Employing correlation function techniques, Ranninger considered the lattice thermal conductivity, As, of dielectric crystals at very high temperatures. At temperatures, T, above the Debye temperature, 6,in a quasiharmonic approximation, the usual 7 variation becomes modified by a factor (l+aT)~(-1). At much higher temperatures, approaching the temperature of lattice instability, T.A, falls rapidly according to the equation Ag -A + B(TK— T)U1, However, in a pair approximation, the diffusive component of the phonon distribution leads to an equation of the form X oc 1 - A{T{ - T)i/2. Published data on the sintered oxides A12O3, MgO, UO2, and (Uo.sTho.sJO, are found to give roughly linear thermal conductivity, A, versus (T- T)in plots with negative gradients conforming to the pair approximation. Here the entire X is attributed to the lattice. On the other hand, values of Ag, deduced from the data on magnesium also those on sintered ThO2 and (U0,5Np0.5)O2 give Ag versus (TJ)2 plots with positive gradients, conforming to the quasiharmonic, approximation.
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