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Thermal decomposition process and electrical conductivity of single crystals of NH+4-β- and β″-ferrites
Solid State Ionics 35 (1989) 207-212 North-Holland, Amsterdam
T H E R M A L D E C O M P O S I T I O N P R O C E S S AND E L E C T R I C A L C O N D U C T I V I T Y OF S I N G L E CRYSTALS O F NH~-[I- AND [I"-FERRITES
K. UCHINOKURA, S. NARIKI, S. ITO and N. Y O N E D A Department c!flndustrial Chemistry. Faculty of Science and Technology, Science University qf Tokyo, Noda-shi, Chiba-ken 278, Japan Received 5 October 1988
The thermal decomposition process and the ionic and electronic conductivities of NH + -13-ferriteand Cd stabilized NH2-~3"ferrite single crystals were investigated. TG curves of both ferrites exhibited the two steps of weight loss at 50-200°C and 300430°C. The first weight loss was based on dehydration, while the second was due to the liberation of NH2 ions. The ~ and 13" structures decomposed with the loss ofNH + ions to form a-Fe203 and Cd doped y-Fe203 respectively.The ionic conductivities of NH+ -13-and 13"-ferrite single crystals were 4 × 10-6 S cm-] and 3 × 10 5 S cm- t at 25 °C, respectively.Temperature dependence of ionic and electronic conductivities is discussed in terms of the thermal decomposition process.
1. Introduction
2. Experimental 2. I. Preparation of N H +-fl- and ~"-[errite crystals
A m m o n i u m and o x o n i u m 13- and [3"-aluminas are known as protonic conductors. Many investigations [ 1 - 2 3 ] have been u n d e r t a k e n on the preparation, thermal stability, electrical conductivity a n d crystal structure of these protonic [3- and [3"-aluminas. The application of these c o m p o u n d s in hydrogen sensors [24] and hydrogen fuel cells [25,26] have been also studied. The authors [ 2 7 - 2 9 ] have previously reported the preparation and some of the properties of the [3- and [3"-alumina type N H +-ferrites. N H +-[3and [3"-ferrite single crystals are obtained by the ion exchange of K + ions in K+-13- and [3"-ferrites with NH + ions, and decomposition at about 3 0 0 - 4 0 0 ° C to form cz-Fe203 or y-Fe203. The N H + -[3- and 13"-ferrites are expected to exhibit protonic c o n d u c t i o n and electronic conduction due to electron hopping between Fe 2+ and Fe 3+ ions. In the present work, we report in more detail, thermal decomposition processes of NH2-[3-ferrite crystals and Cd stabilized NH2-[3"-ferrite crystals. Further, the electrical conductivity of the crystal has been measured and the temperature dependence of the conductivity is discussed in terms of the thermal decomposition process.
K +-[3- and [3"-ferrite crystals were grown from the melt system B 2 0 3 - K 2 0 - K F . CdO was added to the melt as a stabilizing reagent for the [3" phase. The details of the growth procedures of K+-13- and 13"-ferrite crystals are described elsewhere [28,30]. In order to obtain NH +-[3- and 13"-ferrites, the K +[3- and [3"-ferrite crystals (0.1 g) were soaked in molten NH4NO3 ( 5 - 6 g) at 2 0 0 ° C for 12 h. After the exchange treatment, the salt was removed in water and the crystals were dried. The a m o u n t s of K, Cd and Fe in the crystals were determined by atomic absorption and flame analyses.
2.2. Analyses ~f the thermal decomposition process Differential thermal analysis ( D T A ) and thermogravimetric ( T G ) analysis o f N H + -13-and [3"-ferrite crystals were performed in air or in a flow of nitrogen ( 100 m ~ / m i n ) at a heating rate of 1 0 ° C / m i n , using 30 mg of crystals, 1-2 m m in size. The phases of the heat-treated crystals were identified by X-ray powder diffraction. Infrared absorption spectra of the NH~-[3- and [3"-ferrites and the heated speci-
208
K. UHtino/,ura et a/. ,'Sing/o crystals of :Vtt + /J and fl"-lerrttes
mens were measured by the KBr tablet m e t h o d to identify the chemical species in the conduction plane. 2.3. M e a s t t r e m e n t o/electrical conductivilv
w~
Gold wires were b o n d e d with gold paste onto the opposite faces p e r p e n d i c u l a r to the (001) plane o f a single crystal which was ca. 3 m m in d i a m e t e r and ca. 0.2 m m in thickness. The specimen was placed in an electric furnace, in which dry nitrogen gas was introduced. The specimen was held at a given temperature from 25 C t o 600~C for 20 min, and then the impedances and phase angles from 5 Hz to 13 M H z were measured, using a Y H P 4192A LF i m p e d a n c e analyzer. The ionic conductivities o f NH~-[3- and [3"-ferrites were obtained from the complex impedance. The electronic conductivity o f the specimen was measured using a four-probe dc technique.
