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Ordered double perovskite oxides, Sr2MWO6 (M = Mg, Mn, Fe, Ni), were prepared in a ... Purnendu Parhi , Shailesh Upreti , and Arunachalam Ramanan...
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Chem. Mater. 2005, 17, 2310-2316

New Route to Ordered Double Perovskites: Synthesis of Rock Salt Oxides, Li4MWO6, and Their Transformation to Sr2MWO6 (M ) Mg, Mn, Fe, Ni) via Metathesis Tapas Kumar Mandal† and J. Gopalakrishnan*,†,‡ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India, and Jawaharlal Nehru Centre for AdVanced Scientific Research, Bangalore 560 064, India ReceiVed January 12, 2005

We describe a solid-state metathesis route for the synthesis of double perovskite oxides, Sr2MWO6 (M ) Mg, Mn, Fe, Ni), that enabled us to obtain cation-ordered phases at a considerably lower temperature (∼750 °C) than what is employed in conventional synthesis of these materials. The method involves reaction of appropriate rock salt related precursor oxides of the formula Li4MWO6 with anhydrous SrCl2 in the solid state at 750 °C for 1-2 days. For this purpose, rock salt precursor oxides were designed and synthesized for the first time. Cation ordering in the Sr2MWO6 products obtained was investigated by Rietveld refinement of powder X-ray diffraction data. We believe that the strategy described here is general and could be extended for the synthesis of functional double perovskites at relatively mild conditions.

1. Introduction Double perovskite oxides, A2BB′O6 and A3BB′2O9, represent novel variations of the ABO3 perovskite structure where the B and B′ cations could be ordered in the longrange, giving rise to different superstructures.1,2 Besides the novelty of the structures, new materials properties that depend on the identity of B and B′ atoms as well as the extent of long-range order arise in double perovskites. For example, “rock salt” type ordering of B and B′ in A2BB′O6, oxides such as Sr2FeMoO6 and Ba2FeReO6, gives rise to metallic ferrimagnetism3,4 and tunneling magnetoresistance.5,6 Similarly, robust ordering of B-site cations in 1:2 double perovskites such as Ba3ZnTa2O9 makes them important technological materials for microwave telecommunication.7 B-site cation ordering in double perovskites, which is essential for the realization of novel materials properties, is governed by competing thermodynamic and kinetic considerations,8-10 rendering the synthesis of ordered materials * Corresponding author. Telephone: +91 80 2293 2537. Fax: +91 80 2360 1310. E-mail: [email protected]. † Indian Institute of Science. ‡ Jawaharlal Nehru Centre for Advanced Scientific Research.

(1) Anderson, M. T.; Greenwood, K. B.; Taylor, G. A.; Poeppelmeier, K. R. Prog. Solid State Chem. 1993, 22, 197. (2) Mitchell, R. H. PeroVskites: Modern and Ancient; Almaz Press: Thunder Bay, Canada, 2002. (3) Nakagawa, T. J. Phys. Soc. Jpn. 1968, 24, 806. (4) Sleight, A. W.; Weiher, J. F. J. Phys. Chem. Solids 1972, 33, 679. (5) Kobayashi, K.-I.; Kimura, T.; Sawada, H.; Terakura, K.; Tokura, Y. Nature 1998, 395, 677. (6) Gopalakrishnan, J.; Chattopadhyay, A.; Ogale, S. B.; Venkatesan, T.; Greene, R. L.; Millis, A. J.; Ramesha, K.; Hannoyer, B.; Marest, G. Phys. ReV. B 2000, 62, 9538. (7) Vanderah, T. A. Science 2002, 298, 1182. (8) Woodward, P.; Hoffmann, R.-D.; Sleight, A. W. J. Mater. Res. 1994, 9, 2118. (9) Davies, P. K. Curr. Opin. Solid State Mater. Sci. 1999, 4, 467. (10) Shimada, T.; Nakamura, J.; Motohashi, T.; Yamauchi, H.; Karppinen, M. Chem. Mater. 2003, 15, 4494.

