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Chemistry, structure and properties of bismuth copper titanate pyrochlores
SOSI-13019; No of Pages 6 Solid State Ionics xxx (2013) xxx–xxx
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Chemistry, structure and properties of bismuth copper titanate pyrochlores I.V. Piir a, M.S. Koroleva a, Yu.I. Ryabkov a, E.Yu. Pikalova b, S.V. Nekipelov c, V.N. Sivkov d, D.V. Vyalikh e a
Institute of Chemistry, Komi SC UB RAS, 167982, Pervomayskaya St.48, Syktyvkar, Russia Institute of High-Temperature Electrochemistry, UB RAS, 620990, Academicheskaya St.20, Yekaterinburg, Russia c Komi Pedagogical Institute, 167892 Syktyvkar, Russia d Department of Mathematics, KSC UB RAS, 167892, Chernova St. 3a, Syktyvkar, Russia e Technische Universitat Dresden, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 16 May 2013 Received in revised form 24 August 2013 Accepted 28 August 2013 Available online xxxx Keywords: Copper bismuth titanates Pyrochlore NEXAFS TGA Lattice disordering Electrical properties
a b s t r a c t Copper containing bismuth titanates with pyrochlore type structure were obtained by ceramic procedure over a wide range of compositions. The results of measurement of pycnometric density of the pyrochlores and of X-ray powder diffraction structure refinement point to the preference for copper atoms to occupy the Bi3+-sites. TGA study of a series of synthesized compounds indicates a reversible reduction of the copper (2) to copper (1). The NEXAFS Cu2p spectra of bismuth copper titanate pyrochlores point to copper being mainly in the oxidation state +2 in these solid solutions. Electric properties were studied for single phase pyrochlores based on bismuth titanates. The results of investigations show the presence of oxygen ionic conductivity in these samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been considerable interest in bismuth niobate and bismuth titanate cubic pyrochlore phases owing to their unusual crystal chemistry that has led to their designation as so-called “misplaced-displacive” pyrochlores [1–9]. The relatively high dielectric constants and low dielectric losses over a considerable range of frequencies around room temperature coupled with their relatively low sintering temperatures provide a variety of possibilities for practical applications of these materials [1–5]. Careful phase analysis studies of Bi2(MxNb2 − x)O7, where M — M+ 2 or M+ 3, show that they are significantly Bi-deficient with respect to their ideal stoichiometries and require the presence of small M2 + or M3 + cations on the A- and the B-sites of the ideal A2B2O6O′ pyrochlore type structure [4–6]. Stoichiometric Bi2Ti2O7 and Bideficient pyrochlores are usually synthesized at T b 650 °C. Hector and co-worker [7] obtained stoichiometric pyrochlore Bi2Ti2O7 by the co-precipitation procedure and found that the structure was stable only in the narrow temperature range 540–560 °C. Later the group of Nino [10] modified the phase diagram of Bi2Ti2O7 presented earlier [11], substantiated the thermodynamic instability of bismuth titanate-pyrochlore, and showed that Bi2Ti2O7 and bismuth deficient compound Bi1.34Ti2O7 decomposed to Bi4Ti3O12 and E-mail address: [email protected] (I.V. Piir).
Bi2Ti4O7 at T N 650 °C by the peritectic reaction [10]. Introduction of 3d metal atoms that are capable of occupying A and B sites of A2B2O7 pyrochlore results in an increase in the thermal stability of metal-doped bismuth titanates and a wide set of useful properties, depending on the nature and quantity of the doping metal. Heterovalent substitution in both cation sublattices along with the specified deficit of bismuth in comparison with the ideal composition A2B2O7 promotes occurrence of vacancies in the positions of the mobile oxygen (O′) and is likely to contribute to an appearance of ionic (oxygen) conductivity. In a recent study of manganese– bismuth titanate pyrochlore we have synthesized a broad range of solid solutions by solid state reaction method at the temperatures 1000–1150 °C [12]. These temperatures are much lower than the synthesis temperatures of the titanates of rare earth elements Ln2Ti2O7 with pyrochlore structure, some of them exhibit hightemperature ionic conductivity [13–15]. Copper-doped bismuth titanates with a pyrochlore structure have been studied and discussed in this work. 2. Experimental The series of samples with compositions Bi2O3:CuO:TiO2 as y:x:2, where y varies from 0.6 to 1 and x — in the range of 0.01–1, were prepared by solid state reaction method. The ground powders were calcined at 650 °C for 6 h then pressed into disks 10 mm in diameter
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Please cite this article as: I.V. Piir, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.08.041
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I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx
10000
* Bi 4Ti 3O12 8000
I
6000
x
0.70 0.61
4000
0.58 0.44
2000 111
*
0 10
** 20
113 222 004 133
*
30
*
333
**
40
044 135
*
0.08
226
*
50
0.06 60
2 /o
10000
* Bi 4Ti 3O12 8000
I
6000
x
0.70 0.61
4000
0.58 0.44
2000 111
*
0 10
** 20
113 222 004 133
*
30
*
40
333
**
044 135
* 50
0.08
226
*
was measured for a number of samples and compared with calculated density. The samples were examined by X-ray diffraction method by using Shimadzu XRD-6000 diffractometer, and lattice parameter refinement was performed with the Rietveld refinement program FullProf [17]. Patterns were recorded under ambient conditions using CuKα emission over the range of 10–80° with 0.050 step and 2 s count time. DSC, TGA measurements were made by NETZSCH STA 409 PC/PG equipment. The electrical conductivity measurements were carried out by employing the four-point dc technique in air in range 450–900 °C. As an electrode Pt wires (0.1 mm in diameter) were applied in four points across the sample 25 × 5 × 5 in size, following covering by Pt paste and sintering 1 h at 900 °C for a better contact. The dependence of conductivity on oxygen partial pressure pO2 was measured at 750 °C in range 10−18–0.21 atm on special experimental set with automatically varied and controlled pO2 by means of Zirconia-318 connected with electrochemical pump and sensor on the base of YSZ [18]. The NEXAFS (near-edge X-ray absorption fine structure) of the Cu2p-absorption spectrum of bismuth copper titanate pyrochlores and copper oxides was obtained using synchrotron radiation from the Russian–German beamline at BESSY-II (Berlin) device [19]. All the spectra were recorded in a total electron yield (TEY) mode [20]. Scanning electron microscopy was performed using VEGA TESCAN 35 BU microscope to characterize the microstructure of the synthesized compounds. 3. Results and discussion
0.06 60
3.1. Structural properties
2 /o Fig. 1. X-ray patterns of copper containing bismuth titanate Bi1.6CuxTi2O7 − δ. The most intense reflections of Bi4Ti3O12 impurity are marked (*).
