Theory predicts and measurements confirm that anharmonic properties such as the lattice thermal resistivity, the Gr¿neisen parameter, and the coefficient of thermal expansion generally decrease as a function of increasing density upon compression of solids. The Debye-Gr¿neisen continuum model further predicts a discontinuous decrease of anharmonic properties during a phase transition with a density increase, in contradiction with available experimental data on alkali halides. We present results calculated from a model based on interatomic potentials. This model correctly predicts an increase in anharmonic properties for simple compounds transforming from a six to an eight-coordinated structure. These results stress the importance of interatomic spacing as well as density in determining changes thermodynamic properties due to polymorphism. The change in crystal structure across a phase transition also affects the thermal conductivity via Brillouin zone summations over the interacting phonon modes. We have used in the calculations either the bulk sound speed, or the Debye average velocity calculated from the longitudinal and transverse velocities (in both cases. theoretically derived from the interatomic potentials). We have found that the pressure dependence of thermal conductivity determined from the bulk sound speed agrees much better with experimental data for each phase than that determined from the average velocity. Apparently, shear modes contribute less to the change of thermal conductivity with compression than has been thought.