  We report a joint experimental and theoretical investigation of the
high-pressure structural and vibrational properties of terbium sesquioxide
(Tb2O3). Powder x-ray diffraction and Raman scattering measurements show that
cubic (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa. We observe a
first irreversible reconstructive transition to the monoclinic (B-type) phase
at ~7 GPa and a subsequent reversible displacive transition from the monoclinic
to the trigonal (A-type) phase at ~12 GPa. Thus, Tb2O3 is found to follow the
well-known C-B-A phase transition found in other cubic rare-earth sesquioxides
with cations of larger atomic mass than Tb. Our ab initio theoretical
calculations predict phase transition pressures and bulk moduli for the three
phases in rather good agreement with experimental results. Moreover,
Raman-active modes of the three phases have been monitored as a function of
pressure and lattice-dynamics calculations have allowed us to confirm the
assignment of the experimental phonon modes in the C-type phase as well as to
make a tentative assignment of the symmetry of most vibrational modes in the B-
and A-type phases. Finally, we extract the bulk moduli and the Raman-active
modes frequencies together with their pressure coefficients for the three
phases of Tb2O3. These results are thoroughly compared and discussed in
relation to those reported for rare-earth and other related sesquioxides. It is
concluded that the evolution of the volume and bulk modulus of all the three
phases of these technologically relevant compounds exhibit a nearly linear
trend with respect to the third power of the ionic radii of the cations.
