LATTICE THERMAL-CONDUCTIVITY OF SOME TRANSITION-METALS AT VERY HIGH-TEMPERATURES

The lattice thermal conductivity lambda(g) of monatomic dielectric cry stals at very high temperatures, approaching their melting points, was considered by Ranninger, employing correlation function techniques. I n the quasi-harmonic approximation, the rise of temperature above the Debye theta increasingly affects the phonon variables, especially the phonon frequency, so that the lattice thermal resistivity takes the fo rm aT + bT2. At still higher temperatures approaching a critical tempe rature of lattice instability (T(c)), lambda(g) should drop rapidly fo llowing the equation lambda(g) = A + B (T(c) - T)1/2. However, in a pa ir approximation, the diffusive component of the phonon distribution s hould result, at very high temperatures approaching T(c), in a rise in lambda(g) according to the equation lambda(g) is-proportional-to 1 - A (T(c) - T)1/2. Our analysis of the high temperature thermal conducti vity data on some transition metals shows that the lambda(g) vs (T(c) - T)1/2 plots for tantalum and niobium (T(c) being taken as the meltin g point) have negative slopes, conforming to the pair approximation in Ranninger's theory. On the other hand, the lambda(g) vs (T(c) - T)1/2 plot for molybdenum has a positive (numerically nearly equal) slope, conforming to this quasi-harmonic approximation. It is shown that vaca ncy contribution to lambda(g) is comparatively small in these cases.