Synthesis and characterization of Aln 2 O 4 indates, A ˆ Mg, Ca, Sr, Ba

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1 JOURNAL OF MATERIALS SCIENCE LETTERS 17 (1998) 619±623 Synthesis and characterization of Aln 2 O 4 indates, A ˆ Mg, Ca, Sr, Ba S. ESTHER DALI, V. V. S. S. SAI SUNDAR, M. JAYACHANDRAN, M. J. CHOCKALINGAM Electrochemical Materials Science Division, Central Electrochemical Research Institute, Karaikudi , India Transparent conducting oxides, due to their high transparency and conductivity of the order of 10 2 Scm 1, are competing candidates for a variety of technologically important applications such as liquid crystal displays, computer monitors etc. Conventional materials include indium tin oxide (ITO), zinc oxide (ZnO) and tin oxide (SnO 2 ). Higher transparency and=or higher conductivity than those of the materials commercially available are the need of the day in the development of photoelectronic devices with higher functions. It has been reported that MgIn 2 O 4, a spinel oxide, has high electrical conductivity and is capable of transmitting more ultraviolet radiation than ITO [1]. MgIn 2 O 4 crystallizes in cubic symmetry having space group Fd3m and lattice parameter a ˆ 0:8864 nm (Z ˆ 8) [2]. The structure of MgIn 2 O 4 is basically constituted by the close packing of oxygen ions. Oxygen occupies the corners of the polyhedra. The structure consists of rutile chains running parallel to the (1 1 0) direction. The chains are connected to each other by the cations existing in the tetrahedral sites. Spinel has many unoccupied cation sites in the unit cell. There are 64 equivalent tetrahedral cation sites and 32 octahedral sites in the unit cell. Out of these seven-eighths of the tetrahedral sites and half of the octahedral sites are not occupied [3]. MgIn 2 O 4 ± Zn 2 In 2 O 5 lms prepared using the magnetron sputtering technique were found to have high transparency with a resistivity of 3: Ù cm [4]. Besides MgIn 2 O 4, the indates of calcium, strontium and barium are of interest from the viewpoint of practical applications. CaIn 2 O 4 crystallizes in the orthorhombic symmetry system having space group Pca2 1 or Pbcm [5]. Reid has shown that CaIn 2 O 4 has a CaFe 2 O 4 type structure and is isomorphous with CaSc 2 O 4 and with NaScTiO 4 [6]. SrIn 2 O 4 also has orthorhombic symmetry with space group Pnam [7]. Lalla and Muller-Buschbaum have shown that BaIn 2 O 4 crystallizes with monoclinic symmetry having space group P2 1 =a, and it represents a new structure type with relationships to BaSc 2 O 4 [8]. Systematic study of the oxide indate ceramics is expected to provide fundamental understanding of the crystal chemistry of these materials and consequently better device design prospects. Many researchers are now attempting to prepare these materials using special processing techniques such as the sol±gel method and combustion techniques, which will be useful either in the reduction of the synthesis temperature or better control over the particle size, thereby achieving improved properties. Combustion synthesis (CS) was found to be successful in preparing many metastable phases and also conventional materials at low temperatures [9]. In conventional solid state reaction (SSR), the oxides of the starting materials are mixed together and red at higher temperatures. However, atomic level mixing is not possible and also agglomeration may occur. In the CS route, the nitrates of the starting materials are in solution form, which facilitates ionic level mixing of the constituents and leads to the formation of mixed oxides at lower temperatures than by the solid state reaction. It is of interest to compare these two methods in the synthesis of mixed oxide powders and their in uence on monophase formation and crystallization of the powders. In this letter, the synthesis of various indates with general formula AIn 2 O 4 (A ˆ Mg, Ca, Sr, Ba) by SSR and CS is reported. Characterization carried out using X-ray diffraction (XRD) and diffuse re ectance spectra is also reported. In the SSR route, the indates of magnesium, calcium, barium and strontium were prepared by taking stoichiometric quantities of the respective carbonates and indium oxide. The powders were mixed thoroughly in acetone. All samples were calcined at 900 8C for 24 h in air using a muf e furnace. Intermediate grinding was carried out to obtain uniformity and better reactivity. Samples were made into pellets of diameter 12 mm by applying a pressure of 2 t cm 2. The pellets were sintered at 950 8C, 1000 and 0 8C for 48 h followed by further heating at C for 60 h. For the CS route, the indates were prepared by mixing stoichiometric quantities of their respective nitrates, indium nitrate and a calculated amount of urea as fuel in the ratio 1:2:7 [9]. Indium nitrate was prepared by dissolving the appropriate quantity in hot HNO 3 solution (1:1). The clear solutions, in 200 ml beakers, were placed in a furnace at 500 8C. As the reaction was an exothermic one, the solution mixture underwent combustion for about 30 s. The resultant buffy mass, after grinding, was heated at 900 8C for 36 h followed by further heating at C for 6 h. The XRD patterns were recorded in a Jeol JDX X-ray diffractometer with CuK á radiation in the 2è range 10±85 8C. The particle sizes of the powders were analysed using a Malvern Instruments Easy Particle Sizer (M3.0). Lattice parameters were computed from 2è values using a least squares re nement program. KBr was used as the internal # 1998 Chapman & Hall 619

