CHEM. RES. CHINESE UNIVERSITIES 2010, 26(4), 517521

*Corresponding author. E-mail: shfeng@mail.jlu.edu.cn Received May 20, 2009; accepted September 15, 2009. Supported by the National Natural Science Foundation of China(Nos.20631010, 90922034 and 20771042).

Hydrothermal Synthesis and Characterization of Double Perovskites RSrMnFeO6(R=La, Pr, Nd, Sm)

ZHANG Gang-hua, YUAN Hong-ming, CHEN Yan, LI Wei-juan, YANG Mei-qi and FENG Shou-hua* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P. R. China

Abstract A series of double perovskites RSrMnFeO6(R=La, Pr, Nd, Sm) was synthesized under mild hydrothermal conditions. Crystal growths of the samples were sensitive to alkalinity, temperature, filling fraction, and composition of initial reaction mixture. The desired series of compounds belongs to the class of AABBO6 perovskites with a random distribution of Mn and Fe atoms over the B-cation sub-lattice. Their structures show the distorted orthorhom-bic symmetry with space group Pnma. The shapes and sizes of the crystals were analyzed on a Rigaku JSM-6700F by scanning electron microscopy. Analysis done by XPS, Mssbauer spectroscopy and iodometric titration reveals that Mn and Fe ions have +4 and +3 oxidation states, respectively. Keywords Double perovskites; Orthorhombic; Hydrothermal synthesis Article ID 1005-9040(2010)-04-517-05

1 Introduction

Double perovskite oxides with a general formula A2BBO6 or AABBO6(where A and A are alka-line-earth and/or rare-earth metals and B and B are transition metals) have been widely investigated for their catalytic[1,2], magnetic[3], dielectric properties[4] and colossal magnetoresistance(CMR). After the dis-covery of room temperature CMR and tunnelling magnetoresistance(TMR) in the double perovskites Sr2FeMoO6 and Sr2FeReO6, respectively[5,6], there have been growing interests worldwide in researching for effective methods to make double perovskite ma-terials[79]. On the basis of B-cation arrangement, double perovskite can be classified into three general classes[10], namely, (1) the distorted or random pe-rovskite[1114]; (2) the rock salt type[11,15]; and (3) the layered type [16].

Ramesha et al.[13] reported cubic/pseudo-cubic perovskite structure for ALaMnFeO6(A=Ca, Ba, Sr) and were unable to find the doubling of the primitive perovskite cell from their X-ray powder diffraction analysis. Subsequently, Shaheen[14] confirmed the structure of LaCaMnFeO6 by neutron powder diffrac-tion method. They described that the crystal structure of the former had a random distribution of Mn and Fe atoms over the B-cation sub-lattice. However, besides La ions, other rare earth cations substitutions at the

A-site of such Fe-doped manganites were rare re-ported.

The approaches conventionally used for fabri-cating double perovskites rely on high temperature solid state reactions and sol-gel processes of oxides, nitrates and/or carbonate precursors[14,17,18]. But these need a much higher temperature, and usually have inhomogeneous chemical components and such in-homogeneity cannot be avoided. The partial substitu-tion of the divalent cation by the trivalent lanthanide one at the A-site of the perovskite manganates under hydrothermal conditions has been reported before[19,20]. To the best of our knowledge, there have been no pre-vious reports on the hydrothermal synthesis of the manganites with Fe doped at the Mn site. In this study we employed hydrothermal technique for the first time to prepare a family of double perovskites RSrMnFeO6 (R=La, Pr, Nd, Sm). The motivation of this study is to develop new preparative methods by which chemical operation could be readily conducted and perfect crystals could be obtained.

2 Experimental The synthesis was performed from the solutions

of metal salts in aqueous potassium hydroxide, and carried out in an 80-mL Teflon-lined stainless steel autoclave with a filling capacity of 80%. In order to

518 CHEM. RES. CHINESE UNIVERSITIES Vol.26

ensure intimate mixing of reagents, the solutions of the metal salts were prepared prior to mixing form 0.4 mol/L MnCl2, 0.1 mol/L KMnO4, 0.2 mol/L Fe(NO3)2, 0.2 mol/L R(NO3)3(R=La, Pr, Nd, Sm) and 0.2 mol/L Sr(NO3)2, respectively.

