Thermal diffusivity (
D) of garnets with diverse chemical compositions was measured using the laser-flash technique, which is accurate (±2%) and
isolates the lattice component from direct radiative transfer. Temperatures ranged from ~290 to ~1,600 K (unless limited by
melting). Seven synthetic (e.g., YAG, GGG) and 15 natural garnets with two types of ionic substitution [Ca
3(Fe,Al)
2Si
3O
12 and (Mg,Fe,Ca)
3Al
2Si
3O
12] and varying amounts of OH
- were examined. Cation substitution or hydroxyl incorporation lowers
D from end-member values. Thermal diffusivity is constant once the temperature (
T) exceeds a critical value (
T
sat) of ~1,100 to 1,500 K. From ~290 K to
T
sat, the measurements are best represented by 1/
D=A+B
T+C
T
2 where A, B, and C are constants. These constants vary little among diverse chemical compositions, suggesting that the oxygen
sublattice controls heat transport. Higher order terms are needed only when
T
sat is low, such as Ant Hill garnet wherein 1/
D=0.049403+0.0032299
T−2.3992
T
2×10
−6+6.0168
T
3×10
−10(1/
D in s/mm
2;
T in K). The mean free path (λ, computed from
D and sound velocities) is slightly larger than the lattice parameter above
T
sat, in accord with phonon–phonon interactions requiring non-localized modes. At most temperatures, λ is nm-sized. Large values
of λ are obtained by extrapolation to a few Kelvins, suggesting that boundary scattering can only be important at extremely
cold temperatures. The observed behavior with
T and chemical composition is consistent with the damped harmonic oscillator model. Phonon transport is best represented by
inverse thermal diffusivity wherein 1/
D goes as
T
n where
n is between 1 and 3 up to ~200 K, depends on a quadratic or cubic polynomial at moderate
T, but is constant above
T
sat. The predicted and observed temperature response of 1/
D mimics the well-known form for heat capacity, in that acoustic modes control heat transport near cryogenic temperatures,
optic phonons dominate above ambient temperature, and a limit analogous to that of Dulong and Petit is reached at very high
temperature, due to full population of discrete phonon states.
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