UNCORRECTED PROOF Vacuum ] (]]]]) ]]]–]]] The role of metastable states in the temperature driven phase changes of molecular nano-clusters A. Proykova Faculty of Physics, University of Sofia, 5 J. Bourchier Blvd., Sofia-1126, Bulgaria Received 4 May 2001; received in revised form 2 February 2002; accepted 9 April 2002 Abstract The dynamics of finite-size systems is determined by their potential energy surface: a shallow landscape hampers the observation of dynamically coexisting phases as the dynamics of clusters containing the same number of TeF 6 or SF 6 molecules shows. For the case of SF 6 clusters the relative energies of most of the linked minima differ only slightly, the barriers between them are low and the competition between metastable states facilitates the system to escape fast from each minimum. The global minimum of both substances corresponds to a strained monoclinic structure with orientationally ordered molecules. There are several metastable states (local minima) corresponding to various solid- like phases. Depending on the rate of cooling, orientationally disordered body-centered cubic, partially orientationally ordered monoclinic or orthorhombic structures can be formed by those clusters below the freezing point. Unlike SF 6 clusters, the TeF 6 clusters readily exhibit coexistence of the ordered and disordered forms, which makes this substance promising for technological purposes. Both substances, SF 6 and TeF 6 , have global minima (0 K), which correspond to fully orientationally ordered strained monoclinic structures. r 2002 Published by Elsevier Science Ltd. PACS: 02.70.Ns; 64.60.Ht; 64.60.My; 64.70.Kb 1. Phase changes in finite-size systems Among the most spectacular macroscopic events in nature are transformations between the various states of matter. Such transformations, or phase transitions, appear to be of both fundamental and technological importance. Material science, che- mical technology, biophysics are very dependent on the deep understanding of critical phenomena. Recently, the field has expanded from the classical transitions of matter to a variety of phase changes typical for small systems: superparamagnetism of clusters, topological ordering, helix-coiling of proteins, and cosmological quark confinement [1,2]. Abrupt changes, discontinuities, and strong fluctuations as a consequence of a cooperative phenomenon characterize phase transitions in bulk. Statistical mechanics tells us that singular behavior of the free energy—a phase transition— can occur in thermodynamic limit only. In finite systems the transition is rounded and shifted over some region [3]. In what follows, we use terms ‘phase-change’ [4] and ‘phase-transformations’ [5] for small systems, keeping the term ‘phase transi- tions’ for bulk. The sharp phase transitions of bulk materials, with their precise coexisting curves, transform, in their small counterparts, into broad ARTICLE IN PRESS 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 3B2v7:51c GML4:3:1 VAC : 2551 Prod:Type:COM pp:128ðcol:fig::NILÞ ED:SusanKoshy PAGN: thara SCAN: VIJAYKUMAR E-mail address: anap@phys.uni-sofia.bg (A. Proykova). 0042-207X/02/$-see front matter r 2002 Published by Elsevier Science Ltd. PII:S0042-207X(02)00292-0