Heterobimetallic Ba–Co aminopolycarboxylate complexes as precursors for BaCoO 3-d oxides; towards a one-stage-deposition of cobaltite films† Ion Bulimestru, a Olivier Mentr  e, * b Nathalie Tancret, b Aur elie Rolle, b Nora Djelal, b Laurence Burylo, b Nicoleta Cornei, c Nelea Popa a and Aurelian Gulea a Received 12th April 2010, Accepted 18th August 2010 DOI: 10.1039/c0jm01012j Aminopolycarboxylate (APC) ligands have been selected to prepare seven new heterobimetallic barium-cobalt complexes as molecular precursors for BaCoO 3-d oxide materials. The use of APC ¼ nitrilotriacetate (nta 3 ), ethylenediaminetetraacetate (edta 4 ), cyclohexane-1,2- diaminetetraacetate (cdta 4 ) and diethylenetriaminepentaacetate (dtpa 5 ) polydentate chelating agents enables the possible incorporation of Ba and Co II or Co III in a 1 : 1 ratio. The crystal structure of the new BaCo II (nta)CH 3 COO$2H 2 O and BaCo(cdta)$5H 2 O have been refined from single crystal X-ray diffraction data. Our choice for the barium-cobalt system is based on the existence of a number of distinct BaCoO 3-d hexagonal perovskite polytypes, depending on the oxygen stoichiometry. It yields an easy probing of the cobalt oxidation state after thermal treatments. Temperature controlled X-ray diffraction (TCXRD) technique has been used to characterize the phases in competition upon thermal cycles of the precursors. The influences of the nature of ligands, initial cobalt oxidation state, temperature and working atmosphere on the process of formation of the mixed-oxide have been investigated. All the APC-precursors show appearance of a mixture of crystallized barium carbonate and Co 3 O 4 in various ratio during the degradation process under air or flowing oxygen at 350–450  C. Further thermal treatment under flowing oxygen leads to pure submicronic 2H-BaCoO 3-d above 650  C in the case of cdta complexes, i.e. in softer conditions compared to the solid state route performed under high oxygen pressure. No essential influence of the cobalt valence in the precuror complexes was observed regarding the target BaCoO 3-d oxygen non stoichiometry. Finally, with a view to access porous electrodes for SOFCs, we demonstrated the possibility of one-stage-deposition of BaCoO 3- d layers on dense YSZ substrates, with a relatively good adherence, via the high temperature degradation of these complexes. Introduction The development of electro-catalytic performances of transition metal mixed-oxides is associated with the necessity to increase their specific areas, generally mediated by the preparation method. 1 In this context, the sol–gel or co-precipitation tech- niques progressively turned out to provide sizeable particles at low or medium temperature compared to the standard solid state route. An alternative strategy, used to tune the microstructure, is the single-source molecular-precursor method which presents obvious advantages. 2 This method not only allows the mixing of the different metal species at an atomic scale but also consider- ably reduces the time and temperature of reaction. It has recently been shown that the use of molecular precursors often favours the formation of multimetallic crystalline oxides under conditions even milder than what can be achieved by sol–gel or co-precipitation routes. 3 In that context, carboxylates or aminopolycarboxylates (APC) complexes have been demonstrated to be promising single-source precursors due to the wide variety of available ligands fully removable upon adequate thermal treatment. Even more, the presence of bridging or chelating ligands in these compounds has been assumed to avoid undesired metal segregation during the oxide formation. 4 Thus, different alkaline earth/transition metal mixed oxides have been obtained from carboxylate-based precursors: BaTi 2 O 5 5a and BaTiO 3 5b - from citrate complexes; CaVO 3+d 5c - from tartrate; M 0 CuO 2 and M 0 MnO 3 (M 0 ¼ Ca, Ba), 5d SrMO 3– d (M ¼ Mn, Fe, Co), 5e CaMnO 3-d 5f and BaFeO 3-d 5g - from malonate based compounds. To our knowledge, these studies mainly focus on the structural aspect while few of these complexes have been considered as potential single-source precursors. Only, MCu(edta)$4H 2 O (M ¼ Ca, Sr, Ba), 6a Cu[Bi(edta)] 2 9H 2 O, 6b [Ni(H 2 O) 6 ][Fe- Cl(edta)H] 2 $4H 2 O, 6c [Sr(H 2 O) 7 ][Ti(O 2 )(edta)]$H 2 O, 6d LiSb(ed- ta)(H 2 O, 6e CaSb 2 (edta) 2 (H 2 O) 8 ] n 6f have been used to prepare MCuO 2 , CuBi 2 O 4 , NiFe 2 O 4 , SrTiO 3 , LiSbO 3 , CaSb 2 O 4 correspondingly. Also three edta-based [Co(NH 3 ) 4 (NO 2 ) 2 ][Bi- (edta)(H 2 O)]$2H 2 O, 6g Co(NxH) 2 (An) 2 ] 2 [Bi(edta)(H 2 O)] 2 $7- H 2 O 6h and [Co(NxH 2 (p-Tol) 2 ][Bi(edta)]$4H 2 O 6h coordination a Moldova State University, Coordination Chemistry Laboratory, 60 Mateevici str, Chisinau, MD, 2009, Moldova b Universit e Lille Nord de France, Unit e de Catalyse et de Chimie du Solide, CNRS UMR 8181, ENSC Lille – UST Lille, BP 90108, 59652 Villeneuve d’Ascq cedex, France. E-mail: olivier.mentre@ensc-lille.fr c Facult e de Chimie, D epartement de Chimie Inorganique, Universit e ‘‘Al. I. Cuza’’, Boulevard Carol I nr. 11, 6600 Iasi, Romania † CCDC reference numbers 772915 and 772916. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c0jm01012j 10724 | J. Mater. Chem., 2010, 20, 10724–10734 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/materials | Journal of Materials Chemistry Downloaded by Université Claude Bernard Lyon on 11 March 2011 Published on 06 October 2010 on http://pubs.rsc.org | doi:10.1039/C0JM01012J View Online