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D. Sert, A. Mergen / Journal of Alloys and Compounds 482 (2009) 396–399
Fig. 3. FT-IR spectrum of Ba(Zn1/3Nb2/3)O3 powders heat treated at different temperatures for 4 h.
in citric acid [C6H8O7·H2O] (99.5–100.5%, Merck) solution. Citric acid mol ratio to
metal cations in the Ba(Zn1/3Nb2/3)O3 was 0.5. After adding stoichiometric amounts
of barium nitrate and zinc acetate into the niobium citrate solution, ethylene glycol
[HOCH2CH2OH] (99%, Merck) with an amount of citric acid:ethylene glycol weight
ratio 60:40 was added to the solution which was kept at 80 ◦C to form a gel. After heat
treatment at 300 ◦C for 2 h, the gel converted into a black porous powder indicating
that charring occurred. The resin was lightly ground into a powder with an agate
rod. The black powder was calcined at different temperatures of 400–1200 ◦C for 4 h
to monitor the phase development.
The thermogravimetric and differential thermal analysis (TG and DTA) of the
gel treated at 300 ◦C were carried out under static air with a heating rate of
10 ◦C min−1 from room temperature to 1000 ◦C (NETZSCH STA 409C/CD) to detect
the thermal behavior of Ba(Zn1/3Nb2/3)O3 powders. After calcination at various
temperatures between 400 and 1200 ◦C, the phase formation and the purity of
Ba(Zn1/3Nb2/3)O3 powders was investigated by X-ray diffractometer with Cu K␣
radiation (Rigaku) in the range 2ꢁ: 20–80◦. The crystallite size of Ba(Zn1/3Nb2/3)O3
powders at different temperatures was calculated by XRD using Scherrer equation:
D = k ꢂ/B cos ꢁ where ꢂ is the wavelength of the X-ray radiation (1.5406 Å for Cu
K␣), k is a constant taken as 0.9, ꢁ is the diffraction angle and B is the full-width
at half-maximum (FWHM). Fourier transform infrared spectrum of the charred gel
heat treated at temperatures between 400 and 1200 ◦C was recorded with a FT-IR
spectrometer (Nicolet 6700) in the range 400–4000 cm−1. The morphology of the
Ba(Zn1/3Nb2/3)O3 powder treated at 1000 ◦C was investigated by scanning electron
microscopy (SEM, JSM 5910LV) and transmission electron microscopy (TEM, Jeol
200V).
the last weight loss step occurred between 700 and 1000 ◦C. These
peaks are formed probably due to the decomposition of the BaCO3
and remaining organics, and also possibly due to the evaporation
of ZnO in the system.
The phase formation starting from the gel charred at 300 ◦C for
2 h was studied by means of XRD. Fig. 2 shows the XRD patterns of
the Ba(Zn1/3Nb2/3)O3 powders calcined at temperatures between
400 and 1200 ◦C for 4 h. Fig. 2 indicates that the charred powder
is amorphous. The formation of Ba(Zn1/3Nb2/3)O3 phase eventu-
ates at 400 ◦C, nevertheless the existence of secondary phases have
not transformed to Ba(Zn1/3Nb2/3)O3 perovskite structure yet is
observed at this temperature. Some of the secondary phases apart
from Ba(Zn1/3Nb2/3)O3 phase at 400 ◦C are formed due to the reac-
tion between reactive Ba2+ ions in the system and CO and CO2 gases
emerged as a result of the citric acid decomposition [11]. In the sys-
tem, small amounts of ZnO phase is determined as a secondary
phase apart from BaCO3. Increased calcination temperature leads
to a reduction in secondary phase intensities and almost complete
tranformation of BaCO3 and ZnO secondary phases to the single
phase Ba(Zn1/3Nb2/3)O3 perovskite at 1000 ◦C. In addition, increas-
ing calcination temperature does not cause the transformation of
Ba(Zn1/3Nb2/3)O3 phase to any other phases even at 1200 ◦C and
single phase perovskite structure is observed to be stable at high
temperatures. The crystallite size measured from the line broaden-
ing of corresponding XRD peaks is 32, 34 and 37 nm for the powders
calcined at 900, 1000 and 1100 ◦C, respectively.
Thermal behavior of Ba(Zn1/3Nb2/3)O3 powders produced by cit-
rate gel method with a metal cation/citric acid mol ratio of 0.5 and
heat treated at 300 ◦C for 2 h was investigated by TG–DTA analysis
(Fig. 1). Three main weight losses occur up to 1000 ◦C in the ther-
mogravimetric curve. First main loss in the range of 40–335 ◦C was
around 4% which was due to the evaporation of water adsorbed by
the powder surface which gave endothermic peaks at 40 and 110 ◦C
in the DTA. The rest of the 4% total weight loss was related to the
endothermic peaks observed at 235 and 330 ◦C in DTA curve and
it was due to the decomposition of weakly bonded organics in the
citrate structure. The second weight loss in the system between 335
and 700 ◦C was around 20% which was due to the combustion of car-
bon that arised as a result of the decomposition of organics in the
system. This weight loss matches with exothermic peaks at 510 and
625 ◦C. Two endothermic peaks at 748 and 845 ◦C were observed in
The FT-IR spectrum of the gel heat treated at 300 ◦C for 2 h
indicates mainly a broad band at 1610 cm−1 which is assigned to
the carboxyl groups of citric acid (Fig. 3). XRD also indicates an
amorphous nature at this temperature. However, higher amounts
of bands formed in powder heat treated at 400 ◦C. Bands at 1440
and 860 cm−1 decrease in intensity with temperature increase,
but the bands at 1760, 1090 and 700 cm−1 disappear at lower
temperatures of around 600 ◦C. These bands occur due to BaCO3
presence in the material [12]. But the bands at 1440 and 860 cm−1
disappear at higher temperatures of 1100 ◦C which indicates that
the carbonates are present in the material up to 1100 ◦C. How-
ever, although XRD indicates BaCO3 peaks at 800 ◦C, they are
not observed at 900 ◦C possibly due to their low content. Bands
Sert, Duygu
