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The vibrational/configurational split can be simply illustrated by a schematic one‐dimensional representation of interatomic potentials (Figure 18). Contrary to crystals, where these potentials have a long‐range symmetry, glasses have essentially a short‐range order because the bond angles and distances between next‐nearest neighbor atoms are not constant but spread over a range of values. The minima of potential energy, which determine the glass configuration, are separated by barriers with varying heights and shapes [43]. When thermal energy is delivered to the glass, the subsequent temperature rise is associated only with increasing amplitudes of vibration of atoms within their potential energy wells. Like for any solid, the heat capacity of the glass is, therefore, only vibrational in nature.

Figure 18 One‐dimensional schematic representation of interatomic potentials. Inset: potential‐energy landscape for a strong and a fragile liquid

(Source: After [42]).

C: crystal; IG: ideal glass; MC: metastable crystal.

At sufficiently high temperature, thermal energy increases to the point that atoms can overcome the barriers that separate their own from the neighboring potential energy wells (Figure 18). This onset of atomic mobility signals structural relaxation. If the relaxation time is longer than the experimental timescale, however, only the vibrational heat capacity is measured. If the temperature is increased further, or if time is sufficient for the new equilibrium configuration to be attained during the measurement, then the configurational heat capacity is also measured. When integrated over all atoms, the configurational heat capacity represents the energy differences between the minima of the potential energy wells that are explored as temperature increases (Figure 18).

The glass transition can thus be viewed as the point from which atoms begin to explore positions characterized by higher potential energies. Regardless of the complexity of this process at a microscopic level, this spreading of configurations over states of higher and higher potential energy is the main feature of atomic mobility. As a consequence, configurational heat capacities are positive. This feature, in turn, is consistent with the fact that any configurational change must cause an entropy rise when the temperature increases as required by Le Chatelier principle. As for relaxation times, they decrease with rising temperatures because large thermal energies allow potential energy barriers to be overcome more easily.

Another general feature of interatomic potentials is their anharmonic nature: displacements of the vibrating atoms from their equilibrium positions are not strictly proportional to the forces exerted on them. Because increasing vibrational amplitudes result in increasing interatomic distances (Figure 18), the thermal expansion coefficient is generally positive for glasses. In the liquid, it increases markedly when even greater interatomic distances result from configurational changes.