Structural transitions in small molecular clusters

Proykova, Ana; Radev, Rossen; Li, Fengyin; Berry, R. Stephen

doi:10.1063/1.478248

Cited by 16 publications

(21 citation statements)

References 14 publications

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“…This is called displacive-ordering transition. Similar structural transformations have been also detected in small, free clusters consisting of rigid octahedral molecules SF 6 [2] and T eF 6 [3,4] and have been studied by numerical simulations [5][6][7][8][9] and experimentally [10,11]. The change between phases may be continuous or discontinuous depending on the interaction potential.…”

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confidence: 85%

“…In order to account for the broken symmetry at the cluster surfaces, we use sum over the nearest neighbors. Since the molecules of type AF 6 have no low-order electrostatic moments, the Coulomb contribution to the Lennard-Jones potential can be neglected, see fig.2 in [4].…”

mentioning

confidence: 99%

“…Vibrational-rotational coupling was considered qualitatively to explain the two-step process of ordering in some clusters of octahedral molecules, O h symmetry, as the temperature decreases [4,8]. Below the melting temperature, those clusters assume a body-centered cubic structure bcc, O h symmetry, and orientational disorder of the molecules [2,5,3].…”

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Symmetry in order-disorder changes of molecular clusters

2002

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The dynamic orientational order-disorder transition of clusters consisting of octahedral AF6 molecules is formulated in terms of symmetry-adapted rotator functions. The transition from a higher-temperature body-centered-cubic phase whose molecules are orientationally disordered at their sites to lower-temperature, monoclinic, orientationally-ordered phase is a two-step process: first, at temperatures well below the limit of stability for the liquid, a transition occurs to a partially ordered monoclinic phase driven by the rotational-vibrational coupling. This transition has two local minima in the free energy, and hence behaves like a finite-system counterpart of a first-order transition. Further lowering of the temperature initiates another transition, to an orientationally-ordered base-centered monoclinic structure. This last transition is dominated by rotational-rotational interaction and is found from simulations to be continuous. The temperature of this transition predicted by the analytic theory presented here for a 59-molecule cluster of T eF6, 27K, is in good agreement with the 30K result of canonical Monte Carlo calculations.

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Symmetry in order-disorder changes of molecular clusters

2002

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“…For most of the clusters studied here we use the bcc structure as a reference arrangement to decide which atoms are bulk and which are surface ones. Since in the bcc crystal a bulk atom has 8 nearest neighbours, we adopt this number for the clusters and arrange clusters containing more than 8 atoms as spherically as possible following the bcc structure in the manner discussed in detail in [18].…”

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confidence: 99%

Classical simulations of magnetic structures for chromium clusters: Size effects

Proykova

Stauffer

2005

Open Physics

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Classical (Heisenberg) simulations show that the total magnetization of the lowest-energy states of clusters made of antiferromagnetically coupled chromium atoms is planar, rather than collinear, depending on the arrangement of the atoms. Although the model Hamiltonian is not restrictive, many cluster configurations of various numbers of atoms do not use all three directions for the spins. This result confirms the conclusion drawn from the local-spin DFT calculation by Kohl and Bertsch that clusters of N ≤ 13 have non-collinear magnetic moments. The present simulations show non-collinear spin ordering also for bigger clusters, designed to be as spherical as possible following the bcc arrangement, when atoms interact both with the nearest and next-nearest neighbours. Depending on the signs of the coupling constants frustration appears. The advantage of the discrete model, despite the simplicity, is that very large clusters and magnetization at finite temperatures can be studied. This model predicts that clusters with specific numbers of atoms interacting only with the nearest neighbours have collinear spins as in the bulk. We also apply the model to simulate the destruction of the anti-ferromagnetic ordering by thermal fluctuations. This model shows no unique magnetization of mixed F e 0.33 Cr 0.67 , which is consistent with experimental observations.

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“…Clusters have become increasingly popular systems to investigate the kind and mechanism of phase changes [1][2][3][4]. Transitions between different phases are not only commonly encountered in nature but they play a central role in many technological processes in industry, and are also of fundamental interest for theoretical physics in understanding the behavior of a large number of interacting particles.…”

Section: Introductionmentioning

confidence: 99%