after another thorough grinding and sintered in air at 850 °C (10 h) and, then, at 950 °C (10 h). The samples had a small amount of the second phase (Bi4Ti3O12) after calcinated at 850 °C. The content of metals in the obtained single phase pyrochlores was identified by the method of atom-emission spectroscopy (ICP). The pycnometric density
Solid solutions of bismuth copper titanate pyrochlore were obtained by ceramic technique at 950 °C (the temperature of the final high temperature treatment). Different starting compositions of oxides (yBi2O3:xCuO:2TiO2) were used. For equimolar amounts of bismuth and titanium (y = 1) the single-phase solid solutions were obtained for the amount of copper oxide (x) varying in the range of 0.1 ≤ x ≤ 0.5. For bismuth deficient composition (y b 1) the single-phase solid solutions pyrochlores were obtained at y N 0.65 and 0.1 ≤ x ≤ 0.8. In case of y N 0.8 and x b 0.08 along with pyrochlore phase Bi4Ti3O12 was detected. The X-ray diffraction patterns of some of the synthesized
Fig. 2. SEM images of the powders Bi1.61Cu0.08Ti2O6.50 (a) and Bi1.55Cu0.61Ti2O6.93 (b).
Please cite this article as: I.V. Piir, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.08.041
I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx
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3.2. TG and DSC studies of copper bismuth titanate pyrochlores Using differential scanning calorimetry (DSC) and thermogravimetry (TG) it was revealed that a part of copper atoms in copper bismuth titanate compounds with the pyrochlore-type structure is reversibly reduced from Cu2+ to Cu1+ at 910–960 °C. The percentages of the reduced atoms were determined from the weight loss of the samples in the temperature range of 910–960 °C (Fig. 5) in accordance with the reaction (1) [23,24]. 1000−1050 C 1 2СuO → Cu2 O þ O2 ΔH ¼ 75:5 kJ=mol 2
Fig. 3. NEXAFS Cu2p spectra of the copper bismuth titanate pyrochlore samples and the copper oxides.
compounds are shown in Fig. 1. The microstructures of obtained specimens Bi1.61Cu0.08Ti2O6.50 and Bi1.55Cu0.61Ti2O6.93 are shown in Fig. 2. The NEXAFS Cu2p-spectra of copper bismuth titanate pyrochlores and copper oxide analysis (Fig. 3) pointed to the oxidation state of the studied atoms mainly as Cu2+ in bismuth copper titanates. The spectra of copper oxides CuO and Cu2O obtained in our study correspond to the Cu2p-spectra studied earlier [21,22]. The comparison of pycnometric density of the ceramic samples with the theoretical one calculated from formula content with copper oxidation state +2 and unit cell parameters were made to clarify the distribution of copper atoms over the A- and B-sites in the pyrochlore structure. The results of pycnometric density measurements are given in Table 1. The comparison of the experimental density with those calculated for different versions of the copper distribution suggests preference for complete occupation of the vacant positions of bismuth (A-sites) by Cu2+. This suggestion was accepted as a basis for calculating the nominal pyrochlore formula of the solid solution with the possibility of the copper atom distribution in different cation crystallographic positions. The comparison of pycnometric and calculated density in solid solution with equal molar amounts of bismuth and titanium shows distribution of copper atoms over the both A- and B-sites. We made attempt to describe the structure of the Bi1.56Cu0.40Ti2O6.73 sample (Fig. 4) within the theoretical model reported for Bi2Ti2O7 [7]. The results of the pycnometric density measured for a number of samples and compared with theoretical density calculated from XRD-data revealed the copper distribution over A-sites or over the both cationic A- and B-sites for the samples with nearest to equal molar amounts of bismuth and titanium (sample No. 3,Table 1). Rietveld analysis was carried out based on the data of powder diffraction of X-rays (Fig. 4) using the FullProf software package [17]. The fractional occupations of the atom sites and thermal displacement of O′ were fixed. The refinement data for Bi1.56Cu0.40Ti2O6.73 are presented in Table 2.
ð1Þ
The results are presented in Table 3. As far as Bi4O′ sublattice is characterized by rather large interatomic distances and mobile oxygen atoms (O′) can be readily removed from it, it is likely that only the copper atoms which occupy A-sites are liable to reducing at T b 1000 °C. The amount of reduced copper is less than could be expected on the basis of crystal-chemical formula obtained from the comparison of experimental and calculated values of the density of the single-phase samples. 3.3. Electrical properties of copper bismuth titanate pyrochlores The dependencies of the total conductivity of two соpper bismuth titanate pyrochlores on temperature are given in Fig. 6 in comparison with another rare earth metal titanate pyrochlores. The values of total conductivity of Bi1.55Cu0.61Ti2O7 − δ are close to reported data for Yb2Ti2O7 − δ, Gd2Ti2O7 − δ,Tb2Ti2O7 − δ [13–15] and Tb2Hf2O7 − δ [16] with almost equal values of energy of activation. The value of the ionic conductivity in the investigated samples is defined by the level of the oxygen deficiency in the oxygen (O′) sublattice, which grows with an increase in copper content in agreement with the following reaction: =
••
x
2CuO→2CuBi þ V O þ 2OO :
ð2Þ
It was found that a decrease in copper content leads to an increasing Ea from 0.7 to 1.0 eV because of difficulties in oxygen ionic transport by thermal activated hopping mechanism in the structure with lower deficiency or some difference of surrounding atoms. The investigation of a dependence of total conductivity on oxygen partial pressure gives additional information about its nature. It was found that undoped A2Ti2O7 in range of intermediate temperatures (600–900 °C) is a mixed ionic–electronic conductor in the region of the low oxygen partial pressure values and an ionic conductor at the high and intermediate pO2 with relatively low values of ionic conductivity about 10−3 S cm−1 regardless of the nature of A-cation [14]. Above 900 °C remarkable p-type electronic conductivity appears in such materials as a consequence of their structural disorder. The ionic conductivity by acceptor doping in pyrochlore is increased over the order of magnitude caused by the appearance of oxygen vacancies and it becomes dominant in the whole range of pO2.