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3. Results and discussion 3.1. Formation q I ' N H ) - f l - and fl"-/i, rrite crysta/s Flame analysis revealed that the a m o u n t s of K + ion in the NH+-I3 - and !3"-ferrites were extremely small (less than 1% o f K + in the original K+-I3- and ]3"-ferrites). Therefore, it was concluded that K + ions in the K+-I3- and 13"-ferrites had been completely exchanged with N Ha+ ions. 3.2. T h e r m a l stabihty ql?¢tt$-fl-ferrite crystal Fig. 1 shows the t h e r m o g r a v i m e t r i c ( T G ) curve (a) and differential thermal analysis ( D T A ) curves (b and c) for NH2-13-ferrite crystals. Fig. lc is the DTA curve o b t a i n e d in a flow of nitrogen. The T G curve revealed the weight loss of two steps; that is, from 100~C to 200~C and from 350°C to 430°C. Fig. 2 shows the X-ray p o w d e r diffraction patterns o f NHa+-13-ferrite and heat-treated specimens. The product heated above 4 0 0 : C was identified as ctF e , O > relating to the weight loss at a r o u n d 4 0 0 : C on T G curve. The exothermic peak on the DTA curve seems to be due to the crystallization o f a-Fe,O~. Fig. 3 shows the infrared ( I R ) spectra of NH2-I3-
.~3 Sa g~
g
"U 11)0
2'00 360 [460 500 Temperature (°C)
600
700
Fig. 1. Thermal analysis curves of NH2 -13-ferrite: (a) TG and (b) DTA in air, (c) DTA in a flow of nitrogen (100 m~?/min). ferrite and products heated at different temperatures up to 600~C. In the spectrum of the original N H + 13-ferrite, an absorption band at about 1400 cm ' is assigned to the bending mode of N H + ion. The bands at about 1600 cm ' and 3500 cm ~ are assigned to the H - O - H bending m o d e and the stretching m o d e o f the O - H bond, respectively. These O - H bands indicate that NHd -[5-ferrite contains H,O, as in the case of NH4~-13-alumina [19] or NH;-13-gallate [23]. The bands below 900 cm ' are based on the vibrations in the spinel blocks. In the spectrum o f the specimen heated up to 200°C, the characteristic absorption bands o f O - H bond became very weak but the NH~- band was not changed. Therefore, the weight loss at below 200°C on the T G curve may be assigned to the loss o f HzO. The NH + absorption band decreased in the specimens heated above 3 5 0 : C and was not observed in the specimen heated at 450 ~C. Thus, the weight loss at a r o u n d 400°C is due to the liberation o f N H ~ ions. The exothermic peak in air (fig. l b ) may be attributed to the oxidation of liberated NH3 in a d d i t i o n to the transformation into c~FeeO~, because the exothermic peak in air was larger than that in nitrogen gas (see fig. l c ) . According to the above results, the thermal decomposition process o f NH2-[3-ferrite crystal m a y be s u m m a r i z e d as follows: H,O" ( N H 4 ) : O " 11Fe20~ ([3-ferrite) ~,o eo~ ,, ( N H 4 ) 2 0 ' l l F e : O ~ ([3-ferrite) + H z O ~5~, a~,~ 1 lct-Fe_,O~ + 2 N H ~ + H e O ,
K. Uchinokura et al / Single crystals q/'A,Wf # and #" :territes
209
(002)
(004)
original(B) original~~
017)
~
Y
.~
200°C(6)
Y
g
L400°C(6,a-Fez03)
4000
o:a-Fe=Os
i
i
30'00 20'00 1600 1200 Wavenumber (cm -~ )
800
40o
Fig. 3. Infrared spectra of NH+-[3-ferrite and its heat-treated specimens.
600°C(a-Fe2Os)
•
.
.
.
.
.
.
.
.
.
.
i0 20 3 0 4 0 5 0 60 70 80 90 i00 ii0 20 (degree, CrK~) Fig. 2. X-ray powder diffraction patterns of N H I q3-ferrite and its heat-treated specimens.
where the chemical composition in each stage was estimated from the weight loss in TG curve. The composition of original NH2-[3-ferrite crystal was H20" (NH4)20' 11Fe203. At about 200°C, the NH2-[3-ferrite containing an stoichiometric amount of NH~ ion was formed by dehydration. The NHf-[3-ferrite released NH3 and H20 at above 350°C and transformed into ct-Fe203 with an exothermic reaction. The decomposition of ( N H 4 ) 2 0 in NH~--[3-alumina and NH+-[~-gallate proceeds in two steps [ 6,16,231; that is, NH~- -[3-alumina and NH~- -[3-gallate release NH 3 to form H+-[3-alumina (or H30 ÷13-alumina) and H+-13-gallate, respectively, and then the H+-13-alumina and H+-[3-gallate lose H20 to yield an intermediate.phase with the composition of A1203 and Ga203. On the other hand, the NH$-13-ferrite
directly decomposed into a - F e 2 0 3 without the formation of H+-13-ferrite or H30+-[3-ferrite. The column oxygen in the conduction layer seems to be excluded from the loss of NH7 above 350°C.
3.3. Thermal stability of Cd stabilized NH) -#"ferrite crystals In the case of the [3" phase, TG curve (a) and DTA curves in air (b) and a flow of nitrogen gas (c) are shown in fig. 4. From the TG curves, the weight loss
~0
v3 1 22 Y~
l
.~ 3 4
1()0
2;0
4;0
5;0
Temperature
3;0
(°C)
6(~0 7;0
Fig. 4. Thermal analysis curves of NH2-[3"-ferrite: (a) TG and (b) DTA in air, (c) DTA in a flow of nitrogen ( 100 m£/min).
210
K. L"chinokura et al. /Single crystals" o / N H )
fl and fl" :/brriles
(oo3)
original (13")
(oo6) (0111)
i
200°C(13 ")
4000
30'00 20'00 16'00 1 2 ; 0
8;0
350°C(13 " ,;C-Fe203)
400
Wavenumber (em-~) Fig. 5. Infrared spectra of' NH +-[3''-ferrite and its heat-treated specimens.
