- Email: [email protected]
CuX2O6 and Ba3CuX2O9 (X = Nb, Ta): influence of the preparation conditions on phase formation and phase composition
SOLID STATE
&yg
EL..%VIER
Solid State Ionics 101-103
(1997) 579-584
CuX,O, and Ba,CuX,O, (X = Nb, Ta): influence of the preparation conditions on phase formation and phase composition H. Langbein”‘“,
M. Bremerb, I. G-abbes”
“Technische Universitiit Dresden, Institut fiir Anorganische Chemie, D-01062 Dresden, Germany hTU Bergakademie Freiberg, Institut fiir Anorganische Chemie, D-09596 Freiberg, Germany ‘Fraunhofer Institut f iir Keramische Technologien und Sinterwerkstoffe, D-01277 Dresden, Germany
Abstract The title compounds were prepared by conventional solid state reaction and by thermal decomposition of freeze-dried oxalate precursors using different temperature regimes. The thermal decomposition of an appropriately composed oxalate precursor enables the direct synthesis of pure monoclinic CuNb,O,, thermodynamically stable only at 0 5 700°C. The solid state reaction leads to the orthorhombic columbite type phase or to a mixture of the orthorhombic and monoclinic phases. A new metastable CuTa,O,-trirutile phase was synthesized by thermal decomposition of an oxalate precursor. Above 8OO”C, the decomposition results in the already known perovskite-like structure. Tetragonal perovskites Ba,Cu, +YNb,~_AO,_, 5r and Ba,Cu,+,Ta2-,%, 5, were synthesized by solid state reaction and by precursor decomposition. The phase formation and
homogeneity of the perovskites strongly depends on the reaction conditions. Keywords: Precursor; Materials:
Freeze drying; Perovskite;
Columbite;
Trirutile
CuNb?O,; CuTaz06; Ba,CuNb,O,; BalCuTaZO,
1. Introduction The use of wet chemical processes to prepare simple or complex oxides increasingly gained in interest during the last two or three decades. Preparative techniques such as sol-gel processing [l], thermal decomposition of molecular precursors [2] and coprecipitation [3] offer potential advantages over the conventional high temperature techniques. In the field of ceramic materials, chemical preparation of powders offers possibilities in the tailoring of *Corresponding author. Fax: [email protected]
+ 49-351
463
7287;
e-mail:
0167.2738/97/$17,00 0 1997 Elsevier Science B.V. All rights reserved P/I SO167-2738(97)00382-2
materials which result in high purity, high homogeneity and lower processing temperatures. Scientific research is also directed towards the preparation of new materials, which often cannot be obtained using classical synthesis methods. In many cases, metastable phases are obtainable. To get information concerning the phase formation of the title compounds during synthesis in dependence on the preparation conditions, we investigated the solid state reaction in comparison to the thermal decomposition of freeze-dried complex carboxylates. We succeeded in showing that some equalities exist, but also characteristic differences in the reaction behaviour of niobates and tantalates. The same is
580
H. Lmgbein
rt al. I Solid State lonics 101-103
valid for the comparison between solid state reaction and thermal decomposition of molecular precursors.
2. Experimental In the solid state reaction experiments, Nb,O,(Ta,O,), CuO and BaCO, in the appropriate ratios are mixed thoroughly by ball milling in propanol for three hours. After drying the slurry, the reaction is carried out at temperatures of SOO1000°C (reaction time see below). Complex carboxylate solutions were prepared by mixing solutions of ammonium-oxo-tris-oxalato-niobate(oxalato-tantalate) and ammonium-oxalato-cuprate (metal:oxalate ratio of 1:3, total metal concentration 0.01 . . 0.13 mol 1-l for niobates and I 0.01 mol 1-l for tantalates). In the case of perovskite preparation, an appropriate amount of barium acetate was added. To avoid precipitation in barium and tantalum containing solutions, an additional presence of H,O, (0.3 mol I-‘) is required. The resulting solutions were immediately frozen with liquid nitrogen and dried using an Alpha 2-4 freeze-dryer (Christ). The thermal decomposition was investigated by means of a Netzsch thermal analyzer STA 409, coupled with a mass spectrometer QMS 125 (Balzers). X-Ray diffraction was performed using a D5000 powder diffractometer (Siemens) with Cu Ka radiation (step size 20 = 0.02”, step time 2 s, measurements at room temperature).