Table 1 Crystallographic formula, calculated and pycnometric density of the synthesized copper bismuth titanate pyrochlores (□ — vacancy). No.
n(Bi2O3):n(CuO):n(TiO2)
The composition
Crystallographic formula
ρt, (g/cm3)
ρp, (g/cm3)
1 2 3 4 5 6
1:0.6:2 0.8:0.6:2 0.8:0.4:2 0.8:0.5:2 0.8:0.1:2 0.7:0.6:2
Bi1.93Cu0.58Ti2O7 − δ Bi1.55Cu0.61Ti2O7 − δ Bi1.56Cu0.40Ti2O7 − δ Bi1.57Cu0.46Ti2O7 − δ Bi1.61Cu0.08Ti2O7 − δ Bi1.42Cu0.62Ti2O7 − δ
0.75:0.5:2
Bi1.52Cu0.44Ti2O7 − δ
6.89 6.29 6.15 6.69 6.29 6.57 6.44 6.61
6.75(9) 6.27(6) 6.10(5) 6.63(9) 6.26(8) 6.61(9)
7
(Bi1.67Cu0.25□0.08)(Cu0.25Ti1.75)O6O′0.53□0.47 (Bi1.41Cu0.37□0.21)(Cu0.18Ti1.82)O6O′0.58□0.42 (Bi1.44Cu0.22□0.34)(Cu0.15Ti1.85)O6O′0.23□0.77 (Bi1.56Cu0.44)(Cu0.02Ti1.98)O6O′0.72□0.28 (Bi1.61Cu0.08□0.31)Ti2O6.5 (Bi1.41Cu0.61)(Cu0.02Ti1.98)O6O′0.68□0.32 (Bi1.29Cu0.36□0.35)(Cu0.18Ti1.82)O6O′0.13□0.87 (Bi1.43Cu0.3□0.27)(Cu0.11Ti1.89)O6O′0.33□0.67
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6.56(6)
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Fig. 4. Theoretical, experimental and differential profiles of X-ray pattern of Bi1.56Cu0.40Ti2O7 − δ.
Fig. 7 shows the dependence of conductivity of the bismuth titanate pyrochlore samples with different copper contents on the partial oxygen pressure at 750 °C. The conductivity of Bi1.56Cu0.40Ti2O7 − δ and Bi1.55Cu0.61Ti2O7 − δ at the intermediate pO2 values remains constant and its level increases with Cu-content in agreement with Eq. (2) from 2.5 × 10−3 S cm−1 to 12.6 × 10−3 S cm−1. It should be noted that the total conductivity of both samples increases with a decrease of pO2 from 0.21 to 1 × 10−4 atm. It can be connected with partial reduction
of copper ions from Cu2+ to Cu1+ state by following formation of additional oxygen vacancies: =
==
••
2CuBi →2CuBi þ V O þ 1=2O2 :
ð3Þ
The dependencies of conductivity on pO2 were identical in both measurements under the pump-out and pump-in of oxygen [18]. It clearly shows that all the structural changes in pyrochlores are
Please cite this article as: I.V. Piir, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.08.041
I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx Table 2 Refined crystallographic parameters for Bi1.56Cu0.4Ti2O6.72 (Fd3m).
5
Table 3 The TG data of the copper bismuth titanate pyrochlores.
Atom
Site
x
y
z
U11 U22
U33
Frac.
Bi/Cu Ti/Cu O O′
16c 16d 48f 8a
0 1/2 1/8 1/8
0 1/2 1/8 1/8
0 1/2 0.445(2) 1/8
0.29(1) 0.08(3) 0.50(4) 0.1
0.29(1) 0.08(3) 2.67(2) 0.1
0.78/0.22 0.99/0 1.00 0.72
Thermal displacement parameters are in units of Å × 100, a = 10.3298(8) Å; Rp = 6.36%; Rwp = 8.22%; χ2 = 4.45.
No.
n(Bi2O3):n(CuO): n(TiO2)
The compositiona
T, °C
m, mg
−Δm, %
ω(Cu+1), %
1 2 3 5 6 7 8
1:0.6:2 0.8:0.6:2 0.8:0.4:2 0.8:0.5:0.2 0.7:0.6:2 0.75:0.5:2
Bi1.93Cu0.58Ti2O6.53 Bi1.55Cu0.61Ti2O6.93 Bi1.56Cu0.40Ti2O6.73 Bi1.57Cu0.46Ti2O6.82 Bi1.42Cu0.62Ti2O6.75 Bi1.52Cu0.44Ti2O6.72 CuO
912 937 941 954 940 954 950
30.23 60.95 60.20 60.05 60.40 60.05 11.80
0.20 0.26 0.14 0.18 0.19 0.20 9.22
17 31 25 50 21 19 99
a The compositions were determined by atomic emission spectroscopy and have been assumed at the titanium index 2.
reversible and these materials are stable in a wide range of oxygen partial pressures.
