Phase evolution in mixture of cobalt and fullerene deposited from vapor V. Lavrentiev a, * , A. Stupakov b , M. Barchuk c, d , I. Lavrentieva a , J. Pokorný b , J. Vacik a , P.  Capkov  a c , A. Dejneka b a Nuclear Physics Institute AS CR, Rez-130, Husinec 250 68, Czech Republic b Institute of Physics AS CR, Na Slovance 2, Prague 182 21, Czech Republic c The University of J.E. Purkyn e, Pasteurova 1, 400 96 Ústí nad Labem, Czech Republic d Institute of Materials Science, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany article info Article history: Received 13 November 2015 Received in revised form 11 March 2016 Accepted 19 March 2016 Available online 21 March 2016 abstract Recently we reported the evidence of solubility of Co in solid C 60 found from magnetization of the Co x C 60 mixture films (Phys. D: Appl. Phys. 48, 2015, 335002). In the present study we identified the Co solubility limit (x c ¼ 2) and specified phase evolution in the Co x C 60 mixtures in a wide interval of the Co con- centrations (0 < x < 50) that was carried out through systematic characterization of the film surface morphology, chemical and phase composition. The effect of air exposure allowed us to recognize three intervals of x where phase evolution is controlled by rather different mechanisms. In the first interval (0 < x < 2, dilute mixtures) the phase evolution yields two separated phases, namely fcc-C 60 and Co 2 C 60 . The dilute films do not include Co clusters and almost insensitive to the air exposure. Within the second and third intervals (supersaturated mixtures) designated as x c < x < x m and x m < x, respectively (x m ¼ 10÷15), the formation of the Co clusters dominates in the phase evolution, which are found to be sensitive to the air exposure. The Co clustering in the mixtures from the second interval is completed by formation of critical nuclei. Within the third interval of x the phase evolution is controlled by growth of the Co clusters due to coalescence process. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Intercalation of metal into fullerene network is a challenging subject promising new functional materials with attractive prop- erties [1]. The most convincing results within this subject concern to the A-C 60 systems (A is alkali metal), where the formation of the conductive and superconductive alkali fullerides A n C 60 (n ¼ 1,2,3,4,6) was established [1e5]. As for the latter, the super- conductive transition at the temperature T c ¼ 18 K was reported firstly for the K 3 C 60 fulleride [6]. This discovery yields whole set of the superconductive A n C 60 fullerides with even higher T c [7,8] that revealed a remarkable application potential of the metal-fullerene compounds. A highest transition temperature (T c ¼ 38 K) was re- ported for the Cs 3 C 60 fulleride synthesized by a solvent-controlled method [9]. The attempts to create the fulleride phases in the MeeC 60 systems (here Me is d- or f-metal) using sequential deposition (applied for creation of the A-C 60 systems) [1,2] were found to be useless due to relatively high cohesive energy of the metals [1,10,11]. The concept of cohesive energy was used also to explain the formation of the composite nanostructure in the Me x C 60 mixture produced by simultaneous deposition [12e14]. Despite the difficulties in access of the homogeneous Me fullerides in such mixtures (due to easy Me precipitation), the Me x C 60 composite nanostructure is of enhanced attention owing to the formation of the Me clusters as a precursor of the intriguing properties of the materials [15e17]. The bright example here is the CoeC 60 nano- composites. Although the previous study of conductivity in such materials did not reveal a superconductive transition [18,19], the unique coexistence of the Co clusters and the C 60 -based matrix yielded their remarkable magnetic and magnetotransport proper- ties suggesting valuable applications [20e22]. Evidently, variation of the Co concentration in the Co x C 60 mixture will influence the phase composition [18,22] and, respectively, the material proper- ties. In our recent report we demonstrated the dramatic difference * Corresponding author. E-mail address: lavrent@uj.cas.cz (V. Lavrentiev). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2016.03.045 0008-6223/© 2016 Elsevier Ltd. All rights reserved. Carbon 103 (2016) 425e435