a nontrivial issue. Ordering is often achieved by annealing the samples at appropriate high temperatures for long duration, where the thermodynamic and kinetic factors are optimized. Thus, synthesis of ordered Sr2AlTaO6 requires annealing at 1400-1600 °C for several hours.8 Similarly, formation of ordered double perovskites, Sr2MWO6 (M ) Mg, Mn, Fe, Ni), requires annealing at high temperatures (1200-1400 °C) for long duration (1-7 days).11-13 Cation ordering in Ba3ZnTa2O9 and related materials also involves multiple heat cycles (with intermediate grindings and repelletizing) at temperatures as high as 1575 °C and duration as long as 2 weeks.14,15 Synthesis at high temperatures for long duration could lead to volatilization of certain constituents, resulting in product inhomogenity as well.14 Accordingly, we perceived that there is a need to develop an alternate route for the synthesis of double perovskites that yields ordered phases at relatively low temperatures and short duration. Recently we described16 a metathesis-based synthesis route that enabled us to prepare a variety of ABO3 type functional perovskites, including PbTi1-xZrxO3, La1-xCaxMnO3, and BaPb1-xBixO3. The method is based on metathetical reaction of rock salt related metal oxides with appropriate metal salts, a typical example being Li2TiO3 + PbSO4 f PbTiO3 + Li2SO4. Here we describe an extension of this approach for the synthesis of ordered double perovskites, Sr2MWO6 (M (11) Steward, E. G.; Rooksby, H. P. Acta Crystallogr. 1951, 4, 503. (12) Iwanaga, D.; Inaguma, Y.; Itoh, M. Mater. Res. Bull. 2000, 35, 449. (13) (a) Azad, A. K.; Ivanov, S.; Eriksson, S.-G.; Rundlo¨f, H.; Eriksen, J.; Mathieu, R.; Svedlindh, P. J. Magn. Magn. Mater. 2001, 237, 124. (b) Azad, A. K.; Eriksson, S.-G.; Mellergård, A.; Ivanov, S. A.; Eriksen, J.; Rundlo¨f, H. Mater. Res. Bull. 2002, 37, 1797. (14) Bieringer, M.; Moussa, S. M.; Noailles, L. D.; Burrows, A.; Kiely, C. J.; Rosseinsky, M. J.; Ibberson, R. M. Chem. Mater. 2003, 15, 586. (15) Lufaso, M. W. Chem. Mater. 2004, 16, 2148. (16) Mandal, T. K.; Gopalakrishnan, J. J. Mater. Chem. 2004, 14, 1273.

10.1021/cm050064e CCC: $30.25 © 2005 American Chemical Society Published on Web 04/08/2005

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Table 1. Synthesis of Rock Salt Precursors Li4MWO6 for Double Perovskites precursor

synthesis condition

structure

Li4MgWO6 Li4NiWO6

Li4MgReO6 type: monoclinic (P21/n), a ) 5.092(3) Å, b ) 8.794(5) Å, c ) 5.061(4) Å, β ) 109.96(6)° Li4MgReO6 type: monoclinic (P21/n), a ) 5.090(3) Å, b ) 8.810(4) Å, c ) 5.079(1) Å, β ) 109.60(5)°

Li4FeWO6

600 °C/24 h/air 550 °C/24 h; 750 °C/48 h; 780 °C/48 h/air 600 °C/48 h/argon

Li4MnWO6

900 °C/24 h /argon

Li4WO5 type: triclinic (P1h), a ) 5.110(2) Å, b ) 7.760(2) Å, c ) 5.070(1) Å, R ) 101.63(2)°, β ) 101.40(3)°, γ ) 108.73(3)° Li4MgReO6 type: monoclinic (P21/n), a ) 10.331(1) Å, b ) 17.666(3) Å, c ) 10.266(1) Å, β ) 109.75(1)°

