Notes on Structural Phase Transitions

Structural Phase Transitions

• Definition: A structural phase transition is a change in structure to a new, distinct phase. Examples include:
  • Crystal to crystal transitions
  • Crystal to liquid (melting)
  • Crystal to glass transitions
• Inducement: Transitions may occur due to changes in temperature, pressure, electric field, or magnetic field.

Classification of Phase Transitions

• By Mechanism:

  • Reconstructive Transitions:
  • Involves breaking and forming many new bonds.
  • Often slow due to large activation energy.
  • No symmetry relationship between phases.
  • Displacive Transitions:
  • Involves distortion of the structure rather than breaking bonds.
  • Typically fast with low activation barriers.
  • Symmetry relationship present between phases.

• Thermodynamic Classification:

  • First Order Transitions:
  • Characterized by discontinuity in first derivative of free energy with temperature/pressure.
  • Associated with volume change at phase transition.
  • Formula: $\frac{dS}{dT} = -\frac{dG}{dP}$, $ext{At the transition, } riangle G = 0$.
  • Second Order Transitions:
  • Characterized by discontinuity in second derivatives of free energy.
  • Changes in heat capacity, thermal expansion but not volume/enthalpy.
  • No latent heat observed.

Phenomena at Phase Transitions

• First Order Transition:
  • Discontinuities in enthalpy and entropy curves.
• Second Order Transition:
  • Disorder transitions occur below transition temperature.
  • Significant heat capacity changes can occur prior to transition (known as the