10  Anharmonicity and Thermal Expansion

NoteLearning Objectives

• Explain why a strictly harmonic crystal has no thermal expansion.
• Introduce the quasi-harmonic picture and define the mode and average Grüneisen parameters.
• Derive the Grüneisen relation for the thermal expansion coefficient \(\alpha(T)\).
• Interpret the difference between \(C_P\) and \(C_V\) in a solid and connect it to thermal expansion.
• Explain qualitatively how anharmonicity gives phonons finite lifetimes and thereby produces thermal resistance.