2 standard for correcting the instrumental errors. The density of the pellets was estimated using the weight loss technique. The diffuse re ectance spectra were recorded using a Hitachi U-3400 spectrophotometer with alumina as standard in the wavelength region of 200±800 nm. Although the various indates studied have the same general formula AIn 2 O 4, they crystallize in different structures. This is mainly attributed to the variation in the cationic sizes since monophase formation occurs at lower temperature for the larger cationic size compounds. Compared with all other indates, MgIn 2 O 4 has smaller cationic size and its synthesis temperature is higher. By SSR, monophase of magnesium indate, calcium indate, strontium indate and barium indate occurs at 1300, 0, 1000 and 950 8C, respectively, but by CS, up to C, phase formation does not occur in the case of magnesium indate and only 46.29% of phase formation occurs for calcium indate. Monophase formation of strontium indate and barium indate occurs at 700 8C and C, respectively. Subsequent heat treatment only leads to an increase in the intensity of the peaks and better crystallinity of the samples. The relative phase formation is estimated from the relations [10]: AIn 2 O 4 (%) ˆ (I a =I a I b ) In 2 O 3 (%) ˆ (I b =I a I b ) where I a is the maximum intensity of the corresponding Aln 2 O 4 peak and I b is the maximum intensity of the In 2 O 3 peak. The results are listed in Table I. Powder X-ray patterns of magnesium indate, calcium indate, strontium indate and barium indate were indexed on the basis of cubic, orthorhombic, orthorhombic and monoclinic structures, respectively [2, 5, 7, 8]. Figs 1±7 give the XRD patterns of magnesium indate, calcium indate, strontium indate and barium indate, heat treated at different temperatures, both by SSR and CS. The lattice parameters, density and optical band gap values are given in Tables II and Table III. The agreement of the lattice parameters with the literature is good. The density of the samples prepared by SSR was found to be more than 80% of the theoretical density, while that for the powders prepared by the CS route was more than 90%. The particle sizes of the samples are found to be in the range 7±28 ìm. The diffuse re ectance spectra of the samples prepared by both SSR and CS are shown in Figs 8 and 9. A decrease of re ectance was noted in the case of MgIn 2 O 4 from 500 to 800 nm, caused by conduction electrons. The fundamental absorption edge of all indates appeared between 250 and 400 nm, which is a shorter wavelength than ITO [1]. This shows that the optical band gaps of all indates are larger than that of In 2 O 3. The optical transmittance of all indates was much higher than that of ITO in the blue region (400±500 nm), which is important for application in full coloured LCDs. In the case of calcium indate, two peaks were observed. These were attributed to the multiphasic (e) TA B LE I Percentages of various indate phases Synthesis Composition Temperature Indate route (8C) (%) SSR MgIn 2 O CaIn 2 O SrIn 2 O BaIn 2 O CS CaIn 2 O SrIn 2 O 4 As prepared BaIn 2 O Figure 1 Powder XRD patterns of MgIn 2 O 4 heated at different temperatures by SSR C, 950 8C, C for 24 h, 0 8C for 48 h and (e) C for 60 h. (u), MgO;, In 2 O 3.

