### Abstract

The characteristics of glasslike transitions in supercooled liquids and plastic crystals are discussed and a nearest neighbor interaction scheme, termed the "bond lattice" model, is proposed to account for the thermodynamic aspects of the phenomenon. Although the excitations of the bond lattice have features in common with those of the Ising lattice, the bond lattice has a zeroth order (noncooperative) case in which the thermodynamic properties are meaningful and readily evaluated. The zeroth order expressions appear adequate to account for the configurational heat capacity of the covalent bonded liquid ZnCl_{2}, but are clearly incapable of explaining the transition behavior of most molecular and hydrogenbonded glasses. In these cases it is believed the configurational excitations have considerable cooperative character. While the complex cooperative problem is not properly treated in this paper, we show how an ad hoc introduction of temperature-dependent bond energies (a sort of Bragg-Williams approximation) modifies the bond lattice model properties in the direction demanded by experiment. A second order transition between supercooled liquid and glass is not required by the treatment. An expression for transport properties, based on a postulated exponential relationship between rearrangement probabilities and degree of configurational excitation, is evaluated. The VTF (Fulcher) equation, commonly used to describe transport properties of viscous liquids, results as a good approximation. The VTF T_{0} parameter is determined mainly by the bond strength parameter of the model, while the B parameter can be related to the change of heat capacity at T_{g}. It is implied that glasses with little or no thermal manifestation of T_{g} will have large and almost Arrhenius temperature dependences, as seems characteristic of the strong network glasses, BeF_{2}, GeO_{2}, and perhaps SiO_{2}. The VTF transport parameters for the intermediate case of ZnCl_{2} are well predicted using the same molecular excitation parameters which account for the configurational heat capacity.

Original language | English (US) |
---|---|

Pages (from-to) | 470-481 |

Number of pages | 12 |

Journal | The Journal of Chemical Physics |

Volume | 57 |

Issue number | 1 |

State | Published - 1972 |

Externally published | Yes |

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### ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics

### Cite this

*The Journal of Chemical Physics*,

*57*(1), 470-481.

**Configurational excitations in condensed matter, and the "Bond Lattice" model for the liquid-glass transition.** / Angell, Charles; Rao, K. J.

Research output: Contribution to journal › Article

*The Journal of Chemical Physics*, vol. 57, no. 1, pp. 470-481.

}

TY - JOUR

T1 - Configurational excitations in condensed matter, and the "Bond Lattice" model for the liquid-glass transition

AU - Angell, Charles

AU - Rao, K. J.

PY - 1972

Y1 - 1972

N2 - The characteristics of glasslike transitions in supercooled liquids and plastic crystals are discussed and a nearest neighbor interaction scheme, termed the "bond lattice" model, is proposed to account for the thermodynamic aspects of the phenomenon. Although the excitations of the bond lattice have features in common with those of the Ising lattice, the bond lattice has a zeroth order (noncooperative) case in which the thermodynamic properties are meaningful and readily evaluated. The zeroth order expressions appear adequate to account for the configurational heat capacity of the covalent bonded liquid ZnCl2, but are clearly incapable of explaining the transition behavior of most molecular and hydrogenbonded glasses. In these cases it is believed the configurational excitations have considerable cooperative character. While the complex cooperative problem is not properly treated in this paper, we show how an ad hoc introduction of temperature-dependent bond energies (a sort of Bragg-Williams approximation) modifies the bond lattice model properties in the direction demanded by experiment. A second order transition between supercooled liquid and glass is not required by the treatment. An expression for transport properties, based on a postulated exponential relationship between rearrangement probabilities and degree of configurational excitation, is evaluated. The VTF (Fulcher) equation, commonly used to describe transport properties of viscous liquids, results as a good approximation. The VTF T0 parameter is determined mainly by the bond strength parameter of the model, while the B parameter can be related to the change of heat capacity at Tg. It is implied that glasses with little or no thermal manifestation of Tg will have large and almost Arrhenius temperature dependences, as seems characteristic of the strong network glasses, BeF2, GeO2, and perhaps SiO2. The VTF transport parameters for the intermediate case of ZnCl2 are well predicted using the same molecular excitation parameters which account for the configurational heat capacity.

AB - The characteristics of glasslike transitions in supercooled liquids and plastic crystals are discussed and a nearest neighbor interaction scheme, termed the "bond lattice" model, is proposed to account for the thermodynamic aspects of the phenomenon. Although the excitations of the bond lattice have features in common with those of the Ising lattice, the bond lattice has a zeroth order (noncooperative) case in which the thermodynamic properties are meaningful and readily evaluated. The zeroth order expressions appear adequate to account for the configurational heat capacity of the covalent bonded liquid ZnCl2, but are clearly incapable of explaining the transition behavior of most molecular and hydrogenbonded glasses. In these cases it is believed the configurational excitations have considerable cooperative character. While the complex cooperative problem is not properly treated in this paper, we show how an ad hoc introduction of temperature-dependent bond energies (a sort of Bragg-Williams approximation) modifies the bond lattice model properties in the direction demanded by experiment. A second order transition between supercooled liquid and glass is not required by the treatment. An expression for transport properties, based on a postulated exponential relationship between rearrangement probabilities and degree of configurational excitation, is evaluated. The VTF (Fulcher) equation, commonly used to describe transport properties of viscous liquids, results as a good approximation. The VTF T0 parameter is determined mainly by the bond strength parameter of the model, while the B parameter can be related to the change of heat capacity at Tg. It is implied that glasses with little or no thermal manifestation of Tg will have large and almost Arrhenius temperature dependences, as seems characteristic of the strong network glasses, BeF2, GeO2, and perhaps SiO2. The VTF transport parameters for the intermediate case of ZnCl2 are well predicted using the same molecular excitation parameters which account for the configurational heat capacity.

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M3 - Article

AN - SCOPUS:0039354130

VL - 57

SP - 470

EP - 481

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 1

ER -