4. Conclusions Copper containing bismuth titanates with pyrochlore structure BiyCuxTi2O7 − δ were obtained by ceramic procedure in the concentration range 0.08 ≤ x ≤ 0.6 and 1.4 ≤ y ≤ 2. By the NEXAFS method the copper atoms were found to be in the oxidation state Cu2+. It is shown that copper atoms are distributed over two cationic positions (A, B) in the pyrochlore structure of the investigated concentration range. The ability of the copper atoms to be distributed in A-sites or in both A- and B-sites increases the stability of the pyrochlore crystal structure of the bismuth copper titanates, causes the formation of solid solutions in a rather wide range of compositions. Thus compounds may be formed with partially vacant A (bismuth)-sites. A part of copper atoms reversibly reduce to Cu1+ at 930–960 °C. It is more likely that such reduction relates to the copper ions substituted on A-site. Distribution of copper atoms over both bismuth (A) and titanium (B) sites along with a deficit of the bismuth amount relative to the stoichiometric molar ratio in compounds leads to a higher degree of oxygen deficiency associated with mobile oxygen ions (O′) and causes the ionic (oxygen) conductivity. There is mixed conductivity in copper bismuth titanate pyrochlores. The dependence of the total conductivity of the samples in the temperature region T N 450 °C obeys Arrhenius law. The activation energy of conductivity in this temperature range corresponds to the activation energy of oxygen ion
conductivity in oxide ionic conductors. Below 1 × 10−5 atm the conductivity of Bi1.55Cu0.61Ti2O7 − δ and Bi1.56Cu0.40Ti2O7 − δ remains constant and equal to the 12.6 × 10−3 S cm−1 and 2.5 × 10−3 S cm−1, respectively. Obviously, these values correspond to the ionic (oxygen) conductivity of the compounds. Acknowledgments The financial support of the Russian Foundation for Basic Research (project No. 13-03-00132 and No. 12-02-00088), UB RAS (Program 12M 23 2052) and the bilateral program “Russian–German laboratory at BESSY” are acknowledged. References [1] R.L. Withers, T.R. Welberry, A.K. Larsson, Y. Liu, L. Noren, H. Rundlof, F.J. Brink, J. Solid State Chem. 177 (2004) 231–244. [2] Hai B. Nguyen, L. Norén, Y. Liua, Ray L. Withersa, X. Weib, Margaret M. Elcombec, J. Solid State Chem. 180 (2007) 2558–2565. [3] B. Nguyen, Y. Liu, R.L. Withers, J. Solid State Chem. 180 (2007) 549–557. [4] T.A. Vanderah, M.W. Lufaso, A.U. Adler, I. Levin, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3467–3477. [5] M.W. Lufaso, T.A. Vanderah, I.M. Pazos, I. Levin, R.S. Roth, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3900–3910. [6] T.A. Vanderah, T. Siegrist, M.W. Lufaso, M.C. Yeager, J.C. Nino, S. Yates, Eur. J. Inorg. Chem. (2006) 4908–4914. [7] A.L. Hector, S.B. Wiggin, J. Solid State Chem. 177 (2004) 139. [8] D.P. Shoemaker, R. Seshdri, A.L. Hector, et al., Phys. Rev. B 81 (2010) 144113. [9] I. Radosavljevic, J.S.O. Evans, A.W. Sleight, J. Solid State Chem. 136 (1998) 63–66.
Fig. 5. TG and DSC curves of the copper bismuth titanate pyrochlore Bi1.55Cu0.61Ti2O6.93 in air.
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I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx
-1
0
Yb2Ti 2O7 Tb2Hf 2O7 Bi 1.55Cu0.61Ti 2O7Bi 1.56Cu0.40Ti 2O7-
-2
Bi1,55Cu0,61Ti 2O7Bi1,56Cu0,4Ti 2O7-
lg( /Scm-1)
lg ( /Scm-1)
-1 -3
-4
-2
-3 -5 8
9
10
11
12
13
14
-1
lg ( /Scm-1)
-2
-3
-4
-5 9
10
11
-12
-10
-8
-6
-4
-2
0
Fig. 7. The dependence of conductivity of the Bi1.55Cu0.61Ti2O7 − δ and Bi1.56Cu0.40Ti2O7 − δ samples on lg pO2 at 750 °C.
Yb2Ti 2O7 Tb2Hf 2O7 Bi 1.55Cu0.61Ti 2O7Bi 1.56Cu0.40Ti 2O7-
8
-14
lg pO2/atm
104T-1/K-1
12
13
14
104T-1/K-1 Fig. 6. The temperature dependence of total conductivity of the synthesized copper titanate bismuth pyrochlores Bi1.55Cu0.61Ti2O7 − δ, Bi1.56Cu0.40Ti2O7 − δ, Yb2Ti2O7 [13], and Tb2Hf2O7 [24].
[10] J.R. Esquivel-Elizondo, B.B. Hinojosa, J.C. Nino, Chem. Mater. 23 (2011) 4965–4974. [11] E.I. Speranskaya, I.S. Rez, L.V. Kozlova, V.M. Skorikov, V.I. Slavov, Neorg. Mater. 1 (1965) 232.
