Thermochimica Acta 481 (2009) 12–19 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Thermal magnetic investigation of the decomposition of Ni x Mn 1-x C 2 O 4 ·2H 2 O B. Donkova a,∗ , B. Kotzeva b , P. Vasileva a , D. Mehandjiev b a Department of Inorganic Chemistry, Faculty of Chemistry, University of Sofia, 1 J. Bourchier Av., Sofia 1164, Bulgaria b Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, Acad. G. Bonchev Str., bl 11, Sofia 1113, Bulgaria article info Article history: Received 3 August 2008 Received in revised form 14 September 2008 Accepted 17 September 2008 Available online 2 October 2008 Keywords: Decomposition Magnetic properties Solid solutions Thermal analysis abstract The systems Ni x Mn 1-x C 2 O 4 ·2H 2 O(x =0.11, 0.34) are characterized by XRD, SEM, TG/DTA, EGA-MS and magnetic measurements. The last confirmed that the studied samples are real solid solutions. The SEM reveals that the morphology depends on both the excess of C 2 O 4 2- and the initial ratio Ni/Mn. The thermal magnetic investigations (in situ) show that: (i) the presence of Ni in Ni x Mn 1-x C 2 O 4 ·2H 2 O leads to decreas- ing in the decomposition temperature in regard to that of the manganese oxalate; (ii) upon increasing the Ni content the temperature of decomposition (in air) is growing up; (iii) the presence of Ni stabilizes the manganese with respect to oxidation, in spite of the occurring process of decomposition. © 2008 Elsevier B.V. All rights reserved. 1. Introduction During the last years the interest in the utilization of 3d- transition metal oxalates of the so-called “magnesium series” (Mg, Mn, Fe, Co, Ni, Zn) as precursors for oxide materials has been grow- ing considerably. The reasons for this are the following: (i) the low temperature of decomposition; (ii) the liberation of a large amount of volatile substances—H 2 O, CO 2 and CO, resulting in final products of highly developed specific surface area; (iii) their iso- structural feature leading to their co-crystallization. The latter fact enables obtaining various ratios between the metal ions in the oxalate precursor with high precision, as well as achieving their uniform distribution in the crystal lattice. This option is especially important in view of obtaining spinels. There is a great interest in the system Ni(II)–Mn(II)–C 2 O 4 –H 2 O and in the stoichiometric and non-stoichiometric spinels obtained from it [1–18]. They exhibit a negative temperature coefficient of resistance [12–16] as well as a catalytic activity [17,18]. In order to use the oxalates as precursors it is necessary to con- trol the initial composition, to know well their crystallographic structure, the influence of the conditions of preparation and the mechanism of decomposition. The changes in the parameters of the crystal lattice and the thermal behavior of the precur- sor Ni x Mn 1-x C 2 O 4 ·2H 2 O were observed as a function of the ratio ∗ Corresponding author. Tel.: +359 2 8161214; fax: +359 2 8705024. E-mail addresses: nhbd@inorg.chem.uni-sofia.bg, nhbd@wmail.chem.uni-sofia.bg (B. Donkova). nickel–manganese as early as in one of the first works [3]. The stud- ies [6,7] considered the effect of different amounts of Cu or Zn on the crystallographic structure of the oxalate phase [7], as well as on the morphology and thermal behavior [6,7]. The work [4] moni- tored the influence of the hydrodynamic conditions of the synthesis upon the shape and size of Ni x Mn (1-x) C 2 O 4 ·2H 2 O(x = 0.24) by car- rying out the synthesis in different types of reactors. It has been pointed out that the difference in the homogeneity of the precur- sor in shape and size is connected with the stages of formation of the solid phase (nucleus formation, growth, agglomeration). As the conditions of obtaining the precursor exert substantial influence on the technological properties of the oxide materials it is of essen- tial importance to know the factors, which are controlling them. This has not been commented so far in the literature what is the influence of the surplus of oxalate ions on the structure, morphol- ogy and thermal behavior of Ni x Mn 1-x C 2 O 4 ·2H 2 O However in Ref. [19] it has been shown that the concentration of the oxalate ions and the time of contact with the mother solution is the basic factor determining the polymorphous form of the oxalates of the mag- nesium series. Our investigations on the mechanism of inclusion of 3d-elements (Mn, Ni, Co, Cu) in the zinc oxalate dihydrate [20] have established that the co-crystallization processes are also connected with the concentration of C 2 O 4 2- ions in the initial system. As it was stated above, in case of using 3d-transition metal oxalates as precursors it is very important to know the process of decomposition. A series of investigations have been reported on this topic—most of them summarized in Refs. [21,22]. A specific feature of these systems is the liberated CO, which is capable of reducing the obtained oxide. This way the oxide with a lower oxidation state 0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2008.09.021