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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

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 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...

pV-Diagrams

pV-Diagrams

The pV diagram, which is a graph of pressure versus volume of the gas under study, is helpful in describing certain aspects of the substance. When the substance behaves like an ideal gas, the ideal gas equation describes the relationship between its pressure and volume. On a pV diagram, it is common to plot an isotherm, which is a curve showing p as a function of V with the number of molecules and the temperature fixed. Then, for an ideal gas, the product of the pressure of the gas and its...

Phase Diagrams

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. These diagrams indicate the physical states that exist under specific conditions of pressure and temperature and also provide the pressure dependence of the phase-transition temperatures (melting points, sublimation points, boiling points). Regions or areas labeled solid, liquid, and gas represent single phases, while lines or curves represent...

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

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Pressure induced phase transitions in PbTiO(3).

P Ganesh¹, R E Cohen

¹Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015, USA.

Journal of Physics. Condensed Matter : an Institute of Physics Journal

|July 1, 2011

Summary

Density functional theory (DFT) and X-ray diffraction reveal a high-pressure morphotropic phase boundary (MPB) in lead titanate (PbTiO3), uncovering new monoclinic phases and supporting polarization rotation theories for electromechanical materials.

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Area of Science:

Materials Science
Condensed Matter Physics
Computational Materials Science

Background:

Lead titanate (PbTiO3) exhibits complex phase behavior under pressure.
Previous theoretical simulations and experiments suggested a morphotropic phase boundary (MPB).
The precise nature and extent of phases within the MPB region required further investigation.

Purpose of the Study:

To investigate the high-pressure morphotropic phase boundary (MPB) in pure lead titanate (PbTiO3) with high precision.
To reconcile discrepancies between theoretical simulations and experimental findings regarding the phase diagram.
To explore the implications of these findings for the design of novel electromechanical materials.

Main Methods:

High-precision density functional theory (DFT) calculations.
Utilizing the local-density approximation (LDA) and the Wu-Cohen generalized gradient approximation.
Comparison with experimental data from low-temperature, high-energy X-ray diffraction.

Main Results:

Confirmed the existence of a high-pressure MPB in PbTiO3.
Identified multiple monoclinic phases (Pm and Cm) within the MPB region, consistent with experimental observations.
DFT calculations supported the polarization rotation theory for phase transitions.
Provided an explanation for the pressure-dependent stability of the zone-boundary mode.

Conclusions:

The study provides a detailed understanding of the complex phase diagram of PbTiO3 at high pressures.
Findings support the polarization rotation mechanism as a key factor in the MPB.
The research opens avenues for engineering new electromechanical materials through chemical pressure applications.