[12] I.V. Piir, V.E. Grass, N.A. Sekushin, Y.I. Ryabkov, S.V. Nekipelov, V.N. Sivkov, N.V. Chezhina, D.V. Vyalikh, J. Solid State Chem. 225 (2012) 464–470. [13] J.C.C. Abrantes, A.V. Levchenko, A.V. Shlyakhtina, L.G. Shcherbakova, A.L. Horovistiz, D.P. Fragg, J.R. Frade, Solid State Ionics 177 (2006) 1785–1788. [14] S.A. Kramer, H.L. Tuller, Solid State Ionics 82 (1995) 15–23. [15] A.V. Shlyakhtina, I.V. Kolbanev, O.K. Karyagina, L.G. Shcherbakova, Neorg. Mater. 42 (2006) 587–590. [16] A.V. Shlyakhtina, M.I. Boguslavskii, I.V. Kolbanev, A.V. Knotko, O.K. Karyagina, S.A. Borisov, L.G. Shcherbakova, Neorg. Mater. 42 (2006) 579–586. [17] J. Rodriguez-Carvajal, Physica B 19 (1993) 55. [18] E.Yu. Pikalova, V.I. Maragou, A.N. Demina, A.K. Demin, P.E. Tsiakaras, J. Power Sources 181 (2008) 199–206. [19] D.V. Vyalikh, S.I. Fedoseenko, I.E. Iossifov, R. Follath, S.A. Gorovikov, J.-S. Schmidt, S.L. Molodtsov, V.K. Adamchuk, W. Gudat, G. Kaindl, Synchrotron Radiat. News 15 (2002) 26. [20] J. Stöhr, NEXAFS Spectroscopy, Springer, Berlin, 1992. (403 pp.). [21] M. Grioni, J.B. Goedkoop, R. Schoorl, F.M.F. de Groot, J.C. Fuggle, F. Schafers, E.E. Koch, G. Rossi, J.-M. Esteva, E.C. Karnatak, Phys. Rev. B 39 (1989) 1541–1545. [22] L.H. Tjeng, C.T. Chen, S.W. Cheong, Phys. Rev. B 45 (1992) 8205–8208. [23] A. Lupu, J. Therm. Anal. Calorim. 2 (4) (1970) 445–458. [24] P.A. Korzhavyi, B. Johansson, Technical Report TR-10-30, Applied Materials Physics Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, 2010, p. 28.
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Chemistry, structure and properties of bismuth copper titanate pyrochlores I.V. Piir a, M.S. Koroleva a, Yu.I. Ryabkov a, E.Yu. Pikalova b, S.V. Nekipelov c, V.N. Sivkov d, D.V. Vyalikh e a
Institute of Chemistry, Komi SC UB RAS, 167982, Pervomayskaya St.48, Syktyvkar, Russia Institute of High-Temperature Electrochemistry, UB RAS, 620990, Academicheskaya St.20, Yekaterinburg, Russia c Komi Pedagogical Institute, 167892 Syktyvkar, Russia d Department of Mathematics, KSC UB RAS, 167892, Chernova St. 3a, Syktyvkar, Russia e Technische Universitat Dresden, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 16 May 2013 Received in revised form 24 August 2013 Accepted 28 August 2013 Available online xxxx Keywords: Copper bismuth titanates Pyrochlore NEXAFS TGA Lattice disordering Electrical properties
a b s t r a c t Copper containing bismuth titanates with pyrochlore type structure were obtained by ceramic procedure over a wide range of compositions. The results of measurement of pycnometric density of the pyrochlores and of X-ray powder diffraction structure refinement point to the preference for copper atoms to occupy the Bi3+-sites. TGA study of a series of synthesized compounds indicates a reversible reduction of the copper (2) to copper (1). The NEXAFS Cu2p spectra of bismuth copper titanate pyrochlores point to copper being mainly in the oxidation state +2 in these solid solutions. Electric properties were studied for single phase pyrochlores based on bismuth titanates. The results of investigations show the presence of oxygen ionic conductivity in these samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been considerable interest in bismuth niobate and bismuth titanate cubic pyrochlore phases owing to their unusual crystal chemistry that has led to their designation as so-called “misplaced-displacive” pyrochlores [1–9]. The relatively high dielectric constants and low dielectric losses over a considerable range of frequencies around room temperature coupled with their relatively low sintering temperatures provide a variety of possibilities for practical applications of these materials [1–5]. Careful phase analysis studies of Bi2(MxNb2 − x)O7, where M — M+ 2 or M+ 3, show that they are significantly Bi-deficient with respect to their ideal stoichiometries and require the presence of small M2 + or M3 + cations on the A- and the B-sites of the ideal A2B2O6O′ pyrochlore type structure [4–6]. Stoichiometric Bi2Ti2O7 and Bideficient pyrochlores are usually synthesized at T b 650 °C. Hector and co-worker [7] obtained stoichiometric pyrochlore Bi2Ti2O7 by the co-precipitation procedure and found that the structure was stable only in the narrow temperature range 540–560 °C. Later the group of Nino [10] modified the phase diagram of Bi2Ti2O7 presented earlier [11], substantiated the thermodynamic instability of bismuth titanate-pyrochlore, and showed that Bi2Ti2O7 and bismuth deficient compound Bi1.34Ti2O7 decomposed to Bi4Ti3O12 and E-mail address: [email protected] (I.V. Piir).
