Thermal Conductivity and Spin‐Phonon Interactions in GdCl3

The thermal conductivity of GdCl3 has been measured from 0.3 to 175 K as a function of temperature. Between 0.3 and 4 K the conductivity was also measured as a function of magnetic field. No critical phonon scattering was observed at T c. In the ordered phase the interaction between the spin system and the lattice was observed to lead to resonant phonon scattering. This could be accounted for by direct magnon‐phonon relaxation involving the optical magnons. The next nearest neighbor exchange constant was inferred to be 0.051 ± 0.008 K in substantial agreement with that found from pair spectra and high frequency relaxation experiments.