Synthesis and characterization of cerium substituted hematite by sol–gel method Nimai Chand Pramanik a, * , Tarequl Islam Bhuiyan b , Makoto Nakanishi b , Tatsuo Fujii b , Jun Takada b , Sang Il Seok a a Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea b Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700-8530, Japan Received 7 March 2005; accepted 30 June 2005 Available online 16 August 2005 Abstract Fine particles of cerium substituted hematite with different compositions were prepared by the sol – gel method from the aqueous solution of their corresponding metal salts. Mixed hydroxides of Fe 3+ and Ce 4+ were obtained by reacting aqueous NH 3 with the aqueous solution of FeCl 3 and Ce(NH 3 ) 2 (NO 3 ) 6 at pH 2.5 – 3.0) and they were heat treated at different temperatures between 400 and 1200 -C in air for 2 h to get the mixed oxide product. Thermal analysis and X-ray diffraction studies revealed the coexistence of Fe 2 O 3 and CeO 2 phase separately up to 900 -C, and covalent interaction between them above 1000 -C, which was evidenced by the shifting of the XRD peaks. Unit cell parameters and the cell volumes (V) of the samples derived between 1000 and 1200 -C were found to be in the range a = 5.045 – 5.048 T 0.007 A ˚ , c = 13.774 – 13.815 T 0.041 A ˚ and V = 303.608 – 304.874 A ˚ 3 respectively. It was observed that both the lattice parameters and the cell volumes were higher than that of the pure a-Fe 2 O 3 (a = 5.036 A ˚ , c = 13.749 A ˚ , V = 301.976 A ˚ 3 ) system. The expansion of lattice parameters further supported the incorporation of the large Ce 4+ ion into the a-Fe 2 O 3 matrix. The lattice parameter increased with increasing Ce 4+ ion loading. FESEM studies showed the growth of the particles with increasing temperature though it was suppressed due to the presence of Ce 4+ ions as compared to the pure system. D 2005 Elsevier B.V. All rights reserved. Keywords: Cerium substituted Fe 2 O 3 ; Sol – gel method; XRD; Crystal growth 1. Introduction Oxides of iron are common compounds which are widespread in nature and also readily synthesized in the laboratory. Hematite (a-Fe 2 O 3 , called Fbengara_ in Japa- nese), a well-known compound of iron, which is chemically stable, self-preservative in character and most abundant in nature. It is one of the most primitive pigments which exhibited various shades of colors ranging from yellow Y red dark brown Y purple Y black [1]. In 1000 BC, the paintings of Ajanta and Ellora caves in India were drawn by utilizing the red–yellow–black color of the hematite. The existence of wide range of colors of the hematite appears mainly due to the variation in crystallinity [2], particle size [3], shape and extent of aggregation [4], and also cation substitution [2,5]. Out of the various factors affecting the color of hematite, cation substitution has received current interest [6–9] due to the fact that the properties of the substituted hematite, such as the position of the visible and infrared absorption bands, magnetic and electrical properties, crystal size, etc. change regularly with increasing the substitution. For example, substitution of Al and Si changes the color tones of the hematite [2,5], while the substitution of trace of Zr, Nd, Hf, Ta and Ge improves its semiconductivity [10,11] and other fascinating properties. Our earlier work described the influence of Al and Si substitution on the color of hematite [12,13]. The effects of cation substitution on crystal sizes, the magnetic properties and color with the atoms of relatively lower ionic radii have already been 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.06.056 * Corresponding author. Tel.: +82 42 860 7326; fax: +82 42 861 4245. E-mail address: ncpramanik@rediffmail.com (N.C. Pramanik). Materials Letters 59 (2005) 3783 – 3787 www.elsevier.com/locate/matlet