Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Sol-gel synthesis of substoichiometric cobalt ferrite (CoFe 2 O 4 ) spinels: Influence of additives on their stoichiometry and magnetic properties Janio Venturini ⁎ , Rubia Young Sun Zampiva, Sabrina Arcaro, Carlos Pérez Bergmann Department of Materials Engineering, Federal University of Rio Grande do Sul, Osvaldo Aranha 99, Porto Alegre 90035-190, Brazil ARTICLE INFO Keywords: Cobalt ferrite Sol-gel processing Substoichiometry Sol-gel additives ABSTRACT Spinel cobalt ferrites (CoFe 2 O 4 ) with varying levels of substoichiometry were prepared via sol-gel synthesis with different combinations of citric acid, dextrose and PVP (polyvinylpyrrolidone). The gels, prepared from the metal nitrates, were dried at 110 °C, further treated at 850 °C and finally subjected to thorough structural and magnetic characterization in order to correlate the composition of the gel to the structural and magnetic properties displayed by the ferrites. The materials synthesized in the present work have shown to be rather iron- depleted, reaching over 50% deficiency of the metal, which leaves the spinel lattice and forms Fe 2 O 3 instead. The fuel/oxidant ratio and the thermal behavior of the xerogels exert a direct influence on the compositional var- iation of the prepared spinels, which could in turn be correlated to the magnetic properties displayed by the particles. A maximum in coercivity of 2154.4 Oe was achieved without the application of any additive to the nitrate precursors. On the other hand, the magnetic remanence displayed by the ferrites shows a linear re- lationship to the iron content in their chemical formula. The results open up the possibility of fine-tuning the structural and magnetic properties displayed by the spinel product via careful control of the composition of the reaction medium. 1. Introduction The development of novel magnetic nanoparticles is a subject of growing interest, owing to the outstanding properties displayed by such particles at nanoscales [1]. Spinel ferrites (MFe 2 O 4 , M = Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ , …) are a great example hereof [2,3]. Cobalt ferrite (CoFe 2 O 4 ) is a member of this class of ceramics which is widely touted for its applications in medicine and magneto-optical devices [4,5]. Amongst other desirable characteristics for such applications, this ma- terial presents high coercivity, moderate saturation magnetization and an excellent chemical stability over a wide range of temperatures and environmental conditions [6]. Under normal conditions, ferrites crys- tallize in a spinel structure (space group Fd m 3 , n° 227) with the general formula A 2+ B 2 3+ O 4 2- , where the anions are arranged in an fcc lattice with half of the octahedral sites occupied by the trivalent cations whilst 1/8 of the tetrahedral positions are filled by the divalent metal [7]. In inverse spinels, the occupancy of the interstices is inverted, with all of the tetrahedral positions occupied by the trivalent ion while the octa- hedral B sites are occupied by the remaining cations [8]. Cobalt ferrite, however, displays a mixed configuration, with an intermediate struc- ture between normal and inverse spinel. Its structure can be defined as Co 1-x Fe x (Co x Fe 2-x )O 4 with 0 ≤ x ≤ 1, although a strong tendency towards unity is observed for the bulk material, i.e. cobalt ferrite usually crystallizes in an inverse fashion [9]. Nonetheless, kinetic parameters are much more decisive when it comes to the cationic dis- tribution in spinel ferrites [10]. The synthesis parameters utilized in the production of said materials are of paramount importance for the final arrangement of the metal ions [11]. Cobalt ferrite can be prepared in a variety of methods, including co-precipitation [12], polyol [13] and solid-state reaction [14]. Nevertheless, the sol-gel method is one of the most versatile methods for the preparation of transition metal oxide nanoparticles. This method is widely used in the production of oxide nanoparticles, given its low-cost and environmentally friendly char- acter, coupled to the fact that it allows for the fine-tuning of several synthesis parameters. The great control over the sol-gel process trans- lates into the direct manipulation of the desired characteristics of the spinel product [15,16]. The sol-gel method usually applies an organic mineralizer, which aids the incipient polymerization of the framework by complexing the cations involved in the process. Among a variety of additives, citric acid (C 6 H 8 O 7 ) is one of the most commonly applied gelation promoters [17,18]. Its low cost, high solubility and facile de- composition are some of the characteristics explaining its widespread utilization. Several other organic additives are also applied in sol-gel processes in order to influence the size, aspect ratio and composition of https://doi.org/10.1016/j.ceramint.2018.04.026 Received 21 February 2018; Received in revised form 2 April 2018; Accepted 4 April 2018 ⁎ Corresponding author. E-mail address: janio.venturini@ufrgs.br (J. Venturini). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Venturini, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.04.026