3. Results and discussion 3.1. CuNb?O, and CuTa,O, Many complex niobates MNb,O, crystallize with columbite structure, the analogous tantalates with columbite or trirutile structures [4]. An exception is CuTa,O, with a perovskite-like structure (orthorhombic pseudocubic symmetry with four molecules per unit cell, space group Pmmm) [5]. Three copper cations are located in the middle of the cubic edges while the fourth is statistically distributed over the three face centers. Besides the orthorhombic columbite type phase of CuNb,O,, which is usually formed by solid state reaction, a monoclinic form can be obtained by decomposition of CuNbO, in air
(1997) 579-584
at 700°C [6]. Orthorhombic and monoclinic forms differ in the kind of orientation and in the degree of distortion of the CuO,-octahedra [7]. With regard to the direct synthesis of monoclinic CuNb,O, we have decomposed a freeze-dried complex oxalate with a Cu:Nb ratio of 1:2. Fig. la shows the result of thermal analysis. Fig. lb illustrates the corresponding mass spectroscopic analysis of the gaseous decomposition products CO, and H,O, as an example. The decomposition occurs in two clearly separated endothermic steps. First, hydrate water is eliminated below 200°C. Further decomposition takes place between 220 und 320°C giving rise to CO, CO,, H,O, NH,, HCOOH and HCN. From the investigation of single oxalates, it is known that after dehydration the decomposition process starts with the niobium complex (0 > 200°C) followed by the superimposed decomposition of the copper complex (0 > 250°C). The endothermic peak at about 300°C is connected with the decomposition of the intermediately formed oxamide, eliminating HCN in addition to already specified decomposition products. The phase formation was investigated by X-ray powder diffractometry. Decomposing at 300-400°C an amorphous product results. Above 400°C crystallization processes take place. Thermal treatment at 600°C leads to a single phase monoclinic CuNb,O,. Subsequent annealing at 800 or 900°C brings about a partial conversion into the orthorhombic columbite structure. Complete conversion needs a long annealing time at 0 > 900°C. In accordance with the solid state reaction, the direct decomposition of the freezedried precursor at 900°C (3 h) leads to the pure orthorhombic form. To get information about the reversibility of the conversion, the directly formed orthorhombic product was annealed at 700°C (3 h). This results in a partial retransformation into the monoclinic form, but even after a very long annealing time (72 h) the conversion remains incomplete. The results correspond to a thermodynamically stable monoclinic form of CuNb,O, below 700°C and a thermodynamically stable orthorhombic form above 700°C with a large activation barrier of interconversion. The decomposition of an appropriately composed complex Cu-Ta-oxalate (Cu:Ta = 1:2) at 400°C also results in an amorphous oxide. The decomposition process sequence is analogous. The X-ray powder diffractograms of the solid decomposition products
H. Langbein et al. I Solid State Ionics 101-103 (1997) 579-584
TG/%
DTG I % min-’
rel. Intensity, m=18
1
OS5(a)
rel. Intensity, m=44
(W
bl
-
G
TG
I’
-1,d
0
581
1 200
I 400
I 800
---____I
.
200
e
0l”C Fig. I. Thermal analysis of the CuNb20,-oxalate spectroscopic analysis of decomposition products
:I m=44 .I._____,_. ;:\---- _,-I1 400 800 800
WC precursor. Heating rate, 5 K min H,O (m = 18) and CO2 (m = 44).