Table 2. Synthesis of Double Perovskite Oxides Sr2MWO6 by Metathesis reaction

a

conditionsa

Li4MgWO6 + 2SrCl2 f Sr2MgWO6 + 4LiCl

720 °C/24 h/air

Li4MnWO6 + 2SrCl2 f Sr2MnWO6 + 4LiCl

750 °C/24 h/argon

Li4FeWO6 + 2SrCl2 f Sr2FeWO6 + 4LiCl

750 °C/24 h/argon

Li4NiWO6 + 2SrCl2 f Sr2NiWO6 + 4LiCl

750 °C/48 h/air

double perovskite oxide Sr2MgWO6: monoclinic (P21/n); a ) 5.561(6) Å, b ) 5.603(7) Å, c ) 7.858(5) Å, β ) 90.04(9)° Sr2MnWO6: monoclinic (P21/n); a ) 5.660(1) Å, b ) 5.670(2) Å, c ) 7.994(2) Å, β ) 89.79(2)° Sr2FeWO6: monoclinic (P21/n); a ) 5.588(2) Å, b ) 5.617(4) Å, c ) 7.906(9) Å, β ) 89.74(8)° Sr2NiWO6: tetragonal (I4/m); a ) 5.567(1) Å, c ) 7.906(3) Å

The products were washed with water/ethanol to remove LiCl and dried in air (M ) Mg, Ni) at 120 °C/desiccator (M ) Mn, Fe) over anhydrous CaCl2.

) Mg, Mn, Fe, Ni). For this purpose, we first designed and synthesized appropriate rock salt precursor oxides of the general formula Li4MWO6. We then transformed them to the double perovskites via metathesis with SrCl2, Li4MWO6 + 2SrCl2 f Sr2MWO6 + 4LiCl. We could obtain ordered double perovskite phases around 750 °Csa significantly low temperaturesby this method. We believe the method is general and could be applied for the synthesis of several other functional double perovskites. 2. Experimental Section Rock salt related oxides of the formula Li4MWO6 (M ) Mg, Mn, Fe, Ni) were prepared by reacting stoichiometric quantities of Li2CO3, MC2O4‚2H2O, and WO3 at elevated temperatures. Synthesis conditions of the rock salt oxides are summarized in Table 1. Metathesis reaction between the rock salt oxides and anhydrous SrCl2 (Loba Chemie, purity > 99%) was carried out in air (for M ) Ni, Mg) or argon atmosphere (for M ) Fe, Mn) at various temperatures and reaction duration. The products were examined by powder X-ray diffraction (XRD) at different stages to determine the optimal conditions for the formation of single-phase materials. The final products after washing and drying were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. Synthesis conditions together with lattice parameters of the perovskite oxides are summarized in Table 2. Powder XRD patterns were recorded using a Siemens D5005 powder diffractometer (Cu KR radiation). Unit cell parameters were least-squares-refined by the PROSZKI17 program. SEM and EDX analysis were carried out using a JEOL JSM 5600 LV microscope equipped with a Link/ISIS system from Oxford Instruments. Singlephase materials were identified by comparison of the experimental XRD patterns with the simulated patterns for model structures using the program POWDERCELL.18 Rietveld refinement of the powder XRD data for Sr2NiWO6 and Sr2MgWO6 was carried out using the program FULLPROF.19 For this purpose, XRD data were collected in the 2θ range of 5-120° with a step size of 0.02° using a 9 s/step scan speed. The profiles were fitted with pseudo-Voigt function. Profile parameters, scale factor, half-width parameters, and zero error were refined along with unit cell parameters. For Sr2NiWO6, refinement was carried (17) Losocha, W.; Lewinski, K. J. Appl. Crystallogr. 1994, 27, 437. (18) Kraus, W.; Nolze, G. J. Appl. Crystallogr. 1996, 29, 301. (19) Rodrı´guez-Carvajal, J. Physica B 1993, 192, 55.

out using the structural model reported in the literature12 (space group I4/m) for this compound. First, the coordinates for the Sr atom were refined to convergence, followed by thermal parameters for heavy atoms W, Sr, and Ni. Then, in the next step, oxygen positions and thermal parameters were refined. In the final step, occupancies of Ni and W at 2a and 2b sites were refined, keeping the total atom count unity. For Sr2MgWO6, the refinement was carried out using the structural data20 for Sr2MnWO6 (space group P21/n) as the starting model. The profile parameters, atom positions, thermal factors, and occupancies were refined systematically as in the case of Sr2NiWO6. Diffuse reflectance spectra of polycrystalline samples of Sr2MWO6 were recorded with a Perkin-Elmer Lambda 35 double-beam spectrometer over the spectral range 200-800 nm using MgO as standard. Reflectance data were collected and converted to absorbance. Band gap energies were estimated by Shapiro’s method21,22 by extrapolating the onset of absorption to the wavelength axis. The spectra were calibrated using WO3 (Eg ) 2.6 eV) and TiO2, rutile (Eg ) 3.0 eV) as standards.23