In a typical synthesis procedure for RSrMnFeO6 (R=La, Pr, Nd, Sm), 14 mL of KMnO4 and 10 g of KOH were mixed by stirring to form a solution. To this solution was added 9 mL of MnCl2 dropwise un-der constant stirring that was cooled in an ice bath to form a k-birnessite gel precursor. This was followed by an addition of 12.5 mL of Sr(NO3)3, 12.5 mL of R(NO3)3(R=La, Pr, Nd, Sm), 12.5 mL of Fe(NO3)3 and 50 g of KOH. The reaction mixture was stirred with a magnetic stirrer before being transferred into a Teflon-lined stainless steel autoclave. The crystalliza-tion was carried out under autogenous pressure at 260 C for 23 d. After the autoclave was cooled and depressurized, the product was washed with distilled water and sonicated by a direct immersion of a tita-nium horn(Vibracell, 20 kHz, 200 W/cm2). Single- phase RSrFeMnO6 was obtained as dark crystalline powder. Powder X-ray diffraction(XRD) data were collected on a Rigaku D/Max 2500V/PC X-ray dif-fractometer with Cu K radiation(=0.15418 nm) at 50 kV and 250 mA. Scanning electron microsco-py(SEM) was performed on a Rigaku JSM-6700F microscope operated at 10 kV. Inductively couple plasma(ICP) analysis was carried out on a Per-kin-Elmer Optima 3300DV ICP instrument.

The average B-site oxidation state of the powder was determined by iodometric titration as reported previously in the literature[21,22]. The oxidation states of Mn and Fe were further investigated by X-ray pho-toelectron spectroscopy(XPS) on a Thermo ESCALAB 250 spectrometer with a monochromatic aluminum X-ray source. The spectra collected for the C1s, Mn2p, Fe2p, and O1s regions, and the Fe2p3/2, Mn2p3/2 binding energies were corrected versus the C1s reference of 284.6 eV. The 57Fe Mssbauer spectra were recorded on a constant acceleration Mssbauer spectrometer with a 57Co(Pd) source at room tempe- ratures.

3 Results and Discussion Compared to solid state reactions, hydrothermal

reactions may occur at relatively low temperature and allow ready chemical operations[23,24]. We have used

hydrothermal redox reactions of MnO4 and Mn2+ ions to make CMR materials and disproportionation reac-tions to prepare three oxidation state manganese oxides[25,26]. The factors that affect hydrothermal syn-thesis such as precursor, mineral reagent, alkaline concentration, appropriate reaction temperature and pressure play a significant role in the course of the reaction. For instance, in our specific case when the reaction temperature was lower than 260 C, we obtained a mixture containing our title compounds and an impurity of R(OH)3(R=La, Pr, Nd, Sm). The amount of alkali is a key factor in producing the perovskite in hydrothermal processing. KOH not only maintains the alkalinity but also acts as mineralizing agent. When the alkalinity was less than 15 mol/L in our synthesis, the impurity mixture of R(OH)3, KxMnO2(0.50.7)H2O and Fe(OH)3 was formed. This indicates there is some competition between thermodynamically and kinetically stabilized phases. The high alkalinity of the reaction system is necessary to obtain the desired ratios of n(R):n(Sr):n(Mn):n(Fe). The optimal synthetic conditions for preparing the title compounds are listed in Table 1. In the order of from La to Sm, the needed alkalinity of the reactions de-creased. We propose that the effect of alkalinity is due to the fact that R3+ ions show a significant amphoteric characteristic and the solubility of R(OH)3(from R=La to R=Sm) increases in KOH.

Table 1 Optimal synthetic conditions for preparing RSrMnFeO6(R=La, Pr, Nd, Sm)

Compound c(KOH)/(molL1) Reaction temperature/C Time/hLaSrMnFeO6 19 260 48 PrSrMnFeO6 18 260 72 NdSrMnFeO6 15 260 72 SmSrMnFeO6 15 260 72

EDX analysis of selected regions for the sample shows the presence of all four metals in region ana-lyzed with a constant ratio. The ICP composition analysis indicates that the ratios of R:Sr:Mn:Fe were in good agreement with the theoretical value, e.g., n(R):n(Sr):n(Mn):n(Fe)=1:1:1:1 for the perovskite- type oxides (Table 2). The oxygen contents in our

Table 2 Chemical compositions of RSrMnFeO6 (R=La, Pr, Nd, Sm) obtained from ICP and iodometric titration

Compound n(Ln):n(Sr):n(Mn):

n(Fe):n(O) (Theoretical value)

n(Ln):n(Sr):n(Mn): n(Fe)[Calculated

value (ICP)]

Oxygen content

(%) LaSrMnFeO6 1:1:1:1:6 0.98:1.02:1.01:0.99 5.98 PrSrMnFeO6 1:1:1:1:6 0.99:1.01:1.02:0.98 6.00 NdSrMnFeO6 1:1:1:1:6 0.97:1.03:1.02:0.98 5.99 SmSrMnFeO6 1:1:1:1:6 0.98:1.02:1.03:0.97 5.97

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samples were obtained from iodometric titration. Thus the formula of our samples is RSrMnFeO6.

