatinous precipitate was formed by mixing the NbCHPA solution with KOH aq, and it was washed and separated bycentrifugation. Mixing molar ratio (K/Nb) was changed in the rangeof 0.2–3.0. After drying the precipitate at 115 8C for 24 h, 1.0 g ofamorphous potassium salt of NbC-HPA was obtained. Potassiumniobium oxides were obtained by heating the salts as precursors inair at 650–900 8C.
2.2. Characterization
The ultraviolet–visible (UV–vis) absorption spectrum of theNbC-HPA solution was taken using a spectrophotometer (V570,JASCO, Tokyo, Japan). The absorbance at 370 nm was monitored
during the reaction between NbC and H2O2 to estimate the timerequired to complete the reaction. The thermal behavior of theprecursor salt was investigated by thermogravimetry and differential thermal analysis (TG/DTA, DTG-50, Shimadzu, Tokyo, Japan)
at a heating rate of 20 8C/min. The X-ray diffraction patterns weremeasured with a diffractometer (Ultima IV, Rigaku, Tokyo, Japan)
usinggraphite-monochromatizedCuKaradiation(l = 0.15406 nm) at 40 kV and 30 mA. The crystalline parametersof the obtained KNbO3 were reﬁned by the Rietveld method using
the RIETAN-2000 program [34]. The crystallite sizes of KNbO3 weredetermined byp¼

180K lpX(1)

where p is the crystallite size, K is the Scherrer constant (0.9), l is
the wavelength of X-ray, and X is a peak proﬁle parameter, theLorentzian Scherrer broadening obtained in the Rietveld reﬁnement. The K/Nb molar ratios of the specimens were determined by
X-ray ﬂuorescence spectrometry (XRF-1700, Shimadzu, Tokyo,Japan) using the fundamental parameter (FP) method. The
morphology of the potassium niobium oxides after heating wasobserved by a ﬁeld-emission scanning electron microscope (FESEM, S-4800, Hitachi, Tokyo, Japan) with an accelerating voltageof 2 kV.

Fig. 1. UV–vis absorption spectrum of NbC-HPA aqueous solution containing
0.20 mol/L of Nb. Inset shows the time-courses of the absorbance at 370 nm after
mixing NbC with H2O2 solution at the initial temperature of 0 8C and 20 8C.

mixing NbC with H2O2 solution indicated that the reaction wascompleted in 18 min at the initial temperatures of 20–40 8C and in
35 min at the initial temperature of 0 8C. The dependence on theinitial temperatures was relatively low because of the heat releasein the reaction.
The relationship between the K/Nb molar ratio in theprecipitated salt and the mixing molar ratio K/Nb of the solutions
is shown in Fig. 2. The K/Nb ratio in the salt increased withincreasing the mixing molar ratio up to 2.0 and stayed almost
constant at 1.0 above the mixing molar ratio of 2.0. It is interesting
to note that the excess concentration of KOH over 0.40 mol/L (theK/Nb ratio = 2) did not affect the content in the precipitate. In thehydrothermal methods the large excess KOH has been reported tobe essential for dissolving niobium oxide into the aqueous
solution, and the high K/Nb molar ratio of raw materials in therange of 50–800 is a critical requisite for the hydrothermalformation of KNbO3 [4,11]. These earlier studies support ourexperimental fact that the excess amount of KOH over the K/Nbratio of 2 did not affect the composition in the precipitate. In ourprecursor decomposition method, the large excess KOH was notessential because the niobium was dissolved by the reaction with
H2O2 and the role of KOH was simply to provide counter ion for theformation of salt.
The mixing molar ratio K/Nb was set to 2 hereafter because itgave a precipitate having the K/Nb ratio of 1.03 close to that ofKNbO3. Fig. 3 shows the DTA/TG curves of an amorphous precursor
salt precipitated by mixing 100 mL of 0.20 mol/L NbC-HPA aq with
100 mL of 0.40 mol/L KOH aq. After a broad endothermic peak due
to water desorption ranging from room temperature to 200 8C, an[()TD$FIG]

3. Results and discussion
The UV–vis absorption spectrum of NbC-HPA aqueous solution(0.20 mol/L as to Nb) is shown in Fig. 1. A broad absorption band
ranging from 250 to 500 nm was observed, and the molar
extinction coefﬁcient (e) of the NbC-HPA solution at 370 nm
was estimated as 1.84  104 cm2 mol1. The absorption in therange of 250–500 nm is considered to be due to electronictransitions in niobium complex ion with peroxo and oxalategroups as ligands. Time-courses of the absorbance at 370 nm afterFig. 2. Relationship between the K/Nb molar ratio in the precipitated salt and the K/
Nb molar ratio of the mixed solutions.