3. Results and Discussion We investigated the formation of double perovskites, Sr2MWO6, in the metathesis reaction Li4MWO6 + 2SrCl2 f Sr2MWO6 + 4LiCl

(1)

For this purpose, we synthesized the rock salt related oxides of the formula Li4MWO6 by conventional solid-state reaction of the appropriate constituents. To our knowledge, these precursor oxides have not been previously reported in the literature; Li4MgReO6 is the only oxide of this kind that has been structurally characterized recently.24 SEM and EDX data (Figure S1, Supporting Information) show formation of single-phase Li4MWO6 phases with 1:1 M:W ratio. Powder XRD patterns (Figure 1) reveal that Li4MWO6 oxides possess different rock salt related super(20) Mun˜oz, A.; Alonso, J. A.; Casais, M. T.; Martı´nez-Lope, M. J.; Ferna´ndez-Dı´az, M. T. J. Phys.: Condens. Matter 2002, 14, 8817. (21) Shapiro, I. P. Opt. Spektrosk. 1958, 4, 256. (22) Eng, H. W.; Barnes, P. W.; Auer, B. M.; Woodward, P. M. J. Solid State Chem. 2003, 175, 94. (23) Gra¨tzel, M. Nature 2001, 414, 338. (24) Bieringer, M.; Greedan, J. E.; Luke, G. M. Phys. ReV. B 2000, 62, 6521.

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Figure 1. Powder XRD patterns of Li4MWO6: M ) (a) Mg, (b) Ni, (c) Fe, and (d) Mn. Table 3. Simulation of the Powder XRD Pattern of Li4NiWO6 on the Basis of Structural Models Related to Li4MgReO6 (Space Group C2/m) model Ia

model II

atom

x

y

z

occupancy

Li(1) Ni(1) Li(2) Ni(2) Li(3) Ni(3) W(4) O(5) O(6)

0.0 0.0 0.5 0.5 0.0 0.0 0.0 0.2598 0.2658

0.666 0.666 0.315 0.315 0.5 0.5 0.0 0.3445 0.5

0.0 0.0 0.5 0.5 0.5 0.5 0.0 0.7603 0.2297

0.74 0.26 0.84 0.16 0.80 0.20 1.0 1.0 1.0

x

y

z

occupancy

0.0 0.0 0.5

0.666 0.666 0.315

0.0 0.0 0.5

0.5 0.5 1.0

0.0

0.5

0.5

1.0

0.0 0.0 0.0 0.2598 0.3445 0.7603 0.2658 0.5 0.2297

1.0 1.0 1.0

a Model I is exactly the same as that given for Li MgReO ,24 where Mg 4 6 is replaced by Ni and Re is replaced by W.

structures: thus Li4MgWO6 is isostructural with Li4MgReO6 (Table 1 and Table S1, Supporting Information). The Ni compound (Table 1 and Table S2, Supporting Information) adopts a slightly different structure related to Li4MgReO6. We simulated the powder XRD pattern of Li4NiWO6 on two models: model I, Li4MgReO6, and model II, where we place all the Ni in the Li(1) site leaving the other two Li sites (Li(2) and Li(3)) fully occupied (Table 3). We find a better agreement between the experimental and simulated pattern for model II (Figure 2), suggesting that the cations are ordered in the sequence ..., (Li,Ni,W), (Li), (Li,Ni,W), ... along the c-direction, unlike Li4MgWO6 when the cation sequence is ..., (Li,Mg), (Li,Mg,W), (Li,Mg). Powder XRD pattern (Figure 1) suggests that Li4FeWO6 adopts a structure (Table 1 and Table S3, Supporting

Figure 2. Comparison of the powder XRD patterns of (a) Li4NiWO6 and (c) Li4FeWO6 with the corresponding simulated patterns b and d based on Li4MgReO6 and Li4WO5 structures, respectively.

