Glossary of Terms
Order Parameter
An order parameter is a physical quantity that measures the degree of order in a system during a phase transition. It indicates whether the system is in a disordered or ordered state. In the high-temperature disordered phase, the order parameter is zero, while in the low-temperature ordered phase, it takes a nonzero value. Thus, it serves as a “measuring stick” for the material’s level of order.
Quadratic Coefficient
The quadratic coefficient is a factor in the free energy expansion that determines how strongly the order parameter contributes to the system’s stability. Its sign and value control whether the system favors a paraelectric or ferroelectric state.
First-Order Phase Transition
A first-order phase transition is a type of transition where the order parameter changes abruptly at the critical temperature. It is usually accompanied by latent heat, discontinuity in entropy, and phase coexistence. An example is the liquid-to-gas transition in water.
Second-Order Phase Transition
A second-order phase transition (also called a continuous transition) is one where the order parameter changes smoothly and continuously at the critical point. Unlike first-order transitions, it does not involve latent heat but is marked by diverging susceptibilities and critical phenomena. An example is the ferromagnetic to paramagnetic transition at the Curie temperature.
Domain Nucleation
Domain nucleation refers to the process by which small, stable regions (domains) of a new phase form within a material during a phase transition. In ferroelectrics, it involves the creation of regions with uniform polarization, which then grow and determine the material’s macroscopic properties.