Pressure dependence of the thermal Grüneisen parameter, with application to the earth's lower mantle and outer core

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Scientific paper

By considering high-temperature (classical) thermal oscillations of atoms in certain simple crystal structures with purely central interatomic forces, the treatment of anharmonic oscillations is generalised to random three-dimensional motion, yielding the Vashchenko and Zubarev relationship for the Grüneisen ratio γ at any pressure. If one-dimensional atomic oscillations only are considered the equation reduces to the Dugdale-MacDonald expression. To account for non-central forces additional terms must be introduced, giving: γ=1/2dK/dP-5/6+2/9P/K-f/18K+1/6df/dP/1-4/3P/K+f/3K where f = 0 for purely central forces. Calculations of f in terms of the Poisson ratio for different crystal structures have not been made, but for many materials the central-force approximation suffices. This is believed to be true both for the outer core (γ~1.4) and for the close-packed structures of the lower mantle (γ~1.0). For the upper mantle non-central atomic forces are important and we have no estimate of (γ independently of laboratory values for plausible minerals which suggest γ ~ 0.8.

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