  Zircon-type HoPO4 and TmPO4 have been studied by single-crystal x-ray
diffraction and ab initio calculations. We report information on the influence
of pressure on the crystal structure, and on the elastic and thermodynamic
properties. The equation of state for both compounds is accurately determined.
We have also obtained information on the polyhedral compressibility which is
used to explain the anisotropic axial compressibility and the bulk
compressibility. Both compounds are ductile and more resistive to volume
compression than to shear deformation at all pressures. Furthermore, the
elastic anisotropy is enhanced upon compression. Finally, the calculations
indicate that the possible causes that make unstable the zircon structure are
mechanical instabilities and the softening of a silent B1u mode.