are shown in Fig. 2. After annealing at 600°C the sample is still nearly amorphous. Thermal treatment at 700°C (24 h) leads to the crystallization of the already known perovskite phase. Moreover, a hitherto unknown phase T and some peaks of CuTa,,O,, [8] (L) are observed. After annealing at 1000°C nearly single phase copper tantalate with perovskite-like structure is formed. The traces of CuTa,00Z6 refer to an exact stoichiometry Cu,,,,Ta,,,,O,, determined by Wa Ilunga [9]. Increasing the Cu portion in the precursor the portion of phase T after annealing at 700°C is increased. Decomposition of a precursor with a Cu/Ta ratio of 4:l results in a sample only containing CuO and phase T. After dissolving CuO in hydrochloric acid, a crystalline yellow powder remains which is isotypical with the trirutile NiTa,O,. A Cu/Ta ratio of I:2 was determined by chemical analysis. Fig. 3a shows the X-ray powder diffractogram of this sample. Annealing the yellow sample at 1000°C results in a
’, argon. (a) DTA-, TG- and DTG-curves,
(b) mass
complete conversion of the trirutile sample to a green sample with the perovskite-like structure and, in agreement with the small difference in composition, some CuTa,,O,, is formed simultaneously (Fig. 3~). The reaction seems to be irreversible. This refers to a metastable character of the trirutile CuTa,O,. Therefore, the precursor decomposition at relatively low temperature allows the crystallization of a metastable copper tantalate with trirutile structure. The favourable influence of an excess of CuO on the phase formation is hitherto not clarified. 3.2. The perovskites
BaCu,,,Nb,,,O,
and
BaCu,,,Ta,,,O,
The compounds were prepared and characterized as tetragonal perovskites first by Kapyshev et al. in 1966 [lo] and, in recent times, by Fesenko et al. [ 1 I]. According to Ono [ 121, the exact composition of the Nb containing compound should be
582
H. Langbein
et al. I Solid State Ionics 103-103
(1997) 579-584
I
1466
T,
1346
[[
1I u)
600 “C
I J_+kLLLa 13
.
LL
c)
L
,.........‘....‘.........,....,.........I....L
20
25
30
35
40
45
50
55
60
2- theta- scale Fig. 2. X-ray powder diffractograms of CuTa,-precursors, decomposed at different temperatures (room temperature measureperovskite (Cu, ,,5Ta, q2106), ments), annealing time 24 h, P T trirutile (CuTa,O,), L CuTa,,O,,.
BGu,.2Nb,.8%.7; a single phase perovskite BaCu,,,Nb,,,O, could not be obtained. In the hitherto existing investigations, only solid state reaction of mixtures of the simple or pseudobinary oxides at 0 2 1000°C was used for synthesis. With regard to a decrease in the reaction temperature and to a determination of exact composition or rather homogeneity ranges, some compositions were prepared by thermal decomposition of freeze-dried oxalate precursors. During the decomposition of Ba containing carboxylate precursors the formation of BaCO, can be expected. Surprisingly, the thermal decomposition of the precursors is finished after decomposition at 800 . 900°C (24 h). In the X-ray powder diffractograms only very weak BaCO, peaks
10 2-theta-
20
30
40
50
60
70
80
90
100
scale
Fig. 3. X-ray powder diffractograms of (a) CuTa,O, (trirutile, precursor decomposition at 7OO”C), (b) CuTa,O,, 100 h, 8OO”C, after (a), (c) perovskite and phase L, 100 h, IOOO”C, after (a) (room temperature measurement).
can be observed intermediately. The perovskite formation begins at about 600°C and is finished at about 900°C. The X-ray powder diffractograms of both, the niobate and tantalate perovskite Ba,CuNb(Ta),O,, show relatively broad peaks with insufficiently resolved tetragonal splitting. This fact is the same also after a long time of annealing the samples at 1000°C. The X-ray powder diffractogram of Ba,CuNb,O, after an annealing time of 100 h at 1000°C is shown in Fig. 4. An analogous treatment of a Ba,Cu,,,Nb, *08,, precursor (perovskite composition described by Ono [12]) results in a sample with a well resolved X-ray powder diffractogram. Further increasing the Cu/Nb-ratio results in the formation of a new phase with a hitherto unknown structure beside the perovskite (diffractogram 3 in
H. Langbein
2 Theta- Scale , r ‘I
25 l
,
,
35
45
et al. I Solid State tonics
,
55
,
101-103
(1997)
Fig. 4). At a Cu/Nb-ratio of 1:l only the peaks of this phase, for the first time described by Ono [12], are observed. The results illustrated in Fig. 4 allow us to conclude that a homogeneity range of the perovskite Ba,Cu, +xNb,_xO,_, ,5x with 0 5 x I 0.3 exists. In the case of the Ta-containing perovskite the homogeneity range is extended from x = 0 to x = 0.1. In all the other facts, the results are completely analogous. To find the reason for the insufficient resolution of the perovskite peaks of samples with 1:2 Cu/Nb(Ta) ratios, the solid state reaction was investigated systematically. Fig. 5 illustrates some results for the compound Ba,CuTa,O,. Upon preparing the perovskite samples by annealing mixtures of BaCO,, CuO and Ta,O, step by step at 800, 900 and lOOO”C, the result is identical with that of the precursor decomposition. Annealing the oxide mixture immediately at 1000°C contrary to this, a sample with a well resolved X-ray diffractogram is obtained. The difference is discussed on the basis of a favourable influence of partial melting processes. In the system Ba-Cu-0 there are eutectic compositions with melting temperatures at about 900°C. Homogeneous samples with the perovskite composition are char-
,
65
tetragonal phase
Fig. 4. X-ray powder diffractograms of annealing products of Ba-Cu-Nb-oxalate precursors of different composition, annealing at lOOO”C, 100 h.