3 Figure 4 Powder XRD patterns of SrIn 2 O 4 heated at different temperatures by SSR C, 950 8C and C for 24 h. (s), SrO;, In 2 O 3. Figure 2 Powder XRD patterns of CaIn 2 O 4 heated at different temperatures by SSR C, 950 8C, C for 24 h and 0 8C for 48 h. (), CaO;, In 2 O Figure 5 Powder XRD patterns of SrIn 2 O 4 heated at different temperatures by CS. As-prepared, 900 8C for 36 h and C for 6 h. (s), SrO;, In 2 O 3. Figure 3 Powder XRD patterns of CaIn 2 O 4 heated at different temperatures by CS. As-prepared, 900 8C for 36 h and C for 6 h. (), CaO;, In 2 O 3. nature of the samples observed in the XRD pattern. The absorption edge of the second peak just coincided with the absorption edge of standard In 2 O 3. The optical band gaps of magnesium indate, 621

4 Figure 6 Powder XRD patterns of BaIn 2 O 4 heated at different temperatures by SSR C, 950 8C and C for 24 h. (3), BaO;, In 2 O 3. calcium indate, strontium indate and barium indate prepared by SSR were 3.1, 3.9, 3.6 and 3.2 ev; those of calcium indate, strontium indate and barium indate prepared by CS were 4.0, 4.2 and 4.0 ev. The variation in band gaps prepared by the two techniques was due to the variation in the particle Figure 7 Powder XRD patterns of BaIn 2 O 4 heated at different temperatures by CS. As-prepared, 900 8C for 36 h and C for 6 h. (3), BaO;, In 2 O 3. size. The band gaps of the various indates reported here were greater than 3 ev, and thse indates may be useful as transparent conducting oxide materials. In conclusion, it was observed that the CS route produced monophase strontium indate powder at a lower temperature compared to that obtained by the SSR route. The synthesis temperature depends on the melting point of the parent oxides. In other words, the synthesis temperature is lower for lower melting point oxides and higher for higher melting point TA B LE I I Lattice parameters, density and band gap of various indate compositions prepared by SSR Composition System Lattice parameter (nm) Density Band gap (ev) a b c Theoretical Experimental TD (g cm 3 ) (gcm 3 ) (%) MgIn 2 O 4 Cubic ± ± (0.887) CaIn 2 O 4 Orthorhombic (0.972) (1.130) (0.324) SrIn 2 O 4 Orthorhombic (0.981) (1.149) (0.325) BaIn 2 O 4 Monoclinic (1.34) (1.00) (0.61) TABLE III Lattice parameters, density and band gap of various indate compositions prepared by CS Composition System Lattice parameter (nm) Density Band gap (ev) a b c Theoretical Experimental TD (g cm 3 ) (gcm 3 ) (%) CaIn 2 O 4 Orthorhombic (0.973) (1.133) (0.323) SrIn 2 O 4 Orthorhombic (0.980) (1.150) (0.328) BaIn 2 O 4 Monoclinic (1.34) (0.98) (0.62) 622

5 Reflectance (%) oxides. Indate powders with general formula AIn 2 O 4 (A ˆ Mg, Ca, Sr, Ba) prepared from a carbonate precursor have a low synthesis temperature compared to those prepared via the oxide precursor route. Acknowledgement This work was supported by the Department of Science and Technology, New Delhi, India, under SP-S2-M Wavelength (nm) Figure 8 Diffuse re ectance spectra of MgIn 2 O 4, CaIn 2 O 4, SrIn 2 O 4 and BaIn 2 O 4 by SSR. Reflectance (%) Wavelength (nm) Figure 9 Diffuse re ectance spectra of CaIn 2 O 4, SrIn 2 O 4 and BaIn 2 O 4 by CS and subsequent annealing at 900 8C for 36 h. References 1. N. UEDA, T. OMATA, N. HIKUMA, K. UEDA, H. MIZOGUCHI, T. HASHIMOTO and H. KAWAZOE, Appl. Phys. Lett. 61 (1992) JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS, Card (JCPDS, Swarthmore, 3. H. KAWAZOE, N. UEDA, H. UN'NO, T. OMATA, H. HOSONO and H. TANOUE, J. Appl. Phys. 76 (1994) T. MINAMI, S. TAKATA, T. KAKUMU and H. SONO- HARA, Thin Solid Films 270 (1995) JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS, Card (JCPDS, Swarthmore, 6. A. F. REID, Inorg. Chem. 6 (1967) JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS, Card (JCPDS, Swarthmore, 8. A. LALLA and H. K. MULLER-BUSCHBAUM, J. Less- Common Met. 154 (1989) D. A. FUMO, M. R. MORELLIand A. M. SEGADAES, Mater. Res. Bull. 31 (1996) MING-CHRONG JIANG, TIAN-JUE HONG and TAI-BOR WU, Jpn. J. Appl. Phys. 33 (1994) Received 17 October 1997 and accepted 23 January

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