also took place in two steps; that is, from 50°C to 200°C and from 300°C to 4 0 0 : C . The D T A curve in air indicated a very weak exothermic peak at about 400°C. In nitrogen exothermic or endothermic peaks were not found. Fig. 5 shows the IR spectra o f NH +-13,'-ferrite and the materials formed on heating. As in the case o f the 13-phase, the characteristic absorption bands o f the O - H b o n d became weak at 200°C and band o f N H 2 decreased at 300 to 400°C. Fherefore, the weight loss at below 200 ° C on the T G :urve is based on dehydration, and the weight loss From 300 to 400°C is due to the liberation o f NH$ ions. The results o f T G analysis and IR spectra For the [3" phase seem to be almost similar to that o f the 13 phase. However, the phase change o f the 13" ~)hase differed from that o f the 13phase. Fig. 6 shows Ihe X-ray p o w d e r diffraction patterns o f NH2-13"?errite and the heated specimens. The product heated ap to 300°C m a i n t a i n e d 13" phase structure. Above 300:C, reflection intensities from the (003) and (006) planes o f [3" phase decreased, and the 13" phase Iransformed into the i n t e r m e d i a t e phase with spinel ~tructure, which seemed to be Cd d o p e d 7-Fe203. Fbis phase finally d e c o m p o s e d to form a-Fe203 and ~dFe~O4 at 7 0 0 - 9 0 0 ° C . According to the above remits, the thermal d e c o m p o s i t i o n process o f Cd sta)ilized N H +-13''-ferrite crystal m a y be s u m m a r i z e d ~s follows:
•
•
•
@
•
•
•
•
•
•
500°C(y-Fe203)
900°C (a-Fe20a ,CdFe204) o : a-Fe20a
o
~:CdFeaO~
]zxi
o ; 2 ;
" 30
~'
o
o
o
" ; Z " " 40 70 80 28 (degree, CrKa)
oo " 90
" i00 1 0
Fig. 6. X-ray powder diffraction patterns of NH f -~"-ferrite an its heat-treated specimens. 1.8H20" 1.3 ( N H 4 ) 2 0 " 11Fe203 • 1.6CdO ( 13"-ferrite ) SO 200 ('
' 1 . 3 ( N H 4 ) 2 0 ' 11Fe203
• 1.6CdO (13"-ferrite) + 1.8H20 ~l,ll-4o~l (/ 11F%O~ • 1.6CdO (Cd d o p e d y-Fe20~ ) + 2.6NH3 + 1.3H20 7oo ,)oo i 9.4a-Fe2Os + 1.6CdFe204.
K. Uchinokura et al. / Single crystals Qf NItJ 13and 13"-[brrites
The decomposition temperature of the 13" structure was about 50°C lower than that of 13phase. The formation of intermediate phase of ~'-Fe203 is interesting, because such an intermediate phase was not formed in the case of 13 phase. This intermediate phase from 13" phase would be formed by the structural similarity of ~,-Fe203 and the spinel blocks of 13"-ferrite, because the oxygen arrangement of ~,Fe203 is similar to the spinel blocks o f [3"-ferrite rather than those of 13-ferrite [ 31 ]. The D T A curve for [3" phase (see fig. 4) exhibited no significant exothermic peak, in contrast to the [3 phase. This suggests that the transformation of [3" phase into "{-Fe203 is not accompanied by any large rearrangement of atoms. The intermediate phase of ~'-Fe203 was also found in the powdered N H +-[3''-ferrite without stabilized reagent, which was reported in the previous paper [29]. This undoped N H +-13"-ferrite powder yielded ~,-Fe20~ and ct-Fe203 in air above 300°C, and this mixed phase completely transformed into ctFe203 single phase at about 550°C. On the other hand, the intermediate phase obtained from Cd stabilized NHf-[3"-ferrite did not transform up to 700°C and was more stable than the y-Fe203 obtained from undoped NH+-[3"-ferrite; that is, ,{Fe203 seems to be stabilized by Cd 2+ ions.
3.4. Electrical conductivities of NHf-fl- and fl"ferrites Fig. 7 shows the ionic and electronic conductivities of NH+-[3- and 13"-ferrites. The ionic conductivities o f the 13 and [3" phases showed similar behavior, but the conductivity o f 13" phase was higher than that of 13 phase. The ionic conductivities of NH+-13- and 13"-ferrites at 25°C were 4.3X 10 -6 S cm -~ and 2 . 7 × 1 0 -5 S cm -~, respectively. These values were comparable to those of N H +-13-alumina ( 1 . 5 × 1 0 -0 S cm ~) [14] and NH+-13"-alumina ( 3 × 1 0 s S c m - ' ) [19]. The temperature dependence of ionic conductivity was interpreted on the basis o f the thermal change, as in the cases o f N H + -13- and []"-aluminas [ 19 ] and N H + -13-gallate [23]. The ionic conductivities of 13- and 13"-ferrites increased with temperatures from 25 to 150°C. The activation energies at this stage were 0.21 eV for the 13 phase and 0.15 eV for the [3" phase. The conductivities decreased with dehydration above 150 °C.
211
Temperature (°C) 300 I00
500 O ~d
"~-2
-3 i
1.5
~
2.0
i
i
2.5 3.0 103/T (K -I)
i
3.5
Fig. 7. Ionic and electronic conductivities of each single crystals of NH + -13- and 13"-ferrites: (C) ) ion ic ( 13), ( [] ) ion ic (13"), ( • ) electronic (13), (11) electronic (13").
However, the conductivities increased again above 200°C where the liberation of H20 had terminated but N H + ions remained. The activation energies in this temperature region were 0.62 eV for the 13phase and 0.56 eV for the 13" phase. The ionic conductivity of the 13 phase reached a m a x i m u m value of 4 . 9 × 10 4 S cm-L at 350°C. The conductivity of the 13" phase also exhibited a m a x i m u m value of 1.3 × 10 3 S c m - ~at 300°C. On heating above 350°C for the 13 phase and above 300°C for the 13" phase, the conductivities decreased rapidly with the loss o f N H + ions and decomposition of the 13 or 13" phase. On considering the ionic conduction of N H +-[3and [3"-aluminas, several workers [ 19,23 ] have proposed a mechanism of proton transfer between N H + and H20 species in the low temperature region, and a mechanism of NH + ion hopping in the high temperature region. The activation energy in proton hopping is lower than that for N H $ ion hopping. In the cases of NHf-13- and [3"-ferrites, the ionic conductions below 150 ° C seem to be based on a protonic conduction, because the activation energy in this stage is considered to be too small for the conduction due to the hopping of large N H + ions. On the other hand, the conduction in a high temperature region between 200 and 350°C (300°C for 13" phase) would be due to the hopping of NH + ion.