Bi2Ti4O7 at T N 650 °C by the peritectic reaction [10]. Introduction of 3d metal atoms that are capable of occupying A and B sites of A2B2O7 pyrochlore results in an increase in the thermal stability of metal-doped bismuth titanates and a wide set of useful properties, depending on the nature and quantity of the doping metal. Heterovalent substitution in both cation sublattices along with the specified deficit of bismuth in comparison with the ideal composition A2B2O7 promotes occurrence of vacancies in the positions of the mobile oxygen (O′) and is likely to contribute to an appearance of ionic (oxygen) conductivity. In a recent study of manganese– bismuth titanate pyrochlore we have synthesized a broad range of solid solutions by solid state reaction method at the temperatures 1000–1150 °C [12]. These temperatures are much lower than the synthesis temperatures of the titanates of rare earth elements Ln2Ti2O7 with pyrochlore structure, some of them exhibit hightemperature ionic conductivity [13–15]. Copper-doped bismuth titanates with a pyrochlore structure have been studied and discussed in this work. 2. Experimental The series of samples with compositions Bi2O3:CuO:TiO2 as y:x:2, where y varies from 0.6 to 1 and x — in the range of 0.01–1, were prepared by solid state reaction method. The ground powders were calcined at 650 °C for 6 h then pressed into disks 10 mm in diameter
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.08.041
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I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx
10000
* Bi 4Ti 3O12 8000
I
6000
x
0.70 0.61
4000
0.58 0.44
2000 111
*
0 10
** 20
113 222 004 133
*
30
*
333
**
40
044 135
*
0.08
226
*
50
0.06 60
2 /o
10000
* Bi 4Ti 3O12 8000
I
6000
x
0.70 0.61
4000
0.58 0.44
2000 111
*
0 10
** 20
113 222 004 133
*
30
*
40
333
**
044 135
* 50
0.08
226
*
was measured for a number of samples and compared with calculated density. The samples were examined by X-ray diffraction method by using Shimadzu XRD-6000 diffractometer, and lattice parameter refinement was performed with the Rietveld refinement program FullProf [17]. Patterns were recorded under ambient conditions using CuKα emission over the range of 10–80° with 0.050 step and 2 s count time. DSC, TGA measurements were made by NETZSCH STA 409 PC/PG equipment. The electrical conductivity measurements were carried out by employing the four-point dc technique in air in range 450–900 °C. As an electrode Pt wires (0.1 mm in diameter) were applied in four points across the sample 25 × 5 × 5 in size, following covering by Pt paste and sintering 1 h at 900 °C for a better contact. The dependence of conductivity on oxygen partial pressure pO2 was measured at 750 °C in range 10−18–0.21 atm on special experimental set with automatically varied and controlled pO2 by means of Zirconia-318 connected with electrochemical pump and sensor on the base of YSZ [18]. The NEXAFS (near-edge X-ray absorption fine structure) of the Cu2p-absorption spectrum of bismuth copper titanate pyrochlores and copper oxides was obtained using synchrotron radiation from the Russian–German beamline at BESSY-II (Berlin) device [19]. All the spectra were recorded in a total electron yield (TEY) mode [20]. Scanning electron microscopy was performed using VEGA TESCAN 35 BU microscope to characterize the microstructure of the synthesized compounds. 3. Results and discussion
0.06 60
3.1. Structural properties
2 /o Fig. 1. X-ray patterns of copper containing bismuth titanate Bi1.6CuxTi2O7 − δ. The most intense reflections of Bi4Ti3O12 impurity are marked (*).
after another thorough grinding and sintered in air at 850 °C (10 h) and, then, at 950 °C (10 h). The samples had a small amount of the second phase (Bi4Ti3O12) after calcinated at 850 °C. The content of metals in the obtained single phase pyrochlores was identified by the method of atom-emission spectroscopy (ICP). The pycnometric density
Solid solutions of bismuth copper titanate pyrochlore were obtained by ceramic technique at 950 °C (the temperature of the final high temperature treatment). Different starting compositions of oxides (yBi2O3:xCuO:2TiO2) were used. For equimolar amounts of bismuth and titanium (y = 1) the single-phase solid solutions were obtained for the amount of copper oxide (x) varying in the range of 0.1 ≤ x ≤ 0.5. For bismuth deficient composition (y b 1) the single-phase solid solutions pyrochlores were obtained at y N 0.65 and 0.1 ≤ x ≤ 0.8. In case of y N 0.8 and x b 0.08 along with pyrochlore phase Bi4Ti3O12 was detected. The X-ray diffraction patterns of some of the synthesized
Fig. 2. SEM images of the powders Bi1.61Cu0.08Ti2O6.50 (a) and Bi1.55Cu0.61Ti2O6.93 (b).
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I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx
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3.2. TG and DSC studies of copper bismuth titanate pyrochlores Using differential scanning calorimetry (DSC) and thermogravimetry (TG) it was revealed that a part of copper atoms in copper bismuth titanate compounds with the pyrochlore-type structure is reversibly reduced from Cu2+ to Cu1+ at 910–960 °C. The percentages of the reduced atoms were determined from the weight loss of the samples in the temperature range of 910–960 °C (Fig. 5) in accordance with the reaction (1) [23,24]. 1000−1050 C 1 2СuO → Cu2 O þ O2 ΔH ¼ 75:5 kJ=mol 2
Fig. 3. NEXAFS Cu2p spectra of the copper bismuth titanate pyrochlore samples and the copper oxides.
compounds are shown in Fig. 1. The microstructures of obtained specimens Bi1.61Cu0.08Ti2O6.50 and Bi1.55Cu0.61Ti2O6.93 are shown in Fig. 2. The NEXAFS Cu2p-spectra of copper bismuth titanate pyrochlores and copper oxide analysis (Fig. 3) pointed to the oxidation state of the studied atoms mainly as Cu2+ in bismuth copper titanates. The spectra of copper oxides CuO and Cu2O obtained in our study correspond to the Cu2p-spectra studied earlier [21,22]. The comparison of pycnometric density of the ceramic samples with the theoretical one calculated from formula content with copper oxidation state +2 and unit cell parameters were made to clarify the distribution of copper atoms over the A- and B-sites in the pyrochlore structure. The results of pycnometric density measurements are given in Table 1. The comparison of the experimental density with those calculated for different versions of the copper distribution suggests preference for complete occupation of the vacant positions of bismuth (A-sites) by Cu2+. This suggestion was accepted as a basis for calculating the nominal pyrochlore formula of the solid solution with the possibility of the copper atom distribution in different cation crystallographic positions. The comparison of pycnometric and calculated density in solid solution with equal molar amounts of bismuth and titanium shows distribution of copper atoms over the both A- and B-sites. We made attempt to describe the structure of the Bi1.56Cu0.40Ti2O6.73 sample (Fig. 4) within the theoretical model reported for Bi2Ti2O7 [7]. The results of the pycnometric density measured for a number of samples and compared with theoretical density calculated from XRD-data revealed the copper distribution over A-sites or over the both cationic A- and B-sites for the samples with nearest to equal molar amounts of bismuth and titanium (sample No. 3,Table 1). Rietveld analysis was carried out based on the data of powder diffraction of X-rays (Fig. 4) using the FullProf software package [17]. The fractional occupations of the atom sites and thermal displacement of O′ were fixed. The refinement data for Bi1.56Cu0.40Ti2O6.73 are presented in Table 2.
