ISSN 00201685, Inorganic Materials, 2010, Vol. 46, No. 12, pp. 1337–1340. © Pleiades Publishing, Ltd., 2010. Original Russian Text © A.I. Klyndyuk, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 12, pp. 1472–1475.
Structure and Properties of the Layered Oxide GdBaFeNiO5 + δ A. I. Klyndyuk Belarussian State Technological University, ul. Sverdlova 13a, Minsk, 220050 Belarus email:
[email protected] Received August 20, 2009
Abstract—GdBaFeNiO5+δ has been synthesized, and its crystal structure, thermal expansion, and electrical properties have been studied. It has a tetragonal structure (sp. gr. P4/mmm) with unitcell parameters a = 0.3910(2) nm, c = 0.7582(6) nm, and V = 115.9(2) × 10–3 nm3 (δ = 0.53) and is a ptype semiconductor. The linear thermal expansion coefficient of GdBaFeNiO5+δ is 1.32 × 10–5, 1.72 × 10–5, and 1.37 × 10–5 K–1 in the temperature ranges 340–655, 655–870, and 870–1080 K, respectively. DOI: 10.1134/S0020168510120125
INTRODUCTION RBaM'M''O5+δ (R = rareearth metal; M', M'' = 3d transition metal) layered perovskite oxides possess inter esting electrical properties, which make them potential materials for cathodes of solid oxide fuel cells [1], semi conductor gas sensors [2], and thermoelectric converters [3, 4]. The compounds of this family described to date include RBaFeCuO5+δ [3–7], RBaFeCoO5+δ [3, 4, 8, 9], and RBaCuCoO5+δ [4, 8, 10–12]. Data for RBaFeNiO5+δ are not available in the literature. This paper describes the crystal structure of the layered oxide GdBaFeNiO5+δ and its properties (thermal expan sion, electrical conductivity, and thermoelectric power) in air at temperatures from 300 to 1100 K. EXPERIMENTAL GdBaFeNiO5+δ was prepared by a conventional ceramic processing technique [5], using Gd2O3 (reagent grade), BaCO3 (pure grade), Fe2O3 (OSCh 24), and NiO (OSCh 102) as starting materials. Appropriate mixtures were reacted in air for 50 h at temperatures from 1170 to 1470 K. In the final step, the resultant ceramic was oxy genated by firing in air at 1170 K for 5 h. The phase composition of the sample was determined by Xray diffraction (XRD) on a Bruker D8 XRD Advance diffractometer (CuKα radiation). The IR absorption spectrum of the material was measured between 300 and 1500 cm–1 (Δν ≤ 2 cm–1) on a Ther moNicolet Nexus Fouriertransform IR spectrometer using a sample pressed with reagentgrade KBr. The con tent of weakly bound oxygen (δ) was determined iodo metrically (Δδ = ±0.01). The thermal expansion (Δl/l0, electrical conductivity (σ), and thermoelectric power (S) of the ceramic were measured in air at temperatures from 300 to 1100 K during heating and cooling at an average rate of 3–5 K/min, as described elsewhere [3, 5, 6]. The activation energies for conduction (Ea) and thermoelec tric power (E) and the linear thermal expansion coeffi
cient (α) of GdBaFeNiO5+δ were determined from the slope of linear portions in the plots
ln (σT ) = f (1 T ) , S = f (1 T ) , Δl l 0 = f (T ) (correlation coefficient R ≥ 0.999, δ(Ea, E, α) ≤ ±5%). RESULTS AND DISCUSSION After the final firing step, the GdBaFeNiO5+δ sample was phasepure by XRD and had a tetragonal structure. All of the observed XRD peaks could be indexed in space group P4/mmm (Fig. 1), with unitcell parameters a = 0.3910(2) nm, c = 0.7582(6) nm, and V = 115.9(2) × 10–3 nm3. According to iodometry results, the content of weakly bound oxygen in GdBaFeNiO5+δ was δ = 0.53. There fore, the average oxidation state of the 3d transition metal cations in this compound