Thermal Conductivity of Some Ferroelectric Crystals with Hydrogen Bonds

Abstract

The thermal conductivity of KH₂PO₄, KD₂PO₄ and KH₂AsO₄ single crystals was measured in the temperature range from 4.5 to 300°K. For all the samples critical changes of the thermal conductivity are found at the Curie temperatures. The conductivity decreases gradually with falling temperature in the paraelectric state where the phonon mean free paths are limited by the disordered hydrogen atoms to the lattice spacings. It rises steeply just below the Curie temperature, increases exponentially with further temperature fall and attains to a maximum near 10°K. A marked isotope effect is observed in the maximum values of KH₂PO₄ and KD₂PO₄. The former amounts to 600 mW/cm°K and the latter about 5500mW/cm°K. This effect may be attributed to the difference of hydrogen motions in the double-minimum potentials. The peak of KH₂AsO₄ is only 220mW/cm°K. A small anisotropy in the conduction is found in the exponential variation range for these three crystals.