Precursor,
sol. state
reaction,
soLstate
sol. state
~......,.........,.........,.........,.........,.........~.........,........~~ 50 60 20 30 40 2- theta-
Fig. 5. Influence
583
579-584
70
80
8OO”C&
reaction,
reaction,
90
9OO”C,,,
1 OOO”C,,,,
iOOO”C,,,,
1 OOO”C,,,,
1 OOO”C,,
100
scale
of synthesis
methode
on the X-ray powder diffractograms
of Ba,CuTa20,
(room temperature
measurement).
584
H. Lmgbein et al. I Solid State tonics 101-103
acterized by melting temperatures above 1000°C. Therefore, rapid heating of relatively inhomogeneous mixtures of educts can result in a partial melting of the sample. Molten parts then act as mineralizator for the formation of highly homogeneous products with well resolved X-ray powder diffractograms. Annealing the homogeneous and reactive freeze-dried products or at prereaction during the gradual annealing of the oxide mixtures, partial melting processes do not take place, but in the primary steps of crystallization, cluster formation is favoured resulting in samples with submicroscopically inhomogeneous distribution of Cu and Nb (Ta) on the B-sites of perovskite. Because of a high activation barrier of homogenization, the products are characterized by a remarkably high kinetic stability. By enhancement of the oxygen partial pressure over the sample the melting temperature increases. In agreement with this, the solid state reaction in an oxygen atmosphere leads to perovskites with the already discussed broad X-ray diffraction peaks, independently of the heating rate. An increase of the copper content within the homogeneity range of the perovskites lowers the melting temperature of the samples. But, for homogeneous freeze-dried samples no partial melting processes can be observed below 1000°C. Therefore, the explanation for the well resolved X-ray powder diffractograms given above is only valid in the case of a solid state reaction with a high heating rate. On the present stage of investigation, our interpretation of the experimental results is the following: because of the lower valence of copper, substitution of niobium (tantalum) by copper increases the concentration of oxygen vacancies. This should result in an influence on the cationic mobility. The well resolved tetragonal splitting of perovskite peaks of samples rich in copper coincides with a favourable influence of the oxygen vacancies on the cationic mobility. Obviously, the oxygen vacancies lower the activation barrier for the exchange of cations on the B-sites of the perovskite ABO,.
4. Conclusions Freeze-dried complex oxalates of Nb (Ta), Cu and Ba are appropriate precursors for the synthesis of
(1997) 579-584
complex niobates and tantalates at relatively low temperature. The thermal decomposition of appropriately composed precursors allows the direct synthesis of the monoclinic modification of CuNb,O,, thermodynamically stable below about 700°C and of a metastable CuTa,O, with trirutile structure. In some cases, the high homogeneity of precursors is not at all sufficient for the synthesis of desired complex oxides at relatively low temperature. Especially, this is the case, if compounds with a high kinetic stability preferentially are formed in competition to the desired compound. The advantage of the high precursor reactivity is then restricted to the non-desired compound.
Acknowledgements We would like to thank the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie (FCI) for financial support.