ð1Þ
The results are presented in Table 3. As far as Bi4O′ sublattice is characterized by rather large interatomic distances and mobile oxygen atoms (O′) can be readily removed from it, it is likely that only the copper atoms which occupy A-sites are liable to reducing at T b 1000 °C. The amount of reduced copper is less than could be expected on the basis of crystal-chemical formula obtained from the comparison of experimental and calculated values of the density of the single-phase samples. 3.3. Electrical properties of copper bismuth titanate pyrochlores The dependencies of the total conductivity of two соpper bismuth titanate pyrochlores on temperature are given in Fig. 6 in comparison with another rare earth metal titanate pyrochlores. The values of total conductivity of Bi1.55Cu0.61Ti2O7 − δ are close to reported data for Yb2Ti2O7 − δ, Gd2Ti2O7 − δ,Tb2Ti2O7 − δ [13–15] and Tb2Hf2O7 − δ [16] with almost equal values of energy of activation. The value of the ionic conductivity in the investigated samples is defined by the level of the oxygen deficiency in the oxygen (O′) sublattice, which grows with an increase in copper content in agreement with the following reaction: =
••
x
2CuO→2CuBi þ V O þ 2OO :
ð2Þ
It was found that a decrease in copper content leads to an increasing Ea from 0.7 to 1.0 eV because of difficulties in oxygen ionic transport by thermal activated hopping mechanism in the structure with lower deficiency or some difference of surrounding atoms. The investigation of a dependence of total conductivity on oxygen partial pressure gives additional information about its nature. It was found that undoped A2Ti2O7 in range of intermediate temperatures (600–900 °C) is a mixed ionic–electronic conductor in the region of the low oxygen partial pressure values and an ionic conductor at the high and intermediate pO2 with relatively low values of ionic conductivity about 10−3 S cm−1 regardless of the nature of A-cation [14]. Above 900 °C remarkable p-type electronic conductivity appears in such materials as a consequence of their structural disorder. The ionic conductivity by acceptor doping in pyrochlore is increased over the order of magnitude caused by the appearance of oxygen vacancies and it becomes dominant in the whole range of pO2.
Table 1 Crystallographic formula, calculated and pycnometric density of the synthesized copper bismuth titanate pyrochlores (□ — vacancy). No.
n(Bi2O3):n(CuO):n(TiO2)
The composition
Crystallographic formula
ρt, (g/cm3)
ρp, (g/cm3)
1 2 3 4 5 6
1:0.6:2 0.8:0.6:2 0.8:0.4:2 0.8:0.5:2 0.8:0.1:2 0.7:0.6:2
Bi1.93Cu0.58Ti2O7 − δ Bi1.55Cu0.61Ti2O7 − δ Bi1.56Cu0.40Ti2O7 − δ Bi1.57Cu0.46Ti2O7 − δ Bi1.61Cu0.08Ti2O7 − δ Bi1.42Cu0.62Ti2O7 − δ
0.75:0.5:2
Bi1.52Cu0.44Ti2O7 − δ
6.89 6.29 6.15 6.69 6.29 6.57 6.44 6.61
6.75(9) 6.27(6) 6.10(5) 6.63(9) 6.26(8) 6.61(9)
7
(Bi1.67Cu0.25□0.08)(Cu0.25Ti1.75)O6O′0.53□0.47 (Bi1.41Cu0.37□0.21)(Cu0.18Ti1.82)O6O′0.58□0.42 (Bi1.44Cu0.22□0.34)(Cu0.15Ti1.85)O6O′0.23□0.77 (Bi1.56Cu0.44)(Cu0.02Ti1.98)O6O′0.72□0.28 (Bi1.61Cu0.08□0.31)Ti2O6.5 (Bi1.41Cu0.61)(Cu0.02Ti1.98)O6O′0.68□0.32 (Bi1.29Cu0.36□0.35)(Cu0.18Ti1.82)O6O′0.13□0.87 (Bi1.43Cu0.3□0.27)(Cu0.11Ti1.89)O6O′0.33□0.67
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6.56(6)
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Fig. 4. Theoretical, experimental and differential profiles of X-ray pattern of Bi1.56Cu0.40Ti2O7 − δ.
Fig. 7 shows the dependence of conductivity of the bismuth titanate pyrochlore samples with different copper contents on the partial oxygen pressure at 750 °C. The conductivity of Bi1.56Cu0.40Ti2O7 − δ and Bi1.55Cu0.61Ti2O7 − δ at the intermediate pO2 values remains constant and its level increases with Cu-content in agreement with Eq. (2) from 2.5 × 10−3 S cm−1 to 12.6 × 10−3 S cm−1. It should be noted that the total conductivity of both samples increases with a decrease of pO2 from 0.21 to 1 × 10−4 atm. It can be connected with partial reduction
of copper ions from Cu2+ to Cu1+ state by following formation of additional oxygen vacancies: =
==
••
2CuBi →2CuBi þ V O þ 1=2O2 :
ð3Þ
The dependencies of conductivity on pO2 were identical in both measurements under the pump-out and pump-in of oxygen [18]. It clearly shows that all the structural changes in pyrochlores are
Please cite this article as: I.V. Piir, et al., Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.08.041
I.V. Piir et al. / Solid State Ionics xxx (2013) xxx–xxx Table 2 Refined crystallographic parameters for Bi1.56Cu0.4Ti2O6.72 (Fd3m).
5
Table 3 The TG data of the copper bismuth titanate pyrochlores.
Atom
Site
x
y
z
U11 U22
U33
Frac.
Bi/Cu Ti/Cu O O′
16c 16d 48f 8a
0 1/2 1/8 1/8
0 1/2 1/8 1/8
0 1/2 0.445(2) 1/8
0.29(1) 0.08(3) 0.50(4) 0.1
0.29(1) 0.08(3) 2.67(2) 0.1
0.78/0.22 0.99/0 1.00 0.72
Thermal displacement parameters are in units of Å × 100, a = 10.3298(8) Å; Rp = 6.36%; Rwp = 8.22%; χ2 = 4.45.