212
1~ Uchinokura et al. ~Single ct3'stals q / N l l g [] and []" :/brrite,~
The electronic conductivities in both phases were lower than the respective ionic conductivities below 350:C. The Arrhenius plots consisted o f two straight lines. The knee at 3 0 0 - 3 5 0 : C corresponds to the dec o m p o s i t i o n of 13 or 13" phase. The activation energy o f electronic conduction was not affected by the loss of H20 below 2 0 0 : C , because the electronic conduction is not d e p e n d e n t on the change in the alkali layers, but, rather, is due to h o p p i n g o f electrons between Fe 2+ and Fe ~+ ions in spinel blocks.
4. Summary NH2-13-ferrite crystal with an original composition of He0' (NH4)~O' 11Fe203 released H20 at 100200°C. The !3 structure decomposed with the liberation of (NH4)20 to form c~-Fe203 at 350-430°C. On the other hand, Cd stabilized NHg-13"-ferrite crystals with an original composition of 1.8 H20.1.3 (NH4) 20" 11Fe203' 1.6CdO released H20 at 5 0 - 2 0 0 ° C and ( N H 4 ) z O at 3 0 0 - 4 0 0 ° C . Above 400 ° C, the 13" structure d e c o m p o s e d to form the int e r m e d i a t e phase of Cd doped "{-Fe203, which transformed into ct-Fe203 and CdFe204. The formation of this intermediate phase seemed to be due to the structural similarity of y-F%O3 and the spinel block of 13"-ferrite. The ionic conductivities of NH +-13- and 13"-ferr i t e s w e r e 4 . 3 × 1 0 ~'Scm ~ a n d 2 . 7 × 1 0 5 S c m - ~ at 25°C, respectively. The ionic conduction in the temperature range from 25°C to 150°C seemed to be based on a protonic conduction between NH + and H20, while the ionic conduction in the temperature range between 200 and 350°C (300°C for 13" phase) would be due to the hopping of NH + ions.
References [ 1 ] H. Saalfeld, H. Manhies and S.K. Dana, Ber. Dlsch. Keram. Ges. 45 (1968)212. [2] T. Nishikawa, T. Nishida, Y. Katsuyama, Y. Kawakita and I. Uei, Nippon Kagaku Kaishi 1974 (1974) 1048. [ 3 ] S. Komatsu and G. Yamaguchi, Nippon Kagaku Kaishi 1975 (1975) 761.
[4] Ph. Colomban, G. Lucazeau, R. Mercier and A. Novak, J. Chem. Phys. 67 (1977) 5244. [5] M.W. Breiter, G.C. Farrington, W i . Roth and J i . Duf|~', Mat. Res. Bull. 12 (1977) 895. [6 ] Ph. Colomban, J.P. Boilot, A. Kahn and G. Lucazeau, Nouv. J. Chim. 2 (1978) 21. [7] G.C. Farrington, J.L. Briant, M.W. Breiter and W.L. Roth, J. Solid State Chem. 24 (1978) 311. [ 8 ] W . L Roth, M.W. Breiter and G.C. Farrington, J. Solid Slate Chem. 24 (1978) 321. [9] G.C. Farrington and J.L. Briant, Mat. Res. Bull. 13 (1978) 763. [10] W.L. Roth and R.C. DeVries, Mat. Res. Bull. 13 (1978) 1287. [ 11 ] J. Jensen and P. McGeehin, J. Mat. Sci. 13 ( 1978 ) 909. [12] Ph. Colomban and A. Novak, Solid State C o m m u n . 32 (1979) 467. [ 13] T. Nishikawa, T. Nishida and I. Nishimura, Nippon Kagaku Kaishi 1979 (1979) 202. [ 14 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 2 (1980) 107. [15]S. Udagawa. H.lkawa, M. Yamamoto and N. Otsuka, Nippon Kagaku Kaishi 1980 (1980) 1102. [ 16 ] B.C. Tofield, J.M. Newsam and A. Hooper, Solid State lonics 5 (1981)249. [ 17] H. Arribart, H. Carlos and B. Sapoval, Solid State lonics 5 ( 198i ) 253. [ 18 ] H. lkawa, T. Tsurumi, Y. Ohashi, K. Urabe and S. Udagawa, Yogyo-Kyokai-Shi 92 (1984) 473. [ 19 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 13 (1984) 233. [ 20 ] M. Nagai and P.S. Nicholson, Solid State lonics 15 ( 1985 ) 311. [ 2 1 ] H . Ikawa, T. Tsurumi, M. Ishimori, K. Urabe and S. Udagawa, J. Solid State Chem. 60 ( 1985 ) 5 I. [22]K.G. Frase, J.O. Thomas, A.R. McGhie and G.C. Farrington, J. Solid State Chem. 62 (1986) 297. [ 23 ] H. lkawa, T. Tsurumi, K. Urabe and S. Udagawa, Solid State lonics 20 (1986) I. [24] J.S. Lundsgaard and R.J. Brook, J. Mat. Sci. 9 (1974) 2061. [25] P.S. Nicholson, M. Nagai and K. Yamashita, Solid State lonics 15 (1985) 317. [26]T. Tsurumi, H. lkawa, M. lshimori, K. Urabe and S. Udagawa, Solid State Ionics 21 (1986) 3l. [27] S. Ito, N. Kubo, S. Nariki and N. Yoncda, J..Am. Ceram. Soc. 70 (1987) 874. [28] S. Nariki. S. lto, K. Uchinokura and N. Yoneda, J. Cryst. Growth 85 (1987) 483. [ 29 ] S. Nariki, S. Ito and N. Yoneda, Nippon Kagaku Kaishi 1988 (1988) 131. [30] S. Ito, N. Kubo and N. Yoneda. J. Cryst. Growth 71 (1985) 253. [31 ] S. Nariki, S. lto and N. Yoneda, Am. Ceram. Soc. Bull. 66 (1987) 1250.
