Abstract
Thermal expansion is an important property of substances. Its theoretical prediction has been challenging, particularly in cases the volume decreases with temperature, i.e., thermal contraction or negative thermal expansion at high temperatures. In this paper, a new theory recently developed by the authors has been reviewed and further examined in the framework of fundamental thermodynamics and statistical mechanics. Its applications to cerium with colossal thermal expansion and Fe3Pt with thermal contraction in certain temperature ranges are discussed. It is anticipated that this theory is not limited to volume only and can be used to predict a wide range of properties at finite temperatures.
Highlights
It is well known that thermal expansion originates from the effect of anharmonic terms in the potential energy on the mean separation of atoms at a temperature [1], as schematically shown in Figure 1a in the vicinity of equilibrium separation distance
If the separation of the substance at a temperature decreases with the increase in temperature, i.e., thermal contraction or negative thermal expansion, its potential energy must be again asymmetric as shown in Figure 1c, but in the opposite direction of thermal expansion
In our previous works [2,8,9,10], we proposed that a single-phase substance at high temperatures may consist of many states, each with its own potential energy similar to Figure 1a
Summary
It is well known that thermal expansion originates from the effect of anharmonic terms in the potential energy on the mean separation of atoms at a temperature [1], as schematically shown in Figure 1a in the vicinity of equilibrium separation distance. When the temperature is increased, the kinetic energy of atoms increases, and the atoms vibrate and move, resulting in a greater average separation of atoms and thermal expansion, i.e., the vibrational origin of thermal expansion. In our previous works [2,8,9,10], we proposed that a single-phase substance at high temperatures may consist of many states, each with its own potential energy similar to Figure 1a. All these states, except one and its degeneracies, are metastable in terms of total energy.
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