The powder XRD data as shown in Fig.1 could be indexed on the basis of the known perovskite GdFeO3 crystal structure[27] and refined by the Riet-veld method. The products crystallized in the orthor-

hombic system with space group Pnma. The crystal-lographic parameters of the cells(Table 3) of RSrMnFeO6(R= La, Pr, Nd, Sm) decrease in the order of from La to Sm, correlating to their sizes. The se-lected interatomic distances and bond angles are shown in Table 4.

Fig.1 Experimental(dot), calculated(line) and difference(bottom line) X-ray diffraction profiles for RSrMnFeO6[R=La(A), Pr(B), Nd(C), Sm(D)]

Table 3 Unit cell, positional and reliability factors for the Rietveld refinements of RSrMnFeO6 (R=La, Pr, Nd, Sm), in the orthorhombic Pnma space group from XRD data

Compound La/Sr Pr/Sr Nd/Sr Sm/Sr a/nm 0.5479(3) 0.5464(4) 0.5454(2) 0.5443(1) b/nm 0.7739(2) 0.7736(2) 0.7693(2) 0.7690(2) c/nm 0.5478(3) 0.5469(3) 0.5463(2) 0.5465(1) V/nm3 0.23227(8) 0.23117(2) 0.22921(4) 0.22874(7) xa 0.0001(1) 0.0024(9) 0.0067(7) 0.0075(3) ya 0.0006(2) 0.0003(1) 0.0030(2) 0.0014(2) xb 0.4144(3) 0.4294(1) 0.5849(2) 0.5300(4) yb 0.0600(5) 0.0770(4) 0.0613(1) 0.1001(5) xc 0.2545(1) 0.2417(8) 0.2348(9) 0.2808(1) yc 0.0010(6) 0.0002(5) 0.0123(8) 0.0166(2) zc 0.7414(4) 0.7595(9) 0.7660(2) 0.7240(3) Rwp(%) 9.67 10.54 10.22 7.94 Rp(%) 6.09 6.62 7.15 4.66 2 1.58 1.23 1.22 1.28

a. R/Sr 4c(x, 1/4, y); b. Mn/Fe 4b(0, 0, 1/2); O1 4c(x, 1/4, y); c. O2 8d(x, y, z). Table 4 Selected interatomic distances and bond angles for RSrMnFeO6 (R=La, Pr, Nd, Sm) crystal structures

Compound La/Sr Pr/Sr Nd/Sr Sm/Sr d[(Mn, Fe)O1]/nm 0.201(7)2 0.201(6)2 0.200(6)2 0.200(5)2 d[(Mn, Fe)O2]/nm 0.192(2)2 0.192(8)2 0.193(2)2 0.192(7)2 0.195(3)2 0.193(9)2 0.193(9)2 0.196(2)2 Average d[(Mn, Fe)O1]/nm 0.195(3) 0.195(1) 0.195(1) 0.195(6) d[(Sr, R)O1]/nm 0.245(2)1 0.239(7)1 0.243(2)1 0.265(8)1 0.229(4)1 0.234(2)1 0.224(9)1 0.219(6)1

d[(Sr, R)O2]/nm 0.276(9)2 0.277(4)2 0.277(5)2 0.277(2)2 0.270(6)2 0.269(1)2 0.272(1)2 0.265(5)2 0.269(7)2 0.268(2)2 0.256(2)2 0.249(5)2

(Mn, Fe)O1(Mn, Fe)/() 147.0(1) 147.0(8) 146.8(9) 146.9(1) (Mn, Fe)O2(Mn, Fe)/() 176.9(7) 175.9 (2) 170.9(1) 165.0(2)