[()TD$FIG]

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1901

Table 1Reﬁned crystallographic data for KNbO3 obtained by heating the precursor at 650–
900 8C.

Fig. 3. TG and DTA curves of the amorphous precursor at heating rate 20 8C/min inair. The potassium salt of NbC-HPA was obtained by mixing molar ratio at K/Nb = 2.

exothermic peak due to crystallization appeared around 570 8C.
This suggests that a crystalline product can be obtained by heating
the precursor above 570 8C. Additionally, a broad endothermic
peak ranging from 670 to 850 8C was also observed. This isconsidered to be due to fusion of crystallites, because endothermic
reactions at elevated temperatures thermodynamically accompany entropy increase.
The XRD patterns for the samples heated in air for 1 h. at 600,650, 700, 800 and 900 8C are shown in Fig. 4. The amorphous
precursor partly remained in the product obtained at 600 8C
because two broad peaks were observed around 9.48 and 30.68. The
precursor entirely changed into crystal phase over 650 8C, and theXRD patterns for the samples obtained by heating over 650 8C werecoincident with those of the cubic KNbO3 (PDF-080212). The
crystallinity of the KNbO3 was almost the same from 650 to 900 8C
in spite of the increase in heating temperature. The structuralparameters for KNbO3 obtained from the Rietveld reﬁnement aresummarized in Table 1. All the data were well ﬁtted with the spacegroup Pm3m, and the crystallite size of KNbO3 determined from thepeak proﬁle parameters was in the range of 25–35 nm indepen-Cubic; space group; Pm3m (221) Z = 1
AtomSiteK
1a (0, 0, 0)
Nb
1b (1/2, 1/2, 1/2)O
2c (0, 1/2, 1/2)

Occupancy
1.0
1.0
1.0

650 8C700 8C
800 8C
900 8C

Crystallite size: 35.0 nm
Crystallite size: 27.8 nm
Crystallite size: 25.0 nm
Crystallite size: 28.8 nm

a = 4.0129(40)a = 4.0122(28)a = 4.0115(39)a = 4.0136(53)dently of the heating temperature. This indicates that theprecipitation of nano-sized crystallite from the amorphous matrix
occurs at 650–900 8C and its crystal growth does not proceed at thetemperature region. This is coincident with the fact that KNbO3
was conventionally prepared by solid state reactions at 1000 8C ormore [35]. Thus, the crystallite size was independent of the heating
temperatures, because the heating temperatures were lower thanthose for the solid state reactions. Generally, heating amorphous
precursors at temperature regions lower than those for solid state
reactions is a recipe for the preparation of nano-sized particles. The
space groups P4mm and Amm2 were also considered in the ﬁtting
because the formation of tetragonal or orthorhombic phase ofKNbO3 was reported in earlier reports [4,36,37], but the ﬁtting did
not go well. The FE-SEM images of the KNbO3 samples obtained byheating the precursor in air for 1 h at 700 and 900 8C are shown in
Fig. 5a and b, respectively. The aggregates of nano-sized particles
on an agglomerate are observed in both ﬁgures, and theagglomerate is thought to be fused crystallites. The crystallite
sizes estimated from the XRD proﬁles were almost the same in thetemperature
region 650–900 8C, and the fusion of crystallites was[()TD$FIG][()TD$FIG]

Fig. 4. XRD patterns of KNbO3 samples obtained by heating the amorphous
precursor in air for 1 h at 650–900 8C.

Fig. 5. FE-SEM images of KNbO3 samples obtained by heating the amorphous
precursor in air for 1 h at 700 8C (a) and 900 8C (b).

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considered to take place from 670 to 850 8C. Therefore, it isappropriate to heat the precursor around 650 8C to prepare
dispersed nano-sized particles without fusion. Additionally, thetemperature region 670–950 8C is appropriate for the preparationof the agglomerates of nano-sized particles.
4. Conclusions
NbC-HPA solution was prepared by direct reaction between
NbC powder and H2O2 aq. KNbO3 was obtained by heating theamorphous precursor at 650–900 8C, which was precipitated bymixing NbC-HPA solution with KOH aq. All the X-ray diffraction
data at 650–900 8C were well ﬁtted with the space group Pm3m
in the Rietveld analysis. The peak proﬁles in the X-ray diffraction
patterns indicated that the crystallite size was in the range of
25–35 nm, and ﬁeld emission scanning electron microscope
images supported this result. This method can be applied to thepreparation of the other niobium complex oxides such as
LiNbO3, NaNbO3, AgNbO3. Peroxo-precursor decomposition
methods offer attractive prospects for the preparation ofnano-sized complex metal oxides, and the peroxo-precursor is
expected to become a more commonly used precursor in soft
chemical synthesis.
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