T H E R M A L D E C O M P O S I T I O N P R O C E S S AND E L E C T R I C A L C O N D U C T I V I T Y OF S I N G L E CRYSTALS O F NH~-[I- AND [I"-FERRITES
K. UCHINOKURA, S. NARIKI, S. ITO and N. Y O N E D A Department c!flndustrial Chemistry. Faculty of Science and Technology, Science University qf Tokyo, Noda-shi, Chiba-ken 278, Japan Received 5 October 1988
The thermal decomposition process and the ionic and electronic conductivities of NH + -13-ferriteand Cd stabilized NH2-~3"ferrite single crystals were investigated. TG curves of both ferrites exhibited the two steps of weight loss at 50-200°C and 300430°C. The first weight loss was based on dehydration, while the second was due to the liberation of NH2 ions. The ~ and 13" structures decomposed with the loss ofNH + ions to form a-Fe203 and Cd doped y-Fe203 respectively.The ionic conductivities of NH+ -13-and 13"-ferrite single crystals were 4 × 10-6 S cm-] and 3 × 10 5 S cm- t at 25 °C, respectively.Temperature dependence of ionic and electronic conductivities is discussed in terms of the thermal decomposition process.
1. Introduction
2. Experimental 2. I. Preparation of N H +-fl- and ~"-[errite crystals
A m m o n i u m and o x o n i u m 13- and [3"-aluminas are known as protonic conductors. Many investigations [ 1 - 2 3 ] have been u n d e r t a k e n on the preparation, thermal stability, electrical conductivity a n d crystal structure of these protonic [3- and [3"-aluminas. The application of these c o m p o u n d s in hydrogen sensors [24] and hydrogen fuel cells [25,26] have been also studied. The authors [ 2 7 - 2 9 ] have previously reported the preparation and some of the properties of the [3- and [3"-alumina type N H +-ferrites. N H +-[3and [3"-ferrite single crystals are obtained by the ion exchange of K + ions in K+-13- and [3"-ferrites with NH + ions, and decomposition at about 3 0 0 - 4 0 0 ° C to form cz-Fe203 or y-Fe203. The N H + -[3- and 13"-ferrites are expected to exhibit protonic c o n d u c t i o n and electronic conduction due to electron hopping between Fe 2+ and Fe 3+ ions. In the present work, we report in more detail, thermal decomposition processes of NH2-[3-ferrite crystals and Cd stabilized NH2-[3"-ferrite crystals. Further, the electrical conductivity of the crystal has been measured and the temperature dependence of the conductivity is discussed in terms of the thermal decomposition process.
K +-[3- and [3"-ferrite crystals were grown from the melt system B 2 0 3 - K 2 0 - K F . CdO was added to the melt as a stabilizing reagent for the [3" phase. The details of the growth procedures of K+-13- and 13"-ferrite crystals are described elsewhere [28,30]. In order to obtain NH +-[3- and 13"-ferrites, the K +[3- and [3"-ferrite crystals (0.1 g) were soaked in molten NH4NO3 ( 5 - 6 g) at 2 0 0 ° C for 12 h. After the exchange treatment, the salt was removed in water and the crystals were dried. The a m o u n t s of K, Cd and Fe in the crystals were determined by atomic absorption and flame analyses.
2.2. Analyses ~f the thermal decomposition process Differential thermal analysis ( D T A ) and thermogravimetric ( T G ) analysis o f N H + -13-and [3"-ferrite crystals were performed in air or in a flow of nitrogen ( 100 m ~ / m i n ) at a heating rate of 1 0 ° C / m i n , using 30 mg of crystals, 1-2 m m in size. The phases of the heat-treated crystals were identified by X-ray powder diffraction. Infrared absorption spectra of the NH~-[3- and [3"-ferrites and the heated speci-
208
K. UHtino/,ura et a/. ,'Sing/o crystals of :Vtt + /J and fl"-lerrttes
mens were measured by the KBr tablet m e t h o d to identify the chemical species in the conduction plane. 2.3. M e a s t t r e m e n t o/electrical conductivilv
w~
Gold wires were b o n d e d with gold paste onto the opposite faces p e r p e n d i c u l a r to the (001) plane o f a single crystal which was ca. 3 m m in d i a m e t e r and ca. 0.2 m m in thickness. The specimen was placed in an electric furnace, in which dry nitrogen gas was introduced. The specimen was held at a given temperature from 25 C t o 600~C for 20 min, and then the impedances and phase angles from 5 Hz to 13 M H z were measured, using a Y H P 4192A LF i m p e d a n c e analyzer. The ionic conductivities o f NH~-[3- and [3"-ferrites were obtained from the complex impedance. The electronic conductivity o f the specimen was measured using a four-probe dc technique.