was 2.83+. The unitcell parameters of GdBaFeNiO5.53 correlate with those of GdBaFeCuO5.08 (a = 0.3895 nm, c = 0.7993 nm [3, 5]) and GdBaFeCoO5.37 (a = 0.3908 nm, c = 0.7613 nm [3]). The a parameter of the GdBaFeMO5+δ phases is seen to vary little, characteristic of the RBaM'M''O5+δ layered oxides [4], whereas the c parameter decreases in the order Cu > Co > Ni (opposite to the content of weakly bound oxygen, δ). The IR absorption spectrum of the GdBaFeNiO5.53 powder (Fig. 1) shows three prominent absorption bands centered at 367 (ν1), 544–577 (ν2), and 670 cm–1 (ν3), which correspond to the stretching (ν2) and bending (ν1) modes of the metal–oxygen bonds in the [(Fe,Ni)O2] planes and to the stretching mode (ν3) of the apical oxy gen in the (Fe,Ni)–O–(Fe,Ni) bonds in its structure [5, 13]. The metal–oxygen interaction in the [(Fe,Ni)O2] plane of GdBaFeNiO5.53 is seen to be weaker than that
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480 464 544 560 577 670 660 720 749
102
3
110
212 114 210 202 104
111 003 40
30
T
450 600 750 Frequency, cm–1
200 004
100
112
300
002 20
367 372
2
220 204
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1 60
70 2θ, deg
Fig. 1. (1) XRD pattern of GdBaFeNiO5.53 and (2) its IR absorption spectrum in comparison with (3) the spectrum of GdBaFeCuO5.08 [5].
Δl/l0 × 103
10
α3 = 1.37 × 10–5 K–1
8
6
α2 = 1.72 × 10–5 K–1
4
T2 = 870 K
T1 = 655 K
2 α1 = 1.32 × 10–5 K–1 0 450
600
750
900 1050 Temperature, K
Fig. 2. Relative length change as a function of temperature for GdBaFeNiO5+δ.
along the c axis (ν2 < ν3), which fits well with the XRD data for this phase (axial ratio in GdBaFeNiO5.53 c/2a = 0.9696). Comparison of the absorption spectra of the GdBaFeMO5+δ (M = Ni, Cu) phases (Fig. 1) leads us to the following conclusions: 1. The metal–oxygen interaction energy in the [(Fe, M)O2] layers is essentially independent of δ (ν 2 (M⎯ Ni) = (544 + 577)/2 = 560.5 cm–1 ≈ ν2(M– Cu) = 560 cm–1), whereas that along the c axis increases with δ (ν3(M – Сu) = 560 cm–1 < ν3(M – Ni) = 570 cm–1). 2. The splitting of the ν2 singlet (in GdBaFeCuO5.08) into a doublet (in GdBaFeNiO5.53) indicates that the metal–oxygen interaction energy in the [(Fe, Ni)O2] lay ers depends on the cation environment of the oxygen in the layers and differs for the Ni–O–Ni and Fe–O–Fe bonds, which are obviously represented by the lower (544 cm–1) and higher (577 cm–1) frequency components of the ν2 band. The Δl/l0(T) curve of GdBaFeNiO5+δ has two breaks at T1 = 655 K and T2 = 870 K, accompanied by changes in thermal expansion (Fig. 2). The observed rise in the α of GdBaFeNiO5+δ between 655 and 870 K (T1 < T < T2) is most likely associated with the active release of weakly bound oxygen in this temperature range [5, 8, 12]. There fore, the thermal expansion of GdBaFeNiO5+δ in the temperature range 655–870 K has both thermal (increased vibrational anharmonicity) and chemical (increased concentration of oxygen vacancies) contribu tions. INORGANIC MATERIALS
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STRUCTURE AND PROPERTIES OF THE LAYERED OXIDE GdBaFeNiO5 + δ σ, S/m
(а) T3 = 725 K
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S, μV/К 3
2
4 500
103 1 2
102
400 1 300
101 200 100
T4 = 690 K
100
4 3
10–1
0 400
600 800 Temperature, K
1000
400
600 800 Temperature, K
1000
Fig. 3. (a) Electrical conductivity and (b) thermoelectric power as functions of temperature for (1) GdBaFeNiO5+δ, (2) GdBaFeCuO5+δ [4], (3) GdBaFeCuO5+δ [4], and (4) GdBaCuCoO5+δ [4].
