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The thermal conductivity K and the thermoelectric power Q of single-crystal samples of rutile were measured from 2 to 300 °K. Pure, niobium-doped, vacuum-reduced, and hydrogen-reduced specimens were investigated. The thermal conductivity of the pure crystals is similar to that usually found for insulators, with a maximum at about 15°K. Boundary scattering determines K below the maximum, and in the range 25 to 100°K the conductivity varies exponentially with temperature, which is characteristic of umklapp scattering. The Callaway expression for K was used to fit the data over the entire temperature range. From measurements in both the c and an a direction, it was found that the anisotropy ratio K c /K a = 1.5±0.1 above 25°K. The ratio decreases below 25°K to a limiting value of 1.05±0.1. A sample doped with 0.1% Nb has a thermal conductivity quite similar to the pure Ti02. The K of the reduced samples is quite depressed at low temperatures and a discussion of possible processes is included. On oxidation of a reduced sample it was discovered that the thermal conductivity did not recover completely to the original values measured on the same specimen before reduction. For one crystal a comparison of K in the reduced and reoxidized states yields values for the extra thermal resistance which are compatible with the number of point defects introduced by the reduction. The thermoelectric power of the semiconducting samples (Nb-doped and reduced) increases rapidly below 100°K, indicating a large phonon-drag contribution.

The thermal conductivity of single crystalline BaTiO 3 has been measured in the temperature range of 100-500OK. In the neighbourhood of the transition temperature a reduction of the thermal conductivity is observed. This result can be explained in view of a current theory on ferroelectricity which introduces the concept of low frequency ferroelectric modes of lattice vibration.Optical phonon effects in thermal conductivity studies have been reported before [1][2][3].In this note we report measurements on BaTiO 3 near its transition temperatures that differ from previous measurements [4][5][6]. We believe to see the influence of low lying transverse optical phonon branches. Our way of reasoning can also be applied to measurements on other single crystalline ferroelectric materials near their Curie temperature [7,8].Thermal conductivity measurements have been performed by a steady state heat flow method [9] in a temperature range of 100 -500°K. Differential thermocouples of copper-constantan were used. A Leeds and Northrup K3 potentiometer together with a Hewlett and Packard 419-A d.c. null detector were used for the thermocouple measurements. The temperature difference was of the order of one to four degrees centigrades. The sample of roughly three milimeter diameter and one milimeter thickness, was grown by floating zone process [10] with a Sr dope of 1.5%. At room temperature under a polarizing microscope it showed a multidomain structure. The heater and the thermocouples of 0.05 mm diameter were glued to the sample and the sample to the heat sink by a high temperature-setting epoxy resin.The result of the measurements is shown in fig. 1. The curve shows dips in the neighbourhood of the transition temperatures. At the high temperature side it is rising. We expect it to merge to a higher lying curve that has a T-1 dependence at these temperatures.

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