+J
3. Results and discussion 3.1. Formation q I ' N H ) - f l - and fl"-/i, rrite crysta/s Flame analysis revealed that the a m o u n t s of K + ion in the NH+-I3 - and !3"-ferrites were extremely small (less than 1% o f K + in the original K+-I3- and ]3"-ferrites). Therefore, it was concluded that K + ions in the K+-I3- and 13"-ferrites had been completely exchanged with N Ha+ ions. 3.2. T h e r m a l stabihty ql?¢tt$-fl-ferrite crystal Fig. 1 shows the t h e r m o g r a v i m e t r i c ( T G ) curve (a) and differential thermal analysis ( D T A ) curves (b and c) for NH2-13-ferrite crystals. Fig. lc is the DTA curve o b t a i n e d in a flow of nitrogen. The T G curve revealed the weight loss of two steps; that is, from 100~C to 200~C and from 350°C to 430°C. Fig. 2 shows the X-ray p o w d e r diffraction patterns o f NHa+-13-ferrite and heat-treated specimens. The product heated above 4 0 0 : C was identified as ctF e , O > relating to the weight loss at a r o u n d 4 0 0 : C on T G curve. The exothermic peak on the DTA curve seems to be due to the crystallization o f a-Fe,O~. Fig. 3 shows the infrared ( I R ) spectra of NH2-I3-
.~3 Sa g~
g
"U 11)0
2'00 360 [460 500 Temperature (°C)
600
700
Fig. 1. Thermal analysis curves of NH2 -13-ferrite: (a) TG and (b) DTA in air, (c) DTA in a flow of nitrogen (100 m~?/min). ferrite and products heated at different temperatures up to 600~C. In the spectrum of the original N H + 13-ferrite, an absorption band at about 1400 cm ' is assigned to the bending mode of N H + ion. The bands at about 1600 cm ' and 3500 cm ~ are assigned to the H - O - H bending m o d e and the stretching m o d e o f the O - H bond, respectively. These O - H bands indicate that NHd -[5-ferrite contains H,O, as in the case of NH4~-13-alumina [19] or NH;-13-gallate [23]. The bands below 900 cm ' are based on the vibrations in the spinel blocks. In the spectrum o f the specimen heated up to 200°C, the characteristic absorption bands o f O - H bond became very weak but the NH~- band was not changed. Therefore, the weight loss at below 200°C on the T G curve may be assigned to the loss o f HzO. The NH + absorption band decreased in the specimens heated above 3 5 0 : C and was not observed in the specimen heated at 450 ~C. Thus, the weight loss at a r o u n d 400°C is due to the liberation o f N H ~ ions. The exothermic peak in air (fig. l b ) may be attributed to the oxidation of liberated NH3 in a d d i t i o n to the transformation into c~FeeO~, because the exothermic peak in air was larger than that in nitrogen gas (see fig. l c ) . According to the above results, the thermal decomposition process o f NH2-[3-ferrite crystal m a y be s u m m a r i z e d as follows: H,O" ( N H 4 ) : O " 11Fe20~ ([3-ferrite) ~,o eo~ ,, ( N H 4 ) 2 0 ' l l F e : O ~ ([3-ferrite) + H z O ~5~, a~,~ 1 lct-Fe_,O~ + 2 N H ~ + H e O ,
K. Uchinokura et al / Single crystals q/'A,Wf # and #" :territes
209
(002)
(004)
original(B) original~~
017)
~
Y
.~
200°C(6)
Y
g
L400°C(6,a-Fez03)
4000
o:a-Fe=Os
i
i
30'00 20'00 1600 1200 Wavenumber (cm -~ )
800
40o
Fig. 3. Infrared spectra of NH+-[3-ferrite and its heat-treated specimens.
600°C(a-Fe2Os)
•
.
.
.
.
.
.
.
.
.
.
i0 20 3 0 4 0 5 0 60 70 80 90 i00 ii0 20 (degree, CrK~) Fig. 2. X-ray powder diffraction patterns of N H I q3-ferrite and its heat-treated specimens.
where the chemical composition in each stage was estimated from the weight loss in TG curve. The composition of original NH2-[3-ferrite crystal was H20" (NH4)20' 11Fe203. At about 200°C, the NH2-[3-ferrite containing an stoichiometric amount of NH~ ion was formed by dehydration. The NHf-[3-ferrite released NH3 and H20 at above 350°C and transformed into ct-Fe203 with an exothermic reaction. The decomposition of ( N H 4 ) 2 0 in NH~--[3-alumina and NH+-[~-gallate proceeds in two steps [ 6,16,231; that is, NH~- -[3-alumina and NH~- -[3-gallate release NH 3 to form H+-[3-alumina (or H30 ÷13-alumina) and H+-13-gallate, respectively, and then the H+-13-alumina and H+-[3-gallate lose H20 to yield an intermediate.phase with the composition of A1203 and Ga203. On the other hand, the NH$-13-ferrite
directly decomposed into a - F e 2 0 3 without the formation of H+-13-ferrite or H30+-[3-ferrite. The column oxygen in the conduction layer seems to be excluded from the loss of NH7 above 350°C.
3.3. Thermal stability of Cd stabilized NH) -#"ferrite crystals In the case of the [3" phase, TG curve (a) and DTA curves in air (b) and a flow of nitrogen gas (c) are shown in fig. 4. From the TG curves, the weight loss
~0
v3 1 22 Y~
l
.~ 3 4
1()0
2;0
4;0
5;0
Temperature
3;0
(°C)
6(~0 7;0
Fig. 4. Thermal analysis curves of NH2-[3"-ferrite: (a) TG and (b) DTA in air, (c) DTA in a flow of nitrogen ( 100 m£/min).
210
K. L"chinokura et al. /Single crystals" o / N H )
fl and fl" :/brriles
(oo3)
original (13")
(oo6) (0111)
i
200°C(13 ")
4000
30'00 20'00 16'00 1 2 ; 0
8;0
350°C(13 " ,;C-Fe203)
400
Wavenumber (em-~) Fig. 5. Infrared spectra of' NH +-[3''-ferrite and its heat-treated specimens.