Information) closely related to that of Li4WO5.25 Li4WO5 crystallizes in a rock salt superstructure (triclinic, P1h, a ) 5.1094 Å, b ) 7.7159 Å, c ) 5.0609 Å, R ) 101.80°, β ) 101.78°, γ ) 108.77°), where Li and (Li, W) atoms are ordered in alternate (hk0) planes; there is a further 2:1 ordering in the (Li, W) planes. A simulation of the powder XRD pattern of Li4FeWO6 assuming that Fe is statistically distributed in both the Li and (Li, W) planes in the Li4WO5 structure shows good agreement with the experimental data (Figure 2). The powder XRD pattern of Li4MnWO6 (Figure 1d) is different from the patterns of other Li4MWO6 phases. We could however index the pattern of the Mn phase in a monoclinic cell (Table 1 and Table S4, Supporting Information) related to that of Li4MgReO6. We show in Figure 3 the likely cation ordering of Li4MgWO6, Li4NiWO6, and Li4FeWO6 in rock salt related superstructures. 3.1. Double Perovskite Oxides. We investigated the formation of double perovskite oxides Sr2MWO6 (M ) Mg, Mn, Fe, and Ni) by reacting the corresponding rock salt oxides, Li4MWO6 with anhydrous SrCl2 according to the metathesis reaction 1. Examination of the products obtained at various stages of the reaction by powder XRD (Figure 4) showed that the metathesis occurred smoothly around 750 °C in 24-48 h, yielding the desired double perovskites. The details of synthesis conditions are given in Table 2. SEM and EDX analyses of the washed products also show (25) Hoffmann, R.; Hoppe, R. Z. Anorg. Allg. Chem. 1989, 573, 157.

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Figure 3. Cation ordering in (a) Li4MgReO6, (b) Li4NiWO6 (related to Li4MgReO6), and (c) Li4FeWO6 (related to Li4WO5, where Fe is distributed statistically in all the cation sites). Table 4. Powder XRD Data for Sr2MgWO6 and Sr2FeWO6 Sr2MgWO6a h

k

l

1 1 1 2 2 2 0 2 2 3 2

0 1 1 1 0 2 0 1 0 0 2

1 0 2 1 2 0 4 3 4 3 4

dobs (Å) dcalc (Å) 4.538 3.938 2.783 2.371 2.270 1.975 1.964 1.803 1.604 1.513 1.392

4.538 3.946 2.784 2.374 2.269 1.973 1.964 1.804 1.604 1.513 1.392

Sr2FeWO6b Iobs

h

k

l

66 8 100 18 5 11 13 6 8 3 4

1 1 1 2 2 2 2 3 2 3 2

0 1 1 1 0 2 1 1 0 0 2

1 0 2 1 2 0 3 0 4 3 4

dobs (Å) dcalc (Å) 4.578 3.962 2.803 2.388 2.289 1.981 1.818 1.768 1.617 1.524 1.401

4.573 3.962 2.803 2.388 2.287 1.981 1.818 1.768 1.617 1.524 1.401

Iobs 52 8 100 15 3 14 9 4 25 8 14

a a ) 5.561(6) Å, b ) 5.603(7) Å, c ) 7.858(5) Å, β ) 90.04(9)°. b a ) 5.588(2) Å, b ) 5.617(4) Å, c ) 7.906(9) Å, β ) 89.74(8)°.

Figure 4. Powder XRD patterns of Sr2MWO6: M ) (a) Mg, (b) Mn, (c) Fe, and (d) Ni. Asterisks show impurity reflections due to SrWO4.

formation of single-phase materials with the expected metal atom ratios (Figure 5). Lattice parameters obtained by leastsquares refinement of the powder XRD data (Table 2) are in agreement with the corresponding values reported for Sr2MWO6 phases in the literature.11-13 For Sr2MgWO6, only a pseudocubic lattice parameter was reported, although a monoclinic distortion was indicated.11 The XRD pattern (Figure 4) for Sr2MgWO6 prepared by us is indexable in a monoclinic cell, which is similar to that of the M ) Fe

