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Phase Transitions

Phase Transitions

Whether solid, liquid, or gas, a substance's state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the solid pack closely together, generally in a pattern. The particles vibrate about their fixed positions but do not move or squeeze past their neighbors. In liquids, although the particles are closely spaced, they are randomly arranged. The position of the particles are not fixed—that is, they are free to move past their neighbors to occupy...

Phase Transitions

Phase Transitions

A phase transition is the process in which a substance changes from one state of matter to another, like from a solid to a liquid, liquid to gas, or vice versa, at a specific temperature and under given pressure conditions. This change is spontaneous and is affected by alterations in temperature and pressure. These parameters impact the strength of the forces between molecules (intermolecular forces) in the substance.During a phase transition, both the initial and final phases of the substance...

Phase Transitions: Melting and Freezing

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...

Phase Transitions: Vaporization and Condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize (convert into vapor). The process is called vaporization—a surface phenomenon. Vaporization occurs when the thermal motion of the molecules overcome the intermolecular forces, and the molecules (at the surface) escape into the gaseous state. When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules...

Phase Diagram

Phase Diagram

The phase of a given substance depends on the pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region provide considerable insights into the thermal properties of substances. Such plots are known as phase diagrams. For instance, in the phase diagram for water (Figure 1), the solid curve boundaries between the phases indicate phase transitions (i.e., temperatures and pressures at which the phases coexist).

Phase Diagram

Phase Diagram

A phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure. It shows the boundaries between solid, liquid, and gas phases and the conditions at which these phases coexist in equilibrium. An area in a phase diagram represents a single phase, whereas lines or phase boundaries represent the equilibrium between two phases.In the phase diagram of water, the boundary line between the solid and liquid states illustrates...

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Related Experiment Video

Updated: May 15, 2026

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Published on: May 15, 2017

Characterizing Gateway Modes for Solid-Solid Phase Transitions in Organic Crystals: The Thermosalient 4-DBpFO.

Daria Ruth Galimberti¹, Xinyue Li¹, Maarten W de Dreu^1,2

¹Radboud University, Institute for Molecules and Materials, Nijmegen 6500 GL, The Netherlands.

Journal of the American Chemical Society

|May 14, 2026

Summary

Researchers analyzed the 4-DBpFO crystal phase transition using computed low-frequency Raman spectra (LFRS). The study reveals a collective ring motion and the impact of thermal factors on THz spectral shape.

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Area of Science:

Solid-state chemistry
Computational materials science
Spectroscopy

Background:

Thermosalient materials exhibit a "jumping" crystal phase transition upon heating.
Understanding the molecular dynamics governing these transitions is crucial for materials design.
Low-frequency Raman spectra (LFRS) provide insights into collective vibrational modes.

Purpose of the Study:

To elucidate the nature of the 1.2 THz "gateway" mode in the 4-DBpFO thermosalient phase transition.
To investigate the influence of anharmonicity, thermal disorder, and volume fluctuations on the THz spectral shape.
To validate a novel computational method for predicting LFRS in complex systems.

Main Methods:

Computed low-frequency Raman spectra (LFRS) benchmarked against experimental data.
Activity Weighted Velocities (AWV) method integrating classical trajectories with hybrid Density Functional Theory (DFT) Raman activities.
Analysis of anharmonic couplings, thermal disorder, and volume fluctuations.

Main Results:

The 1.2 THz "gateway" mode is identified as a coherent, in-phase collective motion of inner-outer rings.
Anharmonic couplings and cell fluctuations significantly influence the THz spectral shape.
Intermolecular correlations for this mode extend across multiple unit cells.

Conclusions:

The AWV method accurately computes LFRS for disordered systems and entropically stabilized phases.
This approach overcomes limitations of traditional harmonic spectra and DFT-Molecular Dynamics methods.
The findings offer a predictive tool for Raman activity in complex crystalline materials.