Solid State Sciences 12 (2010) 1382e1386
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New double perovskites, LaBaTaNi1xCoxO6: Structural, dielectric and magnetic studies S.L. Samal a, T. Magdaleno b, K.V. Ramanujachary b, S.E. Lofland c, A.K. Ganguli a, * a
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Department of Chemistry, Rowan University, Glassboro, NJ 08028, USA c Department of Physics, Rowan University, Glassboro, NJ 08028, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2010 Received in revised form 15 May 2010 Accepted 18 May 2010 Available online 2 June 2010
Double perovskites of the type LaBaNi1xCoxTaO6 (0 x 1) have been synthesized by solid state method. The compounds crystallize in the tetragonal space group, I4/m. Rietveld refinement has been carried out to determine the phase purity and to study the cation ordering. X-ray photoelectron spectroscopy confirms Co(II) in all compositions. The end members in LaBaNi1xCoxTaO6 show high dielectric constant values. Antiferromagnetic ordering has been observed for all the compositions and the ordering temperature in LaBaNi1xCoxTaO6 gradually decreases with increase in Co doping, which has been attributed to the decrease in covalence of Co/NiO bonds. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskites Rietveld refinement Dielectric properties
1. Introduction The double perovskites of the type A2BB0 O6 are of considerable interest due to their rich structural and physical properties. Ordering of the transition metal ions (B and B0 ) at the B-site of the double perovskite structure (A2BB0 O6) plays a crucial role in determining their properties. It has been established that significant difference in charge and size between the B and B0 cations facilitate the ordering at B-site. Recently, the double perovskites have attracted renewed interest due to the observation of itinerant ferrimagnetism with an ordering temperature of 420 K and a large room-temperature magnetoresistance in Sr2FeMoO6 [1,2]. The itinerant behavior and ferrimagnetism arise from a doubleexchange-type mechanism in which the electronic configuration and ordering of the transition-metal cations play a vital role [3]. Moreover, the recent discovery of magnetocapacitance and magnetoresistance near room temperature in a FM semiconductor, La2NiMnO6 [4] has motivated the search for new double perovskites with novel properties. Structurally, the 3d (B) and 4d or 5d (B0 ) transition-metal cations are ordered in an alternating (rocksalt) manner within a perovskite lattice. Electronically, the 3d cation has a large spin (S ¼ 2 to 5/2 for Fe2þeFe3þ) whereas the 4d/5d cation usually has S 1/2.
There are reports of several double perovskites of the type AA0 BB0 O6 where the B-sites are occupied by both a magnetic and a non-magnetic ion and the A-sites are occupied by rare-earth and alkaline-earth metals [5e12]. However, very few detailed studies have been reported in the literature [13,14] for ordered double perovskites, in which B is magnetic and B0 is nonmagnetic. Recently, Attfield et al. studied the magnetic behavior of LaACoNbO6 (A ¼ Ca, Sr and Ba) [13] where they have observed significant magnetic frustration of the Co2þ ion. A new double perovskite, SrLaMnSbO6 with antiferromagnetic ordering (TN ¼ 8 K) has also been observed [14]. Also double perovskites with d0 transition metals at B site have been synthesized [15]. The modification of structural aspect and properties of ordered double perovskite oxides, caused by the substitution of B-site cations, is of great use when trying to understand the mechanism of variation of the properties [16,17]. Here we discuss the synthesis of new double perovskites of the type, AA0 BB0 O6 (A ¼ La, A0 ¼ Ba, B ¼ Ni/Co, B0 ¼ Ta) containing a magnetic transition element and also a non-magnetic ion at Bsite. Though the Ni end member, LaBaNiTaO6 was known earlier [18], no detailed study had been carried out. Here we report systematic studies of the dielectric and magnetic properties of LaBaNi1xCoxTaO6 as functions of temperature. 2. Experimental
* Corresponding author. Fax: þ91 11 26854715. E-mail address:
[email protected] (A.K. Ganguli). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.05.014
Double perovskites with compositions of the type, LaBaNi1xCoxTaO6 (0.0 x 1.0) were synthesized by the solid-state
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Fig. 1. Powder X-ray diffraction pattern of air-annealed LaBaNi1xCoxTaO6. Fig. 3. Structure of tetragonal LaBaCoTaO6.
