Real Materials

The Thermodynamics of Point Defects

� Point defects increase both the enthalpy and entropy of a crystal lattice.

� Thermodynamic equilibrium at constant temperature and pressure requires the Gibbs function of the system to be a minimum.

� The Gibbs function, G, is given by: G = H - TS = U + PV - TS

where: H = Enthalpy, S = Entropy, U = Internal Energy, V = Volume, P = Pressure,T = Temperature.

� In an arbitrary process the Gibbs function will change such that :

dG = dU + PdV + VdP - TdS - SdT

� With T and P constant: dG = (dU + PdV) - TdS = dH - TdS

� Both the Enthalpy and Entropy change as point defects are created by thermal energy fluctuations in the crystal lattice.