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Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures
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10.1063/1.2773716
/content/aip/journal/jcp/127/12/10.1063/1.2773716
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/12/10.1063/1.2773716

Figures

Image of FIG. 1.
FIG. 1.

Bhatia-Thornton structure factors in the deeply supercooled regime. Number-number (solid lines), number-concentration (dashed lines), and concentration-concentration (dotted lines) structure factors are shown. The concentration-concentration structure factor has been normalized to 1 by plotting . Data are shown at the lowest equilibrated temperature for each given system. Note that all structure factors are finite in the limit and that the first sharp peak of around is roughly system independent.

Image of FIG. 2.
FIG. 2.

Temperature dependence of density along isobaric quenches at (left axis, circles) and of pressure in isochoric quenches (right axis, squares). Data are shown for BMLJ (filled symbols) and WAHN (open symbols). Isochoric quenches were performed at for BMLJ and for WAHN.

Image of FIG. 3.
FIG. 3.

Angell plot of relaxation times of large particles for a selection of AMLJ- mixtures. Results are shown for , 0.70, 0.73, 0.82 along the isobar . The reference temperature is described in the text. The inset shows the isobaric fragility index obtained from generalized VFT equation [see Eq. (5)] against size ratio .

Image of FIG. 4.
FIG. 4.

Angell plot of relaxation times of large particles for BMLJ (black circles) and WAHN mixture (white circles) along isobaric quenches at . The inset shows results at , 10, 20, 50 for BMLJ.

Image of FIG. 5.
FIG. 5.

Angell plot of total diffusion coefficient for a selection of AMLJ- mixtures. Results are shown for , 0.70, 0.73, 0.82 along the isobar . The reference temperature is obtained from fit to Eq. (7). The inset shows the isobaric fragility index obtained from Eq. (7) against size ratio .

Image of FIG. 6.
FIG. 6.

Angell plot of total diffusion coefficient for BMLJ (black circles) and WAHN mixture (white circles) along isobaric quenches at . The inset shows results at , 10, 20, 50 for BMLJ.

Image of FIG. 7.
FIG. 7.

Temperature dependence of the fraction of small particles at the center of (0,0,12) polyhedra in AMLJ- for selected values of . Results are shown for instantaneous configurations (main plot) and local minima (inset) along isobaric quenches at .

Image of FIG. 8.
FIG. 8.

Variation of icosahedral ordering with size ratio in additive mixtures AMLJ-. The fraction of small particles at the center of (0,0,12)-polyhedra in local minima is shown as a function of size ratio , at (black circles) and at the lowest equilibrated temperatures (open circles).

Image of FIG. 9.
FIG. 9.

Examples of locally preferred structures found in local minima of supercooled Lennard-Jones mixtures. Small and large particles are shown as dark and pale spheres respectively. (a) (0,2,8) polyhedron (twisted bicapped square prism) in BMLJ. This is the most frequent chemical coordination, incidentally one or two small particles can form the cap. (b) (0,3,6) polyhedron (capped trigonal prism) in . In this case, one of the caps is often formed by a small particle. (c) (0,0,12) polyhedron (icosahedron) in WAHN. On average, the coordination around the central particle is equimolar.

Image of FIG. 10.
FIG. 10.

Bond-angle distributions around small particles for WAHN (upper plot), BMLJ (middle plot), and (lower plot). The bond-angle distribution is shown as a solid line. Also shown is the bond-angle distribution restricted to small particles which are at the center of the locally preferred structure of the system, as given by Fig. 9. Data refer to the lowest equilibrated temperature of each given system. The sharp peaks in the distributions filtered for locally preferred structures reflect the ideal angles of the corresponding geometry.

Image of FIG. 11.
FIG. 11.

Temperature dependence of the fraction of small particles at the center of selected Voronoi polyhedra in instantaneous configurations (main plot) and local minima (inset). The fraction of (0,2,8) polyhedra in BMLJ (white symbols) and (0,0,12) polyhedra in WAHN mixture (black symbols) is shown along isobaric quenches at . Data for are close to those for BMLJ, but are not shown for clarity.

Image of FIG. 12.
FIG. 12.

Domains formed by locally preferred structures in local minima at the lowest equilibrated temperature at (WAHN: . BMLJ: ). Particles forming (a) (0,2,8) polyhedra in BMLJ and (b) (0,0,12) polyhedra in WAHN are shown as spheres of the same radius, irrespective of chemical species.

Image of FIG. 13.
FIG. 13.

Distribution of the size of domains formed by locally preferred structures in BMLJ (left plots) and WAHN (right plots) at different . Results refer to isobaric quenches at .

Image of FIG. 14.
FIG. 14.

Dynamical impact of locally preferred structures, as identified by the temperature dependence of the ratio (main plot) and (insets) at . See the text for definition of , , and . Upper plot: AMLJ- mixtures for , 0.70, 0.73, 0.82. Lower plot: BMLJ (filled circles), WAHN (open circles), and (stars). The dotted line drawn at 1 indicates the high-temperature limit.

Tables

Generic image for table
Table I.

Parameters of Lennard-Jones potentials for binary mixtures. Also shown are the masses and of the two species and the concentration of particles of species 1. In the case of additive mixtures AMLJ-, the following values of size ratio have been used: 0.60, 0.64, 0.70, 0.73, 0.76, 0.82, 0.88, 0.92, 0.96, 1.00.

Generic image for table
Table II.

Fitted parameters for relaxation times of large particles according to the generalized Vogel-Fulcher-Tammann given by Eq. (5), and for total diffusion coefficient according to Eq. (7). The reference temperature and the onset temperature are described in the text.

Generic image for table
Table III.

Most frequent Voronoi polyhedra around small particles. The percentage is computed with respect to the number of small particles in the system. Also shown is the average number of neighbors of species and . Results refer to local minima along the isobar and are shown for and slightly above the reference temperature , i.e., for the lowest equilibrated temperature.

Generic image for table
Table IV.

Lifetime of most frequent Voronoi polyhedra around small particles. Results are obtained from local minima at the lowest equilibrated temperatures . Also shown is the ratio , where is the relaxation time obtained from the condition .

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/content/aip/journal/jcp/127/12/10.1063/1.2773716
2007-09-26
2014-04-23
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/12/10.1063/1.2773716
10.1063/1.2773716
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