520 CHEM. RES. CHINESE UNIVERSITIES Vol.26

The decreasing values of the (Mn, Fe)O(Mn,

Fe) bond angles on going from R=La to R=Sm indi- cate the increasing tilting distortion of the octahedral framework. The average BO bond lengths were found to be a constant value(ca. 0.195 nm). So the effects of the different rare earth elements on BO bond lengths were slight. Fig.2 shows the polyhedral view of double perovskite structure of RSrMnFeO6 which clearly indicates the tilting of (Mn/Fe)O6 octa-hedra. The Mn and Fe atoms are randomly distributed at B-cation sublattice. Since Fe is one of the nearest neighbors of Mn and the small size of the R3+ and Sr2+ cations, the given cell and tilt system are not compati-ble with an ordering of either B- or A-cations. Similar results have also been reported by other resear- chers[14,17]. Ordering is favored by large differences in ionic radius and formal charges between B and B ca-tions[28,29]. The crystal morphology and purity were confirmed by SEM images as shown in Fig.3. They have the sizes ranging from 5 to 10 m, each in a nar-row size distribution. The cubic-shaped crystallites are clearly shown, but the behavior of crystal growth may

Fig.2 Crystal structure of RSrMnFeO6(R=La, Pr, Nd, Sm) double perovskites Mn, Fe atoms are situated at the octahedra, Sr and R atoms are shown as spheres.

Fig.3 SEM images of crystalline samples (A) LaSrMnFeO6; (B) PrSrMnFeO6; (C) NdSrMnFeO6; (D) SmSrMnFeO6.

be different from each other. The iodometry measurements for Mn and Fe of

all the samples gave an average valence state of +3.48, very close to its theoretical value of 3.50 calculated from the neutrality for the title compound where Sr2+ on the A-site must compensate for the octahedral Mn4+ on the B-site and R3+ for Fe3+. X-Ray photoelectron spectroscopy(XPS) was used to determine the oxida-tion states of Mn and Fe in our products. Fe2p3/2 and Mn2p3/2 binding energies were corrected versus the C1s reference of 284.6 eV. The binding energies of Mn in MnO2(642.2 eV)[30] and Fe in Fe2O3(711.1 eV)[31]

were taken as the references. The Mn2p and Fe2p XPS spectra are shown in Fig.4(A) and (B), respectively, for a representative SmSrMnFeO6 sample.

Fig.4 XPS spectra of SmSrMnFeO6 (A) Fe2p; (B) Mn2p.

The measured binding energies of Mn(642.2 eV) and Fe(710.7 eV) ions of the samples clearly corres-pond to those of MnO2 and Fe2O3, respectively. The oxidation state of Fe in the different samples was also identified by Mssbauer spectra. Fig.5 shows the 57Fe Mssbauer spectra at room temperature for RSrMnFeO6(R=La, Pr, Nd). In all the cases, each spectrum exhibits a doublet pattern, indicating the paramagnetic state of the samples at room temperature. Hyperfine parameters such as isomer shift(IS), qua-drupole splitting(QS) and the width at half height(HWHM) for all the studied samples are listed in Table 5. The isomer shifts for La/Sr, Pr/Sr and Nd/Sr are found to be 0.36, 0.36 and 0.35 mm/s,

No.4 ZHANG Gang-hua et al. 521

respectively. These values are typical of Fe3+ ions in octahedral coordination, being in accordance with previous work[32,33]. Therefore, the oxidation states of Mn and Fe in our samples were +4 and +3, respec-tively.

Fig.5 Mssbauer spectra of RSrMnFeO6 at room temperature a. La/Sr; b. Pr/Sr; c. Nd/Sr.

Table 5 Fitting parameters of Mssbauer spectra Compound IS/(mms1) QS/(mms1) HWHM/(mms1)LaSrMnFeO6 0.36(0) 0.66(0) 0.23(1) PrSrMnFeO6 0.36(1) 0.67(0) 0.24(0) NdSrMnFeO6 0.35(1) 0.72(1) 0.27(0)

4 Conclusions A series of double perovskites RSrMnFeO6(R=La,

Pr, Nd, Sm) was synthesized under hydrothermal con-ditions, in which the crystals had a uniform size of about 510 m. The compounds belong to the class of AABBO6 type perovskites. The Rietveld refine-ment of powder X-ray diffraction data shows that the crystal structures of RSrMnFeO6(R=La, Pr, Nd, Sm) were distorted orthorhombic symmetry with a space group Pnma, Mn and Fe oxidation states equal to +4 and +3, respectively, as determined by XPS and iodometric titration. Mssbauer study indicated that Fe in these compounds was in the trivalent high-spin state. Our results indicate that the hydrothermal me-thod is a promising route to even more complicated double perovskite materials.

