PHONON INTERACTIONS IN CRYSTALS.
Quarterly progress rept. no. 11, 15 Nov 65-14 Feb 66,
IBM THOMAS J WATSON RESEARCH CENTER YORKTOWN HEIGHTS NY
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The thermal decay times of thin indium films evaporated on sapphire and quartz single crystal substrates have been measured over a range of temperatures which, with earlier measurements, now extends from 4-300 K. At the higher temperatures the results were found to be in good qualitative agreement with the predictions of the thermal diffusion theory of heat transport, although there were some quantitative discrepancies. A power series for the free energy of a displacive ferroelectric is derived in the linear anharmonic approximation. This free energy is expressed in terms of a complete set of microscopic order parameters which determine the ionic displacements in the ferroelectric transition. In general this complete set of microscopic order parameters is larger than the set of order parameters used in the phenomenological free energy of Devonshire. However, for those ferroelectric crystals which are characterized by only one soyt optical branch this larger set may be reduced to the same as used by Devonshire. Such a reduction is explicitly carried out for the perovskite structure and the resulting expression is the same as that derived phenomenologically by Devonshire with the coefficients expressed in terms of microscopic lattice parameters.