also took place in two steps; that is, from 50°C to 200°C and from 300°C to 4 0 0 : C . The D T A curve in air indicated a very weak exothermic peak at about 400°C. In nitrogen exothermic or endothermic peaks were not found. Fig. 5 shows the IR spectra o f NH +-13,'-ferrite and the materials formed on heating. As in the case o f the 13-phase, the characteristic absorption bands o f the O - H b o n d became weak at 200°C and band o f N H 2 decreased at 300 to 400°C. Fherefore, the weight loss at below 200 ° C on the T G :urve is based on dehydration, and the weight loss From 300 to 400°C is due to the liberation o f NH$ ions. The results o f T G analysis and IR spectra For the [3" phase seem to be almost similar to that o f the 13 phase. However, the phase change o f the 13" ~)hase differed from that o f the 13phase. Fig. 6 shows Ihe X-ray p o w d e r diffraction patterns o f NH2-13"?errite and the heated specimens. The product heated ap to 300°C m a i n t a i n e d 13" phase structure. Above 300:C, reflection intensities from the (003) and (006) planes o f [3" phase decreased, and the 13" phase Iransformed into the i n t e r m e d i a t e phase with spinel ~tructure, which seemed to be Cd d o p e d 7-Fe203. Fbis phase finally d e c o m p o s e d to form a-Fe203 and ~dFe~O4 at 7 0 0 - 9 0 0 ° C . According to the above remits, the thermal d e c o m p o s i t i o n process o f Cd sta)ilized N H +-13''-ferrite crystal m a y be s u m m a r i z e d ~s follows:
•
•
•
@
•
•
•
•
•
•
500°C(y-Fe203)
900°C (a-Fe20a ,CdFe204) o : a-Fe20a
o
~:CdFeaO~
]zxi
o ; 2 ;
" 30
~'
o
o
o
" ; Z " " 40 70 80 28 (degree, CrKa)
oo " 90
" i00 1 0
Fig. 6. X-ray powder diffraction patterns of NH f -~"-ferrite an its heat-treated specimens. 1.8H20" 1.3 ( N H 4 ) 2 0 " 11Fe203 • 1.6CdO ( 13"-ferrite ) SO 200 ('
' 1 . 3 ( N H 4 ) 2 0 ' 11Fe203
• 1.6CdO (13"-ferrite) + 1.8H20 ~l,ll-4o~l (/ 11F%O~ • 1.6CdO (Cd d o p e d y-Fe20~ ) + 2.6NH3 + 1.3H20 7oo ,)oo i 9.4a-Fe2Os + 1.6CdFe204.
K. Uchinokura et al. / Single crystals Qf NItJ 13and 13"-[brrites
The decomposition temperature of the 13" structure was about 50°C lower than that of 13phase. The formation of intermediate phase of ~'-Fe203 is interesting, because such an intermediate phase was not formed in the case of 13 phase. This intermediate phase from 13" phase would be formed by the structural similarity of ~,-Fe203 and the spinel blocks of 13"-ferrite, because the oxygen arrangement of ~,Fe203 is similar to the spinel blocks o f [3"-ferrite rather than those of 13-ferrite [ 31 ]. The D T A curve for [3" phase (see fig. 4) exhibited no significant exothermic peak, in contrast to the [3 phase. This suggests that the transformation of [3" phase into "{-Fe203 is not accompanied by any large rearrangement of atoms. The intermediate phase of ~'-Fe203 was also found in the powdered N H +-[3''-ferrite without stabilized reagent, which was reported in the previous paper [29]. This undoped N H +-13"-ferrite powder yielded ~,-Fe20~ and ct-Fe203 in air above 300°C, and this mixed phase completely transformed into ctFe203 single phase at about 550°C. On the other hand, the intermediate phase obtained from Cd stabilized NHf-[3"-ferrite did not transform up to 700°C and was more stable than the y-Fe203 obtained from undoped NH+-[3"-ferrite; that is, ,{Fe203 seems to be stabilized by Cd 2+ ions.
3.4. Electrical conductivities of NHf-fl- and fl"ferrites Fig. 7 shows the ionic and electronic conductivities of NH+-[3- and 13"-ferrites. The ionic conductivities o f the 13 and [3" phases showed similar behavior, but the conductivity o f 13" phase was higher than that of 13 phase. The ionic conductivities of NH+-13- and 13"-ferrites at 25°C were 4.3X 10 -6 S cm -~ and 2 . 7 × 1 0 -5 S cm -~, respectively. These values were comparable to those of N H +-13-alumina ( 1 . 5 × 1 0 -0 S cm ~) [14] and NH+-13"-alumina ( 3 × 1 0 s S c m - ' ) [19]. The temperature dependence of ionic conductivity was interpreted on the basis o f the thermal change, as in the cases o f N H + -13- and []"-aluminas [ 19 ] and N H + -13-gallate [23]. The ionic conductivities of 13- and 13"-ferrites increased with temperatures from 25 to 150°C. The activation energies at this stage were 0.21 eV for the 13 phase and 0.15 eV for the [3" phase. The conductivities decreased with dehydration above 150 °C.
211
Temperature (°C) 300 I00
500 O ~d
"~-2
-3 i
1.5
~
2.0
i
i
2.5 3.0 103/T (K -I)
i
3.5
Fig. 7. Ionic and electronic conductivities of each single crystals of NH + -13- and 13"-ferrites: (C) ) ion ic ( 13), ( [] ) ion ic (13"), ( • ) electronic (13), (11) electronic (13").
However, the conductivities increased again above 200°C where the liberation of H20 had terminated but N H + ions remained. The activation energies in this temperature region were 0.62 eV for the 13phase and 0.56 eV for the 13" phase. The ionic conductivity of the 13 phase reached a m a x i m u m value of 4 . 9 × 10 4 S cm-L at 350°C. The conductivity of the 13" phase also exhibited a m a x i m u m value of 1.3 × 10 3 S c m - ~at 300°C. On heating above 350°C for the 13 phase and above 300°C for the 13" phase, the conductivities decreased rapidly with the loss o f N H + ions and decomposition of the 13 or 13" phase. On considering the ionic conduction of N H +-[3and [3"-aluminas, several workers [ 19,23 ] have proposed a mechanism of proton transfer between N H + and H20 species in the low temperature region, and a mechanism of NH + ion hopping in the high temperature region. The activation energy in proton hopping is lower than that for N H $ ion hopping. In the cases of NHf-13- and [3"-ferrites, the ionic conductions below 150 ° C seem to be based on a protonic conduction, because the activation energy in this stage is considered to be too small for the conduction due to the hopping of large N H + ions. On the other hand, the conduction in a high temperature region between 200 and 350°C (300°C for 13" phase) would be due to the hopping of NH + ion.