As seen in Fig. 3, GdBaFeNiO5+δ is a ptype semi conductor. Its conductivity exhibits semiconducting behavior (∂σ ∂ T > 0) below T3 = 725 K and metallic behavior (∂σ ∂ T < 0) at higher temperatures. Its ther moelectric power rises steeply starting at T4 = 690 K, which is due to the thermal dissociation of this com pound [5, 8, 12]. Comparison of the electrical properties of the GdBaM'M''O5+δ (M'M'' = FeCu, FeCo, CuCo) layered perovskite oxides [3–5, 12] leads us to the following con clusions: 1. The shape of the σ(T) curves for GdBaM'M''O5+δ is independent of the nature of the 3d transition metals, and the conductivity of GdBaFeMO5+δ near room tem perature increases in the order Cu < Co < Ni (in the same way as the content of weakly bound oxygen) (Fig. 3a). 2. The thermoelectric power S of GdBaM'M''O5+δ and its variation with temperature strongly depend on the nature of the 3d transition metals. In other words, the conductivity of the RBaM'M''O5+δ layered perovskite oxides (for a given R) depends mainly on their oxygen content, whereas their thermoelectric power is deter mined to a significant degree by the nature of the 3d tran sition metals. In the temperature range studied, the activation energy for conduction in GdBaFeNiO5 + δ is Ea = 0.231 eV, and the activation energy for thermoelectric INORGANIC MATERIALS
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power is E = 0.042 eV. In materials with polaron transport, including layered perovskite oxides [4, 8, 9, 12], the acti vation energy for thermoelectric power is the polaron excitation energy, and the polaron transport energy (W) can be determined as W = Ea − E
(W ≈ 0 for charge transport by large polarons; if W > 0, charge transport is due to smallpolaron hopping) [14]. We obtained W = 0.189 eV, which indicates that the electrical transportin in GdBaFeNiO5+σ is domi nated by small polarons. CONCLUSIONS A new layered oxide, GdBaFeNiO5.53, was synthe sized, and its crystal structure, thermal expansion, and electrical properties were studied. It has a tetragonal struc ture (sp. gr. P4/mmm) with lattice parameters a = 0.3910(2) nm and c = 0.7582(5) nm and is a ptype semi conductor. We determined parameters of electrical transport (car rier excitation and transport energies) in GdBaFeNiO5+δ and its linear thermal expansion coefficient: 1.37 × 10–5, 1.72 × 10–5, and 1.37 × 10–5 K–1 in the temperature ranges 340–655, 655–870, and 870–1080 K, respectively.
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ACKNOWLEDGMENTS This work was supported through the national inte grated research program Crystalline and Molecular Struc tures, project no. 33. REFERENCES 1. Zhou, Q., He, T., He, Q., and Ji Ya, Electrochemical Per formances of LaBaFeCuO5+x and LaBaFeCoO5+x As Potential Cathode Materials for IntermediateTempera ture Solid Oxide Fauel Cells, Electrochem. Commun., 2009, vol. 11, pp. 80–83. 2. Klyndziuk, A., Petrov, G., Kurhan, S., et al., Sensor Prop erties of Some PerovskiteLike Metal Oxides, Chem. Sens., 2004, vol. 20, suppl. B, p. 854–855. 3. Klyndyuk, A.I. and Chizhova, Ye.A., Thermoelectric Properties of the Layered Oxides LnBaCo(Cu)FeO5+δ (Ln – La, Nd, Sm, Gd), Funct. Mater., 2009, vol. 16, no. 1, pp. 17–22. 4. Klyndyuk, A.I., Chizhova, E.A., Sazanovich, N.V., and Krasutskaya, N.S., Thermoelectric Properties of Some Perovskite Oxides, Termoelektrichestvo, 2009, no. 3, pp. 72–80. 5. Klyndyuk, A.I. and Chizhova, E.A., Properties of RBaCuFeO5+δ (R = Y, La, Pr, Nd, Sm–Lu), Neorg. Mater., 2006, vol. 42, no. 5, pp. 611–622 [Inorg. Mater. (Engl. Transl.), vol. 42, no. 5, pp. 550–561]. 6. Pissas, M., Mitros, C., Kallias, G., et al., Synthesis, Ther mogravimetric and 57Fe Moessbauer Studies of the Oxy gen Deficient Perovskite REBaCuFeO5+x Series (RE = Y, Nd, Sm, Gd, Dy, Tm, Lu), Phys. C (Amsterdam, Neth.), 1992, vol. 192, pp. 35–40.
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