References [II R.C. Mehrotra, Structure and Bonding 77 (1992) 1. PI H. Langbein, C. Michalk, K. Knese. P. Eichhorn, J. Eur. Ceram. Sot. 8 (1991) 171. 131 M. Jansen, J. Bredtnauer, Z. Anorg. Allg. Chem. 578 (1989) 143. 141 E. Husson, Y. Repelin, H. Brusset, J. Solid State Chem. 33 (1980) 375. [51 H. Vincent, B. Bochu, J.J. Aubert, J.C. Joubert, M. Marezio, J. Solid State Chem. 24 (1978) 245. WI E. Wahlstrom, B.-O. Marinder, Inorg. Nucl. Chem. Lett. 13 (1977) 559. J. [71 J. Norwig, H. Weitzel, H. Paulus, G. Lautenschlager, Rodriguez-Carvajal, H. Fuess, J. Solid State Chem. 115 (1995) 476. PI P.A. Wa Ilunga, M. Sundberg, Mater. Res. Bull. 19 (1984) 807. [91 P.A. Wa Ilunga, Acta Chem. Stand. A 37 (1983) 117. [lOI A.G. Kapyshev, VV. Ivanova, Y.N. Venevaev, Dokl. Akad. Nauk SSSR 167 (1966) 564. [Ill E.G. Fesenko, W.T. Smotrakov, V.G. Eremkin, L.A. Shilkina, Neorg. Mater. 28 (1992) 2153. [ITI A. Ono, J. Mater. Sci. Lett. 11 (1992) 114.
&yg
EL..%VIER
Solid State Ionics 101-103
(1997) 579-584
CuX,O, and Ba,CuX,O, (X = Nb, Ta): influence of the preparation conditions on phase formation and phase composition H. Langbein”‘“,
M. Bremerb, I. G-abbes”
“Technische Universitiit Dresden, Institut fiir Anorganische Chemie, D-01062 Dresden, Germany hTU Bergakademie Freiberg, Institut fiir Anorganische Chemie, D-09596 Freiberg, Germany ‘Fraunhofer Institut f iir Keramische Technologien und Sinterwerkstoffe, D-01277 Dresden, Germany
Abstract The title compounds were prepared by conventional solid state reaction and by thermal decomposition of freeze-dried oxalate precursors using different temperature regimes. The thermal decomposition of an appropriately composed oxalate precursor enables the direct synthesis of pure monoclinic CuNb,O,, thermodynamically stable only at 0 5 700°C. The solid state reaction leads to the orthorhombic columbite type phase or to a mixture of the orthorhombic and monoclinic phases. A new metastable CuTa,O,-trirutile phase was synthesized by thermal decomposition of an oxalate precursor. Above 8OO”C, the decomposition results in the already known perovskite-like structure. Tetragonal perovskites Ba,Cu, +YNb,~_AO,_, 5r and Ba,Cu,+,Ta2-,%, 5, were synthesized by solid state reaction and by precursor decomposition. The phase formation and
homogeneity of the perovskites strongly depends on the reaction conditions. Keywords: Precursor; Materials:
Freeze drying; Perovskite;
Columbite;
Trirutile
CuNb?O,; CuTaz06; Ba,CuNb,O,; BalCuTaZO,
1. Introduction The use of wet chemical processes to prepare simple or complex oxides increasingly gained in interest during the last two or three decades. Preparative techniques such as sol-gel processing [l], thermal decomposition of molecular precursors [2] and coprecipitation [3] offer potential advantages over the conventional high temperature techniques. In the field of ceramic materials, chemical preparation of powders offers possibilities in the tailoring of *Corresponding author. Fax: [email protected]
+ 49-351
463
7287;
e-mail:
0167.2738/97/$17,00 0 1997 Elsevier Science B.V. All rights reserved P/I SO167-2738(97)00382-2
materials which result in high purity, high homogeneity and lower processing temperatures. Scientific research is also directed towards the preparation of new materials, which often cannot be obtained using classical synthesis methods. In many cases, metastable phases are obtainable. To get information concerning the phase formation of the title compounds during synthesis in dependence on the preparation conditions, we investigated the solid state reaction in comparison to the thermal decomposition of freeze-dried complex carboxylates. We succeeded in showing that some equalities exist, but also characteristic differences in the reaction behaviour of niobates and tantalates. The same is
580
H. Lmgbein
rt al. I Solid State lonics 101-103
valid for the comparison between solid state reaction and thermal decomposition of molecular precursors.