phase13b (Table 4). For M ) Mn, both tetragonal13a and monoclinic20 cells were reported; our XRD data are in agreement with the latter. The pattern of the M ) Ni phase is indexable on a tetragonal cell that has been reported in the literature.12 We have carried out Rietveld refinement of the powder XRD data of Sr2NiWO6 using the tetragonal structure reported for this oxide as the model.12 The refinement (Figure 6) shows that the sample we obtained in metathesis reaction is indeed tetragonal Sr2NiWO6 with an ordered arrangement of Ni and W at 2a and 2b sites. Refinement also shows that the ordering of Ni and W is nearly complete with full occupancies of Ni and W at 2a and 2b sites, respectively. Attempts to refine the structure with lesser ordering (9095%) gave unsatisfactory (higher) reliability factors. The crystallographic parameters obtained from the refinement are in agreement with the literature values.12 The refined atomic coordinates, occupancies, and isotropic thermal parameters are given in Table 5. Figure 7a shows the structure of Sr2NiWO6 drawn from the refined coordinates showing rock salt type ordering of Ni and W in the double perovskite structure. A similar refinement of the powder XRD data for Sr2MgWO6 based on the monoclinic structure of Sr2MnWO6 20 gave a satisfactory profile fit (Figure 8). Here again we see

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Figure 5. SEM pictures of Sr2MnWO6 (top) and Sr2NiWO6 (bottom). The corresponding EDX spectra are shown on the right-side panels.

Figure 6. Rietveld refinement of the structure of Sr2NiWO6 from powder XRD data. Observed (+), calculated (-), and difference (bottom) profiles are shown. The vertical bars represent the Bragg positions.

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Figure 7. Crystal structures of (a) Sr2NiWO6 and (b) Sr2MgWO6 drawn from the refined atomic coordinates.

Figure 8. Rietveld refinement of the structure of Sr2MgWO6 from powder XRD data. Observed (+), calculated (-), and difference (bottom) profiles are shown. The vertical bars represent the Bragg positions. Table 5. Atomic Positions, Occupancy, and Isotropic Temperature Factors for Sr2NiWO6a

Table 6. Atomic Positions, Occupancy, and Isotropic Temperature Factors for Sr2MgWO6a

atom

Wyckoff position

x

y

z

B (Å2)

occupancy

atom

Wyckoff position

x

y

z

B (Å2)

occupancy

Sr Ni W O1 O2

4c 2a 2b 8h 4e

0.0 0.0 0.0 0.287(1) 0.0

0.5 0.0 0.0 0.225(1) 0.0

0.25 0.0 0.5 0.0 0.251(2)

1.1(1) 1.5(1) 0.9(1) 1.0(1) 0.9(1)

1.0 1.0 1.0 1.0 1.0

Sr Mg W O1 O2 O3

4e 2d 2c 4e 4e 4e

0.994(1) 0.5 0.5 0.050(1) 0.733(1) 0.203

0.014(1) 0.0 0.0 0.486(1) 0.287(1) 0.230

0.240(1) 0.0 0.5 0.231(1) 0.026(1) 0.971

0.55(4) 0.93(3) 0.72(3) 0.79(1) 0.89(1) 1.00

1.0 1.0 1.0 1.0 1.0 1.0

a Space group I4/m, a ) 5.566(1) Å, c ) 7.917(1) Å, R Bragg ) 5.6%, Rf ) 4.5%, Rp ) 3.4%, Rwp ) 4.9%, Rexp ) 2.1%.

a near complete ordering of Mg and W in the 2d and 2c sites of the monoclinic cell. The refined atomic coordinates, occupancies, and isotropic thermal parameters are given in Table 6, and the structure is drawn in Figure 7b. The present work shows that metathesis reaction 1 indeed provides a convenient alternate route for the synthesis of ordered double perovskite oxides, Sr2MWO6 (M ) Mg, Mn, Fe, Ni), at much lower temperatures (∼750 °C) than what is reported (1100-1400 °C) in the literature for the formation of these oxides through the ceramic route.11-13 While

a Space group P2 /n, a ) 5.586(1) Å, b ) 5.584(1) Å, c ) 7.942(1) Å, 1 β ) 90.01(1)°, RBragg ) 5.9%, Rf ) 10.2%, Rp ) 4.5%, Rwp ) 6.5%, Rexp ) 2.5%.

the formation of co-produced ionic salt (LiCl) with a high lattice energy is most likely the driving force for the metathesis,26,27 ordering of octahedral site cations at such low temperatures is likely facilitated by the presence of molten LiCl at the reaction condition. Significantly, the (26) (a) Wiley: J. B.; Kaner, R. B. Science 1992, 255, 1093. (b) Gillan, E. G.; Kaner, R. B. Chem. Mater. 1996, 8, 333. (27) Parkin, I. P. Chem. Ind. 1997, 725.

