Abstract
Inelastic neutron-scattering experiments and lattice-dynamical calculations are reported on a series of rareearth
orthophosphates RPO4 R=Tm, Er, Ho, and Tb. The experimental phonon spectra for the compounds are
in good agreement with our model calculations. The lattice-dynamical model is found useful for the calculation
of various thermodynamic properties such as the lattice specific heat, thermal expansion, and equation of state
of these compounds. The RPO4 compounds are known to transform to the scheelite body-centered tetragonal,
I41 /a or monoclinic phase P21 /n at high pressures. Our calculations show that while the scheelite phase
stabilizes at high pressure due to its lower volume, the monoclinic phase may occur as an intermediate phase
depending on the ionic size of the R atom. The latter phase is stabilized at higher temperature at high pressure
due to its high vibrational entropy. A pressure-temperature phase diagram is proposed.