Low Temperature Thermal Conductivity of KTaO3 and KTN Single Crystals
The mixed crystals KTa1-x NbxO3 (KTN) are ferroelectrics with a transition temperature Tc which depends on the Nb content x. The phase diagram for small x, established by dielectric and elastic measurements1,2, shows that crystals with x < 0.8% remain paraelectric at all temperatures. We have measured the thermal conductivity K of several crystals with x varying from 0 to 3%, from 1.3 to 200 K and, in some cases, down to 80mK. The standard stationary flow method was used and the error in K is thought to be less than 7%. The single crystals were grown by means of a flux technique, using a high temperature (1430°C) reaction of Ta2O5, Nb2O5 and an excess of K2CO3. The material, which was contained in a platinum crucible, was held at the maximum temperature for approximately 12 hours, and then slowly cooled (1° to 3°C/hour) between 1430 and 950°C. Self-nucleation usually resulted in the production of 1 to 4 large grains. Cooling from 950°C to room temperature was accomplished in 8 to 10 hours and hot H2O was then used to remove the flux in order to free the crystals. The samples were cut, using a wire saw, as parallelepipeds with cross-sections about 2 × 2 mm2 and length 8 to 16 mm along 100 axis. Typical compositional inhomogeneities can be estimated to about 0.3%2.
KeywordsAcoustic Phonon Phonon Scattering Mixed Crystal Residual Impurity Elastic Measurement
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