No.
n(Bi2O3):n(CuO): n(TiO2)
The compositiona
T, °C
m, mg
−Δm, %
ω(Cu+1), %
1 2 3 5 6 7 8
1:0.6:2 0.8:0.6:2 0.8:0.4:2 0.8:0.5:0.2 0.7:0.6:2 0.75:0.5:2
Bi1.93Cu0.58Ti2O6.53 Bi1.55Cu0.61Ti2O6.93 Bi1.56Cu0.40Ti2O6.73 Bi1.57Cu0.46Ti2O6.82 Bi1.42Cu0.62Ti2O6.75 Bi1.52Cu0.44Ti2O6.72 CuO
912 937 941 954 940 954 950
30.23 60.95 60.20 60.05 60.40 60.05 11.80
0.20 0.26 0.14 0.18 0.19 0.20 9.22
17 31 25 50 21 19 99
a The compositions were determined by atomic emission spectroscopy and have been assumed at the titanium index 2.
reversible and these materials are stable in a wide range of oxygen partial pressures.
4. Conclusions Copper containing bismuth titanates with pyrochlore structure BiyCuxTi2O7 − δ were obtained by ceramic procedure in the concentration range 0.08 ≤ x ≤ 0.6 and 1.4 ≤ y ≤ 2. By the NEXAFS method the copper atoms were found to be in the oxidation state Cu2+. It is shown that copper atoms are distributed over two cationic positions (A, B) in the pyrochlore structure of the investigated concentration range. The ability of the copper atoms to be distributed in A-sites or in both A- and B-sites increases the stability of the pyrochlore crystal structure of the bismuth copper titanates, causes the formation of solid solutions in a rather wide range of compositions. Thus compounds may be formed with partially vacant A (bismuth)-sites. A part of copper atoms reversibly reduce to Cu1+ at 930–960 °C. It is more likely that such reduction relates to the copper ions substituted on A-site. Distribution of copper atoms over both bismuth (A) and titanium (B) sites along with a deficit of the bismuth amount relative to the stoichiometric molar ratio in compounds leads to a higher degree of oxygen deficiency associated with mobile oxygen ions (O′) and causes the ionic (oxygen) conductivity. There is mixed conductivity in copper bismuth titanate pyrochlores. The dependence of the total conductivity of the samples in the temperature region T N 450 °C obeys Arrhenius law. The activation energy of conductivity in this temperature range corresponds to the activation energy of oxygen ion
conductivity in oxide ionic conductors. Below 1 × 10−5 atm the conductivity of Bi1.55Cu0.61Ti2O7 − δ and Bi1.56Cu0.40Ti2O7 − δ remains constant and equal to the 12.6 × 10−3 S cm−1 and 2.5 × 10−3 S cm−1, respectively. Obviously, these values correspond to the ionic (oxygen) conductivity of the compounds. Acknowledgments The financial support of the Russian Foundation for Basic Research (project No. 13-03-00132 and No. 12-02-00088), UB RAS (Program 12M 23 2052) and the bilateral program “Russian–German laboratory at BESSY” are acknowledged. References [1] R.L. Withers, T.R. Welberry, A.K. Larsson, Y. Liu, L. Noren, H. Rundlof, F.J. Brink, J. Solid State Chem. 177 (2004) 231–244. [2] Hai B. Nguyen, L. Norén, Y. Liua, Ray L. Withersa, X. Weib, Margaret M. Elcombec, J. Solid State Chem. 180 (2007) 2558–2565. [3] B. Nguyen, Y. Liu, R.L. Withers, J. Solid State Chem. 180 (2007) 549–557. [4] T.A. Vanderah, M.W. Lufaso, A.U. Adler, I. Levin, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3467–3477. [5] M.W. Lufaso, T.A. Vanderah, I.M. Pazos, I. Levin, R.S. Roth, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3900–3910. [6] T.A. Vanderah, T. Siegrist, M.W. Lufaso, M.C. Yeager, J.C. Nino, S. Yates, Eur. J. Inorg. Chem. (2006) 4908–4914. [7] A.L. Hector, S.B. Wiggin, J. Solid State Chem. 177 (2004) 139. [8] D.P. Shoemaker, R. Seshdri, A.L. Hector, et al., Phys. Rev. B 81 (2010) 144113. [9] I. Radosavljevic, J.S.O. Evans, A.W. Sleight, J. Solid State Chem. 136 (1998) 63–66.
Fig. 5. TG and DSC curves of the copper bismuth titanate pyrochlore Bi1.55Cu0.61Ti2O6.93 in air.
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-1
0
Yb2Ti 2O7 Tb2Hf 2O7 Bi 1.55Cu0.61Ti 2O7Bi 1.56Cu0.40Ti 2O7-
-2
Bi1,55Cu0,61Ti 2O7Bi1,56Cu0,4Ti 2O7-
lg( /Scm-1)
lg ( /Scm-1)
-1 -3
-4
-2
-3 -5 8
9
10
11
12
13
14
-1
lg ( /Scm-1)
-2
-3
-4
-5 9
10
11
-12
-10
-8
-6
-4
-2
0
Fig. 7. The dependence of conductivity of the Bi1.55Cu0.61Ti2O7 − δ and Bi1.56Cu0.40Ti2O7 − δ samples on lg pO2 at 750 °C.
Yb2Ti 2O7 Tb2Hf 2O7 Bi 1.55Cu0.61Ti 2O7Bi 1.56Cu0.40Ti 2O7-
8
-14
lg pO2/atm
104T-1/K-1
12
13
14
104T-1/K-1 Fig. 6. The temperature dependence of total conductivity of the synthesized copper titanate bismuth pyrochlores Bi1.55Cu0.61Ti2O7 − δ, Bi1.56Cu0.40Ti2O7 − δ, Yb2Ti2O7 [13], and Tb2Hf2O7 [24].
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