2. Experimental In the solid state reaction experiments, Nb,O,(Ta,O,), CuO and BaCO, in the appropriate ratios are mixed thoroughly by ball milling in propanol for three hours. After drying the slurry, the reaction is carried out at temperatures of SOO1000°C (reaction time see below). Complex carboxylate solutions were prepared by mixing solutions of ammonium-oxo-tris-oxalato-niobate(oxalato-tantalate) and ammonium-oxalato-cuprate (metal:oxalate ratio of 1:3, total metal concentration 0.01 . . 0.13 mol 1-l for niobates and I 0.01 mol 1-l for tantalates). In the case of perovskite preparation, an appropriate amount of barium acetate was added. To avoid precipitation in barium and tantalum containing solutions, an additional presence of H,O, (0.3 mol I-‘) is required. The resulting solutions were immediately frozen with liquid nitrogen and dried using an Alpha 2-4 freeze-dryer (Christ). The thermal decomposition was investigated by means of a Netzsch thermal analyzer STA 409, coupled with a mass spectrometer QMS 125 (Balzers). X-Ray diffraction was performed using a D5000 powder diffractometer (Siemens) with Cu Ka radiation (step size 20 = 0.02”, step time 2 s, measurements at room temperature).
3. Results and discussion 3.1. CuNb?O, and CuTa,O, Many complex niobates MNb,O, crystallize with columbite structure, the analogous tantalates with columbite or trirutile structures [4]. An exception is CuTa,O, with a perovskite-like structure (orthorhombic pseudocubic symmetry with four molecules per unit cell, space group Pmmm) [5]. Three copper cations are located in the middle of the cubic edges while the fourth is statistically distributed over the three face centers. Besides the orthorhombic columbite type phase of CuNb,O,, which is usually formed by solid state reaction, a monoclinic form can be obtained by decomposition of CuNbO, in air
(1997) 579-584
at 700°C [6]. Orthorhombic and monoclinic forms differ in the kind of orientation and in the degree of distortion of the CuO,-octahedra [7]. With regard to the direct synthesis of monoclinic CuNb,O, we have decomposed a freeze-dried complex oxalate with a Cu:Nb ratio of 1:2. Fig. la shows the result of thermal analysis. Fig. lb illustrates the corresponding mass spectroscopic analysis of the gaseous decomposition products CO, and H,O, as an example. The decomposition occurs in two clearly separated endothermic steps. First, hydrate water is eliminated below 200°C. Further decomposition takes place between 220 und 320°C giving rise to CO, CO,, H,O, NH,, HCOOH and HCN. From the investigation of single oxalates, it is known that after dehydration the decomposition process starts with the niobium complex (0 > 200°C) followed by the superimposed decomposition of the copper complex (0 > 250°C). The endothermic peak at about 300°C is connected with the decomposition of the intermediately formed oxamide, eliminating HCN in addition to already specified decomposition products. The phase formation was investigated by X-ray powder diffractometry. Decomposing at 300-400°C an amorphous product results. Above 400°C crystallization processes take place. Thermal treatment at 600°C leads to a single phase monoclinic CuNb,O,. Subsequent annealing at 800 or 900°C brings about a partial conversion into the orthorhombic columbite structure. Complete conversion needs a long annealing time at 0 > 900°C. In accordance with the solid state reaction, the direct decomposition of the freezedried precursor at 900°C (3 h) leads to the pure orthorhombic form. To get information about the reversibility of the conversion, the directly formed orthorhombic product was annealed at 700°C (3 h). This results in a partial retransformation into the monoclinic form, but even after a very long annealing time (72 h) the conversion remains incomplete. The results correspond to a thermodynamically stable monoclinic form of CuNb,O, below 700°C and a thermodynamically stable orthorhombic form above 700°C with a large activation barrier of interconversion. The decomposition of an appropriately composed complex Cu-Ta-oxalate (Cu:Ta = 1:2) at 400°C also results in an amorphous oxide. The decomposition process sequence is analogous. The X-ray powder diffractograms of the solid decomposition products
H. Langbein et al. I Solid State Ionics 101-103 (1997) 579-584
TG/%
DTG I % min-’
rel. Intensity, m=18
1
OS5(a)
rel. Intensity, m=44
(W
bl
-
G
TG
I’
-1,d
0
581
1 200
I 400
I 800
---____I
.