Table 2 Selected bond lengths (Å) and bond angles ( ) for LaBaCoTaO6.
route. Stoichiometric amount of La2O3 (Aldrich 99.9%), BaCO3 (Loba Chemie 99.0), Ta2O5 (Aldrich 99.9%), NiO (Aldrich 99.9%) and Co3O4 (Aldrich 99.9%) were thoroughly mixed and ground in an agate mortar to obtain a homogeneous mixture. The rare earth oxides were preheated at 900 C before weighing. The mixtures were loaded in a ceramic boat and fired at 950 C for 12 h. The above mixture was then further calcined at 1150 C for 24 h. To avoid the formation of higher-valent Co(III), the compounds were annealed in argon atmosphere. Powder X-ray diffraction (PXRD) data was collected a Bruker D8Advance diffractometer with a Cu-Ka source and a Ni filter. X-ray
Distance (Å)/Bond angle (deg)
hLa/BaeOi TaeO1 (x 2) TaeO2 (x 4) hTaeOi CoeO1 (x 2) CoeO2 (x 4) hCoeOi TaeO(1)eCo TaeO(2)eCo
2.861(3) 2.041(1) 2.107(2) 2.085(2) 1.995(2) 1.998(2) 1.997(2) 180 158.3(1)
data for Rietveld refinement was collected in the 2 q range of 10e90 . A step size of 0.02 and a step time of 10 s per step were used. The refinement was carried out using GSAS software [19]. The background was modeled by a shifted Chebyschev polynomial of the first kind with 12 variables and the peak profile was simulated with the pseudo-Voigt function. The refinement involved the cell parameter, scaling factor, twelve terms of background fitting, unit cell and the isotropic thermal parameters for all atoms. Energy dispersive X-ray analysis data of the sintered samples were obtained with a scanning electron microscope (Carl Zeiss EVO 50 WDS electron microscope). The X-ray photoelectron spectroscopy (XPS) analysis was recorded in an ESCA instrument (VG Scientific) with Mg-Ka radiation with a total resolution of w0.9 eV at 2 109 Torr base vacuum.
Atom
Wyckoff Coordinates symbol x Y
z
La Ba Co/Ta
4d 4d 2b
0.25 0.25 0.5
Ta/Co
2a
O(1) O(2)
4e 8h
0 0 0
0.5 0.5 0.97(1)/ 0.03(1) 0 0 0 0.97(1)/ 0.03(1) 0 0 0.253(1) 1 0.209(2) 0.305(2) 0 1
Rp ¼ 5.19, Rwp ¼ 6.58, Chi square ¼ 1.44.
0.5 0.5 0
Fractional 100*Uiso occupancy 0.42(2) 0.42(2) 0.51(4) 0.63(3) 1.23(5) 1.16(3)
a (Å)
Table 1 Rietveld refinement data of LaBaCoTaO6. Crystal system: tetragonal; Space group: I4/m. Uiso is the isotropic thermal parameter of the atoms.
5.72
8.10
5.70
8.08
5.68
8.06
5.66
8.04
5.64
8.02
5.62
0.0
0.2
0.4
0.6
0.8
1.0
8.00
x Fig. 4. Variation of lattice parameters of LaBaNi1-xCoxTaO6 with x.
c (Å)
Fig. 2. Final Rietveld plots showing the observed, calculated and the difference pattern of LaBaCoTaO6.
Bond/bond angle
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Table 3 Loaded and measured compositions of LaBaNi1xCoxTaO6 from EDX studies. Loaded composition
Measured composition
LaBaNiTaO6 LaBaNi0.8Co0.2TaO6 LaBaNi0.6Co0.4TaO6 LaBaNi0.4Co0.6TaO6 LaBaNi0.2Co0.8TaO6 LaBaCoTaO6
LaBa0.95Ni0.97Ta1.06O7.3 LaBa0.97Ni0.82Co0.17Ta1.05O7.2 LaBa0.95Ni0.59Co0.37Ta1.07O6.2 LaBa0.96Ni0.41Co0.56Ta1.06O6.8 LaBa0.94Ni0.20Co0.72Ta1.08O5.9 LaBa0.93Co1.04Ta1.13O6.2
Fig. 5. X-ray photoelectron spectra of LaBaCoTaO6 showing for Co and Ba.