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Figure 9. Optical absorption spectra of Sr2MWO6 (M ) Mg, Mn, Fe, Ni) oxides. For comparison the corresponding spectrum for WO3 is shown.

double perovskites are not formed in a “one-pot synthesis” of stoichiometric quantities of Li2CO3, MC2O4‚2H2O, WO3, and SrCl2 under the conditions given in Table 2. 3.2. Optical Absorption. Electronic properties of ordered double perovskites crucially depend on B-O-B′ interaction.4,5 While the superexchange interaction between cation d orbitals determines the magnetism of the material, the electron transfer, Bn+-O-(B′)m+ T B(n+1)+-O-(B′)(m-1)+

(2)

would determine the electrical transport. Sr2FeMoO6 and Ba2FeReO6 are rare instances of double perovskites where the charge-transfer energy for this electron transfer, ∆E ∼ 0, gives metallic conduction.4,5 The majority of the double perovskites are semiconducting/insulating,1 where the chargetransfer energy ∆E is expected to be greater than zero. In the family of Ba2MReO6, M ) Mn, Fe, and Ni members are ferrimagnetic, but only the M ) Fe member is a metallic conductor,4 revealing the influence of ∆E on electron transport. ∆E is therefore a crucial quantity that determines the electrical transport properties of double perovskites. To our knowledge, there seems to be no systematic effort made in the literature to determine this quantity for double perovskites

containing transition-metal B and B′ cations; recently these data have been reported for double perovskites containing d0 and non-transition-metal cations (e.g. Sr2AlTaO6).22 In an attempt to probe the magnitude of ∆E in Sr2MWO6 synthesized here, we investigated the optical absorption spectra of these oxides. Reflectance spectra recorded using well-powdered polycrystalline samples are given in Figure 9. For comparison, we have also given the spectrum for WO3 in the same figure. We have estimated the optical band gaps (Eg) from the spectra by extrapolating the onset of the absorption edge to the wavelength axis.21,22 We find sharp absorption edges for Sr2MgWO6 and Sr2NiWO6 corresponding to Eg values of 3.4 and 3.1 eV, respectively. As compared to the Eg of WO3 (2.6 eV), there is a significant increase in the Eg values for the double perovskites. A similar increase has also been found for other d0 double perovskites.22 This has been attributed to isolating the BO6 and B′O6 octahedra in ordered double perovskites, which narrows the bandwidths and increases the band gap.22 We could not estimate the band gap for M ) Mn and Fe compounds from the absorption spectra because absorption in the visible region is dominated by surface states and ligand field transitions. Finite band gaps for the Ni and Mg compounds are consistent with the insulating behavior of these compounds. 4. Conclusions We have developed a low-temperature (∼750 °C) synthetic route for the preparation of ordered double perovskite oxides, Sr2MWO6 (M ) Mg, Mn, Fe, Ni), based on a solid-state metathesis reaction between rock salt precursor oxides, Li4MWO6, and anhydrous SrCl2. Molten LiCl formed at the reaction conditions appears to facilitate not only the formation of the perovskite oxides but also ordering of B-site cations as well. We believed the method could be extended for the synthesis of several other A2BB′O6 and A3BB′2O9 perovskites, where A ) Ca, Ba, and Pb as well as other combinations of B and B′ cations. Acknowledgment. We thank the Department of Science and Technology (DST), Government of India, for support of this research work. Supporting Information Available: SEM image and EDX spectra of Li4MWO6 (M ) Ni, Mn) and indexing of the powder XRD data for Li4MWO6 (M ) Mg, Ni, Fe and Mn) (PDF). This material is available free of charge via the Internet at http://pubs/acs.org. CM050064E