212
1~ Uchinokura et al. ~Single ct3'stals q / N l l g [] and []" :/brrite,~
The electronic conductivities in both phases were lower than the respective ionic conductivities below 350:C. The Arrhenius plots consisted o f two straight lines. The knee at 3 0 0 - 3 5 0 : C corresponds to the dec o m p o s i t i o n of 13 or 13" phase. The activation energy o f electronic conduction was not affected by the loss of H20 below 2 0 0 : C , because the electronic conduction is not d e p e n d e n t on the change in the alkali layers, but, rather, is due to h o p p i n g o f electrons between Fe 2+ and Fe ~+ ions in spinel blocks.
4. Summary NH2-13-ferrite crystal with an original composition of He0' (NH4)~O' 11Fe203 released H20 at 100200°C. The !3 structure decomposed with the liberation of (NH4)20 to form c~-Fe203 at 350-430°C. On the other hand, Cd stabilized NHg-13"-ferrite crystals with an original composition of 1.8 H20.1.3 (NH4) 20" 11Fe203' 1.6CdO released H20 at 5 0 - 2 0 0 ° C and ( N H 4 ) z O at 3 0 0 - 4 0 0 ° C . Above 400 ° C, the 13" structure d e c o m p o s e d to form the int e r m e d i a t e phase of Cd doped "{-Fe203, which transformed into ct-Fe203 and CdFe204. The formation of this intermediate phase seemed to be due to the structural similarity of y-F%O3 and the spinel block of 13"-ferrite. The ionic conductivities of NH +-13- and 13"-ferr i t e s w e r e 4 . 3 × 1 0 ~'Scm ~ a n d 2 . 7 × 1 0 5 S c m - ~ at 25°C, respectively. The ionic conduction in the temperature range from 25°C to 150°C seemed to be based on a protonic conduction between NH + and H20, while the ionic conduction in the temperature range between 200 and 350°C (300°C for 13" phase) would be due to the hopping of NH + ions.
References [ 1 ] H. Saalfeld, H. Manhies and S.K. Dana, Ber. Dlsch. Keram. Ges. 45 (1968)212. [2] T. Nishikawa, T. Nishida, Y. Katsuyama, Y. Kawakita and I. Uei, Nippon Kagaku Kaishi 1974 (1974) 1048. [ 3 ] S. Komatsu and G. Yamaguchi, Nippon Kagaku Kaishi 1975 (1975) 761.
[4] Ph. Colomban, G. Lucazeau, R. Mercier and A. Novak, J. Chem. Phys. 67 (1977) 5244. [5] M.W. Breiter, G.C. Farrington, W i . Roth and J i . Duf|~', Mat. Res. Bull. 12 (1977) 895. [6 ] Ph. Colomban, J.P. Boilot, A. Kahn and G. Lucazeau, Nouv. J. Chim. 2 (1978) 21. [7] G.C. Farrington, J.L. Briant, M.W. Breiter and W.L. Roth, J. Solid State Chem. 24 (1978) 311. [ 8 ] W . L Roth, M.W. Breiter and G.C. Farrington, J. Solid Slate Chem. 24 (1978) 321. [9] G.C. Farrington and J.L. Briant, Mat. Res. Bull. 13 (1978) 763. [10] W.L. Roth and R.C. DeVries, Mat. Res. Bull. 13 (1978) 1287. [ 11 ] J. Jensen and P. McGeehin, J. Mat. Sci. 13 ( 1978 ) 909. [12] Ph. Colomban and A. Novak, Solid State C o m m u n . 32 (1979) 467. [ 13] T. Nishikawa, T. Nishida and I. Nishimura, Nippon Kagaku Kaishi 1979 (1979) 202. [ 14 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 2 (1980) 107. [15]S. Udagawa. H.lkawa, M. Yamamoto and N. Otsuka, Nippon Kagaku Kaishi 1980 (1980) 1102. [ 16 ] B.C. Tofield, J.M. Newsam and A. Hooper, Solid State lonics 5 (1981)249. [ 17] H. Arribart, H. Carlos and B. Sapoval, Solid State lonics 5 ( 198i ) 253. [ 18 ] H. lkawa, T. Tsurumi, Y. Ohashi, K. Urabe and S. Udagawa, Yogyo-Kyokai-Shi 92 (1984) 473. [ 19 ] N. Baffler, J.C. Badot and Ph. Colomban, Solid State lonics 13 (1984) 233. [ 20 ] M. Nagai and P.S. Nicholson, Solid State lonics 15 ( 1985 ) 311. [ 2 1 ] H . Ikawa, T. Tsurumi, M. Ishimori, K. Urabe and S. Udagawa, J. Solid State Chem. 60 ( 1985 ) 5 I. [22]K.G. Frase, J.O. Thomas, A.R. McGhie and G.C. Farrington, J. Solid State Chem. 62 (1986) 297. [ 23 ] H. lkawa, T. Tsurumi, K. Urabe and S. Udagawa, Solid State lonics 20 (1986) I. [24] J.S. Lundsgaard and R.J. Brook, J. Mat. Sci. 9 (1974) 2061. [25] P.S. Nicholson, M. Nagai and K. Yamashita, Solid State lonics 15 (1985) 317. [26]T. Tsurumi, H. lkawa, M. lshimori, K. Urabe and S. Udagawa, Solid State Ionics 21 (1986) 3l. [27] S. Ito, N. Kubo, S. Nariki and N. Yoncda, J..Am. Ceram. Soc. 70 (1987) 874. [28] S. Nariki. S. lto, K. Uchinokura and N. Yoneda, J. Cryst. Growth 85 (1987) 483. [ 29 ] S. Nariki, S. Ito and N. Yoneda, Nippon Kagaku Kaishi 1988 (1988) 131. [30] S. Ito, N. Kubo and N. Yoneda. J. Cryst. Growth 71 (1985) 253. [31 ] S. Nariki, S. lto and N. Yoneda, Am. Ceram. Soc. Bull. 66 (1987) 1250.