For dielectric measurements, cylindrical samples of 8 mm diameter and 1e2 mm thickness were uniaxially pressed and sintered at 1150 C for 12 h. The density of the sintered disks was measured by Archimedes method and was in the range of 90e95% of the theoretical density. Then the disks were coated with silver and dried overnight. The dielectric properties were measured with an HP 4284L LCR meter in the frequency range of 50 Hz to 500 kHz and temperatures in the range of 30e300 C. The dielectric measurements were repeated twice to check the reproducibility of the data. Magnetic data were collected on a Quantum Design, Physical Property Measuring System vibrating sample magnetometer for temperatures between 5 and 300 K. Data was collected during both the cooling and the warming cycles at an applied magnetic field of 0.5 T. 3. Results and discussion All the compositions with the formula, LaBaNi1xCoxTaO6 were found to be monophasic (Fig. 1) and crystallize in the tetragonal
space group I4/m, thus forming a complete solid solution. We note that for x ¼ 0 composition (LaBaNiTaO6), a monoclinic space group (P21/n) was reported earlier [18]. Recently, Attfield et al. studied niobium based double perovskites, LaACoNbO6 (A ¼ Ba, Sr, Ca) and it was observed from the neutron diffraction studies that the Ba compound, LaBaCoNbO6, crystallizes in a tetragonal space group (I4/m) whereas the Ca and Sr analogs, LaCaCoNbO6 and LaSrCoNbO6, crystallize in a monoclinic space group (P21/n) [13]. It has been observed that smaller average size of ions at A-site leads to more distortion and favors the bbcþ three-tilt system whereas greater average size leads to less distortion and stabilizes the a0a0c one-tilt system [13]. In our compounds, the A-site has comparatively larger average size of ions and hence would lead to less distortion and crystallizes in a tetragonal space group as expected from Attfield’s reasoning [13]. Rietveld refinement studies of the X-ray data were carried out on LaBaCoTaO6 to verify the phase purity and the cation ordering. The space group I4/m was used for the refinement. All the atomic positions, structural parameters, isotropic thermal parameters (Biso) and the occupation numbers (n) were refined. The R values and c2, giving different measures of the quality of fits, are small and indicate a good quality of refinement. Rietveld analysis showed Co and Ta to be almost fully ordered with 3% inversion. The goodness of fit is illustrated in Fig. 2, showing an excellent agreement between observed and calculated patterns. The relevant crystallographic parameters are given in Table 1. The crystal structure of LaBaCoTaO6 is illustrated in Fig. 3. Selected bond lengths and bond angles are given in Table 2. The tilting angle of the (Co/Ta)O6 octahedral can be defined as (180 4)/2, where 4 is the CoeOeTa bond angle. The octahedral tilt angle as calculated from the CoeO2eTa bond angle is 10.8(1) which is slightly larger than the octahedral tilt angle, 8.1(2) , as observed for LaBaCoNbO6 [13]. The variation of lattice parameters in LaBaNi1xCoxTaO6 with cobalt concentration is presented in Fig. 4. We observe that both the a- and c-axis lattice parameters increased with Co content due to the larger ionic radius of Co2þ (VI; HS) than Ni2þ [20]. The compositional analysis of all the compounds has been carried out using EDX studies (Table 3) and shows very close match of the obtained values with the loaded compositions. The cobalt oxidation state in LaBaCoTaO6 was confirmed from X-ray photoelectron spectroscopy (XPS) studies. The core-level region (Fig. 5) shows peaks at 779.4 eV and 794.9 eV corresponding to the 2p3/2 and 2p1/2 levels, respectively, and confirms the presence of Co in the II oxidation state [21]. The oxidation state of Co and Nb in LaACoNbO6 (A ¼ Ca, Sr and Ba) has been calculated earlier from bond valence sums (BVS) [13] and was found to be II and V respectively.
