Cerium molybdenum oxides for environmentally benign pigments Kalarical Janardhanan Sreeram a, * , Radhika Srinivasan a , Jeyapragasam Meena Devi b , Balachandran Unni Nair a , Thirumalachari Ramasami a a Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, TN, India b School of Physics, Madurai Kamaraj University, Madurai 625 021, TN, India Received 20 June 2006; received in revised form 3 July 2006; accepted 6 July 2006 Available online 8 September 2006 Abstract A new class of environmentally benign yellow rare earth pigments as alternatives to lead, cadmium and chromium pigments is presented. These pigments are based on cerium molybdenum oxides corresponding to the structural formulae NaCe 0.5 (MoO 4 ) and Ce(MoO 4 ) 2 . The col- oring properties of the compositions prepared by varying the Ce:Mo ratio in the presence and absence of phosphates have been evaluated. The reflectance spectra and photoacoustic spectra indicate the absorption in visible and ultraviolet regions under 500 nm, which could originate from the O 2p eCe 4f and the O 2p eMo 3d double charge transfer transitions, and as a result the pigments show yellow color. In the presence of a phosphate, the products are homogeneous and crystalline at temperatures of 700  C itself and exist as multiphasic with white colored CePO 4 being present in association with cerium molybdenum oxide. Further, the particle size of the cerium molybdenum oxide sample was lower in the presence of phosphate (116 nm). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Inorganic pigments; Ceramic pigments; High temperature pigments; Pigments of cerium; Molybdenum; Phosphate 1. Introduction Inorganic pigments are widely used in various applications such as paints, inks, plastics, rubbers, ceramics, enamels and glasses [1,2]. The pigments for coloring ceramics, usually in- organic products composed of metal oxides or compounds formed from the host of metal oxides, must possess thermal and chemical stabilities at high temperature and must be inert to the chemical action of the molten glaze [3]. The majority of inorganic pigments for the above applications currently em- ployed are derived from toxic metals such as cadmium, lead, chromium or cobalt [1]. Thus, serious need arises to search for materials of environmentally friendly and economically vi- able materials for the replacement of toxic components of pig- ments. Development of safe inorganic alternatives is essential, as the use of high-performance organic pigments have some limitations because of their thermal and ultraviolet (UV) ray radiation instabilities. Pigments based on CeO 2 are inorganic pigments with high temperature stability and in the orange- pink hue when doped with praseodymium [4]. Lanthanum oxide (La 2 O 3 ) and praseodymium dioxide (PrO 2 ) dissolve in cerium oxide at 1300  C forming a solid solution of the three oxides. CeO 2 ePrO 2 eLa 2 O 3 pigments give very interesting red hues in the ceramic glaze. These heat and chemical-resistant pigments have a fluorite structure and represent potential envi- ronmentally friendly inorganic pigments [4]. Recently pigments based on rare earth phosphates [5], Ce 1x M x W 2 O 8 (M ¼ Zr or Ti) were reported in the yellow seg- ment [6], along side the pigments based on Pr-ZrSiO 4 [7] and RE 2 Mo 2 O 9 , where RE ¼ La or Pr [8]. The intense color arises from mostly charge transfer interactions between a donor and acceptor with the metal ions playing the role of an acceptor. Several of these pigments employ high temperature calcina- tions in the synthesis process to form the ZrSiO 4 (zircon) phase, which tends to induce particle growth of the pigments. Consequently, it is difficult to apply the praseodymium yellow * Corresponding author. Tel.: þ91 44 2441 1630; fax: þ91 44 2491 1589. E-mail address: kjsreeram@rediffmail.com (K.J. Sreeram). 0143-7208/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2006.07.021 Dyes and Pigments 75 (2007) 687e692 www.elsevier.com/locate/dyepig