Published: January 19, 2011 r2011 American Chemical Society 2021 dx.doi.org/10.1021/ie101424z | Ind. Eng. Chem. Res. 2011, 50, 2021–2030 ARTICLE pubs.acs.org/IECR Soft Synthesis Route and Characterization of Superparamagnetic Mn 1/2 - Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O and Its Decomposed Product Banjong Boonchom* ,†,‡ and Naratip Vittayakorn ‡,§,|| † King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus, 17/1 M. 6 Pha Thiew District, Chumphon, 86160, Thailand Electroceramic Research Laboratory, ‡ College of KMITL Nanotechnology, and Advanced Materials Science Research Unit, § Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand ) ThEP Center, CHE, 328 Si Ayutthaya Road, Bangkok 10400, Thailand ABSTRACT: The superparamagnetic Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O was synthesized by a soft synthesis method using a Mn- (c)-Fe(c)-H 3 PO 4 system in water-acetone medium at ambient temperature. The synthesized Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O decomposed through the dehydration and the phosphate condensation reactions at high temperature and yielded binary manganese iron cyclotetraphosphate MnFeP 4 O 12 . The XRD and FTIR results of the synthesized Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O and the decomposed MnFeP 4 O 12 indicate the pure monoclinic phase with space group P2 1 /n and C2/c, respectively. The thermal behaviors and superparamagnetic properties of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O and MnFeP 4 O 12 in this work differed from the single compounds (M(H 2 PO 4 ) 2 3 2H 2 O and M 2 P 4 O 12 , where M = Mn, Fe) and the binary compounds (Mn 0.5 Fe 0.5 (H 2 PO 4 ) 2 3 xH 2 O) reported in previous works. The kinetic and thermodynamic functions for thermal decomposition of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O were studied and confirmed as reaction mechanisms. Vibrational frequencies of breaking bonds in two thermal transformation stages were estimated and assigned by comparison with the observed FTIR spectra. 1. INTRODUCTION Binary metal(II) dihydrogenphosphate hydrates M 1-x A x - (H 2 PO 4 ) 2 3 nH 2 O (M and A = Mg, Ca, Mn, Fe, Co, Ni, Zn, or Cu; x =0-1; n =1-4) have been investigated for over 30 years and have been widely applied as potential pigments, selective catalysts, phosphors, and materials for corrosion- resistant coatings, and they are biocompatible and biodegrad- able in tissue. 1-9 This phosphate hydrate group is trans- formed to binary metal(II) cyclotetraphosphate group M 2-y A y P 4 O 12 (y =0-2) via the reactions of dehydration and deprotonation of dihydrogenphosphate groups as well as polycondensation at high temperatures. 10-16 Both phosphate groups are good sources for macro- and micronutrients (P, Mg, Ca, Mn, Fe, Co, Ni, Zn, Cu) required by plants. 3-13 Consequently, these phosphate materials have been become a hot research topic in materials science in recent years. Some binary metal M 1-x A x (H 2 PO 4 ) 2 3 nH 2 O were prepared for the first time by Trojan et al. using corresponding metal carbonates and phosphoric acid at low temperature (313-353 K) with long time periods (2-60 h), 1-16 and releasing toxic gas (CO 2 ). Recently, M 1-x Ni x (H 2 PO 4 ) 2 3 xH 2 O (M = Mg, Mn, Fe, Co, Zn, and Cd) were prepared by corresponding metal carbo- nates and phosphoric acid at 293 K for 2-90 days. 9 More recently, Mn 1-x A x (H 2 PO 4 ) 2 3 xH 2 O (A = Ca, Fe, Co, Ni, and Cu) were prepared by metal sources (Mn (c; complexometric) or MnCO 3 and A(II) = CaCO 3 , Fe, CoCO 3 , NiCO 3 , or CuCO 3 ) and phosphoric acid H 3 PO 4 at ambient temperature for 20 s. These procedures were strong exothermic reactions and evolved gases (CO 2 and H 2 ). 17-21 However, a limited dose of M 1-x A x (H 2 PO 4 ) 2 3 nH 2 O could be synthesized without toxic gases. Synthesis of transition phosphates by a soft solid state reaction in media agents (ethanol, acetone, and water, etc.) at ambient temperature has received a great deal of attention due to their conveniences, cost-effectiveness, and that they are environmen- tally benign. 22-24 Because of its solubility in water and its ability to associate with metal ions in media, solvent has been used as a binder cum gel for shaping materials (bulk, porous, micro- or nanoparticles) and as a matrix for entrapment of ions to generate a gelled precursor, which resulted in obtaining different material or the same material with different size and morphology. The presence of media agent (solvent) reduces strong exothermic reaction and protects the evolved gases, which will be necessary for elaboration of technology to produce transition metal phos- phates. The use of solvent simplifies the process and would provide another alternative process for the environmental and economical synthesis of transition phosphate with different particle size and morphology. Herein, this work reports the fabrication of Mn 1/2 Fe 1/2 - (H 2 PO 4 ) 2 3 2H 2 O through a soft solid state reaction from metals of manganese and iron with phosphoric acid in water-acetone medium at ambient temperature with short time consumption (<30 min). The synthesized Mn 1/2 Fe 1/2 (H 2 PO 4 ) 3 2H 2 O de- composed to binary manganese iron cyclotetraphosphate MnFe- P 4 O 12 at 773 K. Consequently, kinetic (E a , A) and thermo- dynamic (ΔH*, ΔS*, ΔG*) functions of thermal transformation of Mn 1/2 Fe 1/2 (H 2 PO 4 ) 2 3 2H 2 O playing an important role in Received: July 4, 2010 Accepted: December 26, 2010 Revised: December 1, 2010