Fig. 6. Frequency dependent (a) dielectric constant and (b) dielectric loss of LaBaTaCoO6 at various temperatures.
S.L. Samal et al. / Solid State Sciences 12 (2010) 1382e1386
a
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b
Fig. 7. Frequency dependent (a) dielectric constant and (b) dielectric loss at different temperatures of LaBaNiTaO6.
Fig. 8. Temperature variation of (a) dielectric constant and (b) dielectric loss of LaBaCo1xNixTaO6 compounds.
Fig. 6 shows the frequency dependence of the dielectric constant 3 and dielectric loss D of LaBaTaCoO6 at different temperatures. Both 3 and D gradually decrease with frequency and similar behavior is observed for other compositions in the series LaBaTaNi1xCoxO6. It may be noted that for air-annealed samples, we have observed very high dielectric constant at lower frequency region which could be due to the surface oxidation of Co(II). 3 and D have values of 20 and 0.037, respectively, for LaBaCoTaO6 at 500 kHz and room temperature. Fig. 7 shows the dielectric behavior of
LaBaTaNiO6 which shows similar behavior with 3 ¼ 19 and D ¼ 0.17 at 500 kHz and room temperature. Fig. 8 shows temperature variation of dielectric properties of LaBaNi1xCoxTaO6 at 100 kHz. 3 and D values increase with increasing temperature for all the compositions. However, the increase is very sharp for compositions with higher cobalt content. The highest values for 3 are observed for the end members (LaBaCoTaO6 and LaBaNiTaO6). Fig. 9(a) and (b) shows temperature variation of the magnetic susceptibility and its inverse for argon-annealed LaBaTaNi1xCoxO6
Fig. 9. Temperature variation of (a) molar susceptibility and (b) inverse molar susceptibility plots of LaBaNi1xCoxTaO6.
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increase in dielectric loss (due to conduction) in Ni-rich compositions (Fig. 8b).
Table 4 The magnetic data of LaBaNi1xCoxTaO6. Composition
LaBaTaNiO6 LaBaTaNi0.8Co0.2O6 LaBaTaNi0.6Co0.4O6 LaBaTaNi0.4Co0.6O6 LaBaTaNi0.2Co0.8O6 LaBaTaCoO6
Theoretical moment (B.M.)
Ar-annealed Magnetic moment (B.M.)
TN (K)
Magnetic moment (B.M.)
TN (K)
3.11 3.37 3.61 3.84 4.06 4.26
3.42 3.59 3.93 4.65 4.78 4.95
26.5 20 16.5 14 11 9
2.41 3.86 4.18 4.72 4.99 5.26
20.2 19.6 16.2 13.6 11 8.5
Air annealed
compounds. All the compounds show antiferromagnetic ordering, and the ordering temperature gradually decreases with increase in Co content (20e8.5 K). No antiferromagnetic ordering was reported earlier in this pure Ni compound [18], although antiferromagnetic behavior has been reported in similar double perovskites such as LaACoNbO6 (A ¼ Ca, Sr, Ba) at low temperatures (10e17 K) [13]. We have also carried out the magnetic measurement of airannealed samples which show similar behavior as Ar-annealed samples. Note that there is not much difference in the Neel temperature TN (Table 4) for air-annealed and Ar-annealed samples except for LaBaNiTaO6. Table 4 also lists the paramagnetic moment, calculated from the high-temperature region of inverse susceptibility plot. Except for the Ar-annealed Ni-based compound, the experimental magnetic moment value for all compounds is higher than the expected effective moment calculated for Ni2þ (S ¼ 1) and highspin Co2þ (S ¼ 3/2) for Lande spectroscopic factor g ¼ 2.2. This might be due to stoichiometry or perhaps could indicate the presence of a small amount of Co3þ or Ni3þ. The Weiss temperatures are negative