Debye lattice heat capacity
The three-dimensional Debye approximation assigns g(ω)=9Nω²/ω_D³ for 0≤ω≤ω_D so that the mode count is 3N.
Key knowledge
The three-dimensional Debye approximation assigns g(ω)=9Nω²/ω_D³ for 0≤ω≤ω_D so that the mode count is 3N. With Θ_D=ħω_D/k_B, its lattice C_V is 9Nk_B(T/Θ_D)³∫₀^(Θ_D/T) x⁴eˣ/(eˣ−1)²dx, approaching (12π⁴/5)Nk_B(T/Θ_D)³ at low T and 3Nk_B at high T.
Values and equations
C_V=9Nk_B(T/Θ_D)³∫₀^(Θ_D/T)x⁴eˣ/(eˣ−1)²dx; Θ_D=ħω_D/k_B
- C_V
- Lattice heat capacity J K⁻¹
- N
- Atom count
- k_B
- Boltzmann constant J K⁻¹
- T,Θ_D
- Temperature and Debye temperature K
- ω_D
- Debye cutoff angular frequency rad s⁻¹
- ħ
- Reduced Planck constant J s
- x
- Dimensionless integration variable
Conditions and scope
- Ideal harmonic bulk solid, low-frequency acoustic continuum with a fitted or sound-speed-derived cutoff; N is atom count.
- Anisotropic speeds, optical modes and anharmonicity limit the interpolation. Electronic or magnetic heat capacity is separate; the lattice T³ limit is not total heat capacity of every material.
Sources · 1
References
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Evidence and review
Solid State Physics: third-year lecture notes (2012): Sections 2.2.2–2.2.4: mode density 2.3, cutoff count 2.5, energy integral 2.6, low/high temperature limits and listed shortcomings; heat capacity obtained by differentiating the displayed energy integral.
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