200
e
0l”C Fig. I. Thermal analysis of the CuNb20,-oxalate spectroscopic analysis of decomposition products
:I m=44 .I._____,_. ;:\---- _,-I1 400 800 800
WC precursor. Heating rate, 5 K min H,O (m = 18) and CO2 (m = 44).
are shown in Fig. 2. After annealing at 600°C the sample is still nearly amorphous. Thermal treatment at 700°C (24 h) leads to the crystallization of the already known perovskite phase. Moreover, a hitherto unknown phase T and some peaks of CuTa,,O,, [8] (L) are observed. After annealing at 1000°C nearly single phase copper tantalate with perovskite-like structure is formed. The traces of CuTa,00Z6 refer to an exact stoichiometry Cu,,,,Ta,,,,O,, determined by Wa Ilunga [9]. Increasing the Cu portion in the precursor the portion of phase T after annealing at 700°C is increased. Decomposition of a precursor with a Cu/Ta ratio of 4:l results in a sample only containing CuO and phase T. After dissolving CuO in hydrochloric acid, a crystalline yellow powder remains which is isotypical with the trirutile NiTa,O,. A Cu/Ta ratio of I:2 was determined by chemical analysis. Fig. 3a shows the X-ray powder diffractogram of this sample. Annealing the yellow sample at 1000°C results in a
’, argon. (a) DTA-, TG- and DTG-curves,
(b) mass
complete conversion of the trirutile sample to a green sample with the perovskite-like structure and, in agreement with the small difference in composition, some CuTa,,O,, is formed simultaneously (Fig. 3~). The reaction seems to be irreversible. This refers to a metastable character of the trirutile CuTa,O,. Therefore, the precursor decomposition at relatively low temperature allows the crystallization of a metastable copper tantalate with trirutile structure. The favourable influence of an excess of CuO on the phase formation is hitherto not clarified. 3.2. The perovskites
BaCu,,,Nb,,,O,
and
BaCu,,,Ta,,,O,
The compounds were prepared and characterized as tetragonal perovskites first by Kapyshev et al. in 1966 [lo] and, in recent times, by Fesenko et al. [ 1 I]. According to Ono [ 121, the exact composition of the Nb containing compound should be
582
H. Langbein
et al. I Solid State Ionics 103-103
(1997) 579-584
I
1466
T,
1346
[[
1I u)
600 “C
I J_+kLLLa 13
.
LL
c)
L
,.........‘....‘.........,....,.........I....L
20
25
30
35
40
45
50
55
60
2- theta- scale Fig. 2. X-ray powder diffractograms of CuTa,-precursors, decomposed at different temperatures (room temperature measureperovskite (Cu, ,,5Ta, q2106), ments), annealing time 24 h, P T trirutile (CuTa,O,), L CuTa,,O,,.
BGu,.2Nb,.8%.7; a single phase perovskite BaCu,,,Nb,,,O, could not be obtained. In the hitherto existing investigations, only solid state reaction of mixtures of the simple or pseudobinary oxides at 0 2 1000°C was used for synthesis. With regard to a decrease in the reaction temperature and to a determination of exact composition or rather homogeneity ranges, some compositions were prepared by thermal decomposition of freeze-dried oxalate precursors. During the decomposition of Ba containing carboxylate precursors the formation of BaCO, can be expected. Surprisingly, the thermal decomposition of the precursors is finished after decomposition at 800 . 900°C (24 h). In the X-ray powder diffractograms only very weak BaCO, peaks
10 2-theta-
20
30
40
50
60
70
80
90
100
scale
Fig. 3. X-ray powder diffractograms of (a) CuTa,O, (trirutile, precursor decomposition at 7OO”C), (b) CuTa,O,, 100 h, 8OO”C, after (a), (c) perovskite and phase L, 100 h, IOOO”C, after (a) (room temperature measurement).