for all compositions, as expected for antiferromagnetic ordering. The double perovskites with a non-magnetic cation at the B-site and a magnetic ion at B0 -site lead to insulating and antiferromagnetic behavior through super-exchange interactions. A decrease in TN value with increasing average A-cation size hrAi for any combination of B cations has been observed for ordered double perovskites [13]. In our compounds, we have kept average A-cation size constant and substituted at the B site. The interaction between the magnetic ions (Co/Ni) is via 90 Co/NieOeTaeOeCo/Ni exchange pathways. Increase in Ni2þ content increases the covalence, which leads to stronger exchange interaction. This could be the reason for observing an increase in ordering temperature with increase in Ni content. The stronger overlap of orbitals is also reflected in the
4. Conclusions We have successfully synthesized new double perovskites in the system La/BaeTa/Co/NieO crystallizing in the tetragonal space group (I4/m). All the compositions in LaBaNi1xCoxTaO6 show antiferromagnetic ordering, and the ordering temperature gradually increases with increase in nickel content. The increase in ordering temperature could be explained on the basis of the increase in covalence with Ni content. Acknowledgment A.K.G. thanks Department of Science and Technology for funding. S.L.S. would like to thank CSIR for a research fellowship. S.E.L. acknowledges support from NSF MRSEC DMR 0520471. K.V.R. acknowledges the support of CP-STIO program of DST, Government of India. References [1] K.L. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature (London) 395 (1998) 677. [2] J. Gopalakrishnan, A. Chattopadhyay, S.B. Ogale, T. Venkatesan, R.L. Greene, A. J. Millis, K. Ramesha, B. Hannoyer, G. Marest, Phys. Rev. B 62 (2000) 9538. [3] D.D. Sarma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 261. [4] N.S. Rogado, J. Li, A.W. Sleight, M.A. Subramanian, Adv. Mater. 17 (2005) 2225. [5] T. Nakamura, J.-H. Choy, J. Solid State Chem. 20 (1977) 233. [6] G. Blasse, J. Inorg. Nucl. Chem. 27 (1965) 993. [7] M.P. Attfield, P.D. Battle, S.K. Bollen, T.C. Gibb, R.J. Whitehead, J. Solid State Chem. 100 (1992) 37. [8] I. Fernandez, R. Greatrex, N.N. Greenwood, J. Solid Slate Chem. 32 (1980) 97. [9] M. Walewski, B. Buffat, G. Demazeau, F. Wagner, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 18 (1983) 881. [10] J.H. Choy, S.T. Hong, J. Chem. Soc. Dalton Trans. 11 (1989) 2335. [11] T. Nakarnura, T. Sata, J. Phys. Soc. Jap 30 (1971) 1501. [12] M. Yoshimura, K. Kamata, T. Nakamura, Chem. Lett. 8 (1972) 737. [13] J.W.G. Bos, J.P. Attfield, Phys. Rev. B 70 (2004) 174434. [14] T.P. Mandal, A.M. Abakumov, M.V. Lobanov, M. Croft, V.V. Poltavets, M. Greenblatt, Chem. Mater. 20 (2008) 4653. [15] A.K. Ganguli, V. Grover, M. Thirumal, Mater. Res. Bull. 36 (2001) 1967. [16] A. Maignan, B. Raveau, C. Martin, M. Hervieu, J. Solid State Chem. 144 (1999) 224. [17] D. Iwanaga, Y. Inaguma, M. Itoh, J. Solid State Chem. 147 (1999) 291. [18] I. Alvarez, M.L. Lopez, C. Gonzalez, A. Jerez, M. Luisa, C. Pico, Solid State Ionics 63e65 (1993) 609. [19] A.C. Larson, R.B. Von Dreele, General structure analysis system (GSAS), Los Alamos National Laboratory report LAUR 86e748 (2004). [20] R.D. Shannon, Acta Cryst A32 (1976) 751. [21] Handbook of X-ray Photoelectron Spectroscopy. PerkineElmer Corporation,, Eden Prairie, Minnesota, 1979, pp. 78e79.