References

[1] Yamazoe N., Teraoka Y., Catal. Today, 1990, 8, 175 [2] Tejuca L. G., Fierro J. L. G., Properties and Applications of

Perovskite Type Oxides, Marcel Dekker, NewYork, 1993 [3] Hong K. P., Choi Y. H., Kwon Y. U., Jung D. Y., Lee J. S., Lee C. H.,

J. Solid State Chem., 2000, 150, 383 [4] Ganguli A. K., Grover V., Thirumal M., Mater. Res. Bull., 2001, 36,

1967 [5] Kobayashi K. L., Kimura T., Sawada H., Terakura K., Tokura Y.,

Nature, 1998, 395, 677 [6] Kobayashi K. I., Kimura T., Tomioka Y., Sawada H., Terakura K.,

Tokura Y., Phys. Rev. B, 1999, 59, 11159 [7] Garcia-Landa B., Ritter C., Ibarra M. R., Blasco J., Algarabel P. A.,

Mahendiran R., Garcia J., Solid State Commun., 1999, 110, 435 [8] Zeng Z., Fawcett I. D., Greenblatt M., Croft M., Mater. Res. Bull.,

2001, 36, 705 [9] Han H., Kim C. S., Lee B. W., J. Magn. Magn. Mater., 2003, 254/255,

574 [10] Anderson M. T., Greenwood K. B., Taylor G. A., Poeppelmeier K. R.,

Prog. Solid State Chem., 1993, 22, 197 [11] Attfield M. P., Battle P. D., Bollen S. K., Gibb T. C., Whitehead R. J.,

J. Solid State Chem., 1992, 100, 37 [12] Attfield M. P., Battle P. D., Bollen S. K., Kim S. H., Powell A. V.,

Workman M., J Solid State Chem., 1992, 96, 344 [13] Ramesha K., Thangadurai V., Sutar D., Subramanyam S. V., Sub-

banna G. N., Gopalakrishnan, Mater. Res. Bull., 2000, 35, 559 [14] Shaheen R., Bashir J., Rundlf H., Rennie A. R., Mater. Lett., 2005,

59, 2296 [15] Battle P. D., Jones C. W., J. Solid State Chem., 1989, 18, 108 [16] Anderson M. T., Poeppelmeier K. R., Chem. Mater., 1991, 3, 476 [17] Abakumov A. M., Rossell M. D., Seryakov S. A., Antipov E. V.,

J. Mater. Chem., 2005, 15, 4899 [18] Tian S. Z., Zhao J. C., Qiao C. D., Ji X. L., Jiang B. Z., Mater. Lett.,

2006, 60, 2747 [19] Chen Y., Yuan H., Tian G., Zhang G., Feng S., J. Solid State Chem.,

2007, 180, 167 [20] Wang D., Yu R. B., Feng S. H., Zheng W. J., Pang G. S., Zhao H.,

Chem. J. Chinese Universities, 1998, 19(1), 165 [21] Li B., Hakuta Y., Hayashi H., Chem. Commun., 2005, 1732 [22] Mao Y. C., Li G. S., Xu W., Feng S. H., J. Mater. Chem., 2000, 10,

479 [23] Song Z., Li G. H., Yu Y., Shi Z., Feng S. H., Chem. Res. Chinese

Universities, 2009, 25(1), 1 [24] Wang L., Zhang H., Dong W., Shi Z., Feng S. H., Chem. Res. Chi-

nese Universities, 2008, 24(4), 389 [25] Feng S., Xu R., Acc. Chem. Res., 2001, 34, 239 [26] Feng S., Yuan H., Shi Z., Chen Y., Wang Y., Huang K., Hou C., Li J.,

Pang G., Hou Y., J. Mater. Sci., 2008, 43, 2131 [27] Marezio M., Remeika J. P., Dernier P. D., Acta Crystallogr., Sect. B,

1970, B26, 2008 [28] Nakamura T., Choy J. H., J. Solid State Chem., 1977, 20, 233 [29] Galasso F., Pyle J., Inorg. Chem., 1963, 2, 482 [30] Bernal S., Botana F. J., Laachir A., Moral P., Martin G. A., Perrichon

V., Eur. J. Solid State Inorg. Chem., 1991, 28, 421 [31] Seyama H., Soma M., J. Electron Spectrosc. Relat. Phemon., 1987,

42, 97 [32] Da Costa F. M. A., Dos Santos A. J. C., Inorg. Chim. Acta, 1987, 140,

105 [33] Hernandez T., Plazaola F., Rojo T., Barandiaran H. M., J. Alloys

Compd., 2001, 323/324, 440