can be observed intermediately. The perovskite formation begins at about 600°C and is finished at about 900°C. The X-ray powder diffractograms of both, the niobate and tantalate perovskite Ba,CuNb(Ta),O,, show relatively broad peaks with insufficiently resolved tetragonal splitting. This fact is the same also after a long time of annealing the samples at 1000°C. The X-ray powder diffractogram of Ba,CuNb,O, after an annealing time of 100 h at 1000°C is shown in Fig. 4. An analogous treatment of a Ba,Cu,,,Nb, *08,, precursor (perovskite composition described by Ono [12]) results in a sample with a well resolved X-ray powder diffractogram. Further increasing the Cu/Nb-ratio results in the formation of a new phase with a hitherto unknown structure beside the perovskite (diffractogram 3 in
H. Langbein
2 Theta- Scale , r ‘I
25 l
,
,
35
45
et al. I Solid State tonics
,
55
,
101-103
(1997)
Fig. 4). At a Cu/Nb-ratio of 1:l only the peaks of this phase, for the first time described by Ono [12], are observed. The results illustrated in Fig. 4 allow us to conclude that a homogeneity range of the perovskite Ba,Cu, +xNb,_xO,_, ,5x with 0 5 x I 0.3 exists. In the case of the Ta-containing perovskite the homogeneity range is extended from x = 0 to x = 0.1. In all the other facts, the results are completely analogous. To find the reason for the insufficient resolution of the perovskite peaks of samples with 1:2 Cu/Nb(Ta) ratios, the solid state reaction was investigated systematically. Fig. 5 illustrates some results for the compound Ba,CuTa,O,. Upon preparing the perovskite samples by annealing mixtures of BaCO,, CuO and Ta,O, step by step at 800, 900 and lOOO”C, the result is identical with that of the precursor decomposition. Annealing the oxide mixture immediately at 1000°C contrary to this, a sample with a well resolved X-ray diffractogram is obtained. The difference is discussed on the basis of a favourable influence of partial melting processes. In the system Ba-Cu-0 there are eutectic compositions with melting temperatures at about 900°C. Homogeneous samples with the perovskite composition are char-
,
65
tetragonal phase
Fig. 4. X-ray powder diffractograms of annealing products of Ba-Cu-Nb-oxalate precursors of different composition, annealing at lOOO”C, 100 h.
Precursor,
sol. state
reaction,
soLstate
sol. state
~......,.........,.........,.........,.........,.........~.........,........~~ 50 60 20 30 40 2- theta-
Fig. 5. Influence
583
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70
80
8OO”C&
reaction,
reaction,
90
9OO”C,,,
1 OOO”C,,,,
iOOO”C,,,,
1 OOO”C,,,,
1 OOO”C,,
100
scale
of synthesis
methode
on the X-ray powder diffractograms
of Ba,CuTa20,
(room temperature
measurement).
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H. Lmgbein et al. I Solid State tonics 101-103
acterized by melting temperatures above 1000°C. Therefore, rapid heating of relatively inhomogeneous mixtures of educts can result in a partial melting of the sample. Molten parts then act as mineralizator for the formation of highly homogeneous products with well resolved X-ray powder diffractograms. Annealing the homogeneous and reactive freeze-dried products or at prereaction during the gradual annealing of the oxide mixtures, partial melting processes do not take place, but in the primary steps of crystallization, cluster formation is favoured resulting in samples with submicroscopically inhomogeneous distribution of Cu and Nb (Ta) on the B-sites of perovskite. Because of a high activation barrier of homogenization, the products are characterized by a remarkably high kinetic stability. By enhancement of the oxygen partial pressure over the sample the melting temperature increases. In agreement with this, the solid state reaction in an oxygen atmosphere leads to perovskites with the already discussed broad X-ray diffraction peaks, independently of the heating rate. An increase of the copper content within the homogeneity range of the perovskites lowers the melting temperature of the samples. But, for homogeneous freeze-dried samples no partial melting processes can be observed below 1000°C. Therefore, the explanation for the well resolved X-ray powder diffractograms given above is only valid in the case of a solid state reaction with a high heating rate. On the present stage of investigation, our interpretation of the experimental results is the following: because of the lower valence of copper, substitution of niobium (tantalum) by copper increases the concentration of oxygen vacancies. This should result in an influence on the cationic mobility. The well resolved tetragonal splitting of perovskite peaks of samples rich in copper coincides with a favourable influence of the oxygen vacancies on the cationic mobility. Obviously, the oxygen vacancies lower the activation barrier for the exchange of cations on the B-sites of the perovskite ABO,.
4. Conclusions Freeze-dried complex oxalates of Nb (Ta), Cu and Ba are appropriate precursors for the synthesis of
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complex niobates and tantalates at relatively low temperature. The thermal decomposition of appropriately composed precursors allows the direct synthesis of the monoclinic modification of CuNb,O,, thermodynamically stable below about 700°C and of a metastable CuTa,O, with trirutile structure. In some cases, the high homogeneity of precursors is not at all sufficient for the synthesis of desired complex oxides at relatively low temperature. Especially, this is the case, if compounds with a high kinetic stability preferentially are formed in competition to the desired compound. The advantage of the high precursor reactivity is then restricted to the non-desired compound.
Acknowledgements We would like to thank the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie (FCI) for financial support.
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