  The structural properties of Thallium (III) oxide (Tl2O3) have been studied
both experimentally and theoretically under compression at room temperature.
X-ray powder diffraction measurements up to 37.7 GPa have been complemented
with ab initio total-energy calculations. The equation of state of Tl2O3 has
been determined and compared to related compounds. It has been found
experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure
up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being
the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state
after pressure release. To understand the pressure-induced amorphization
process, we have studied theoretically the possible high-pressure phases of
Tl2O3. Although a phase transition is predicted at 5.8 GPa to the orthorhombic
Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic a-Gd2S3-type
structure, neither of these phases were observed experimentally, probably due
to the hindrance of the pressure-driven phase transitions at room temperature.
The theoretical study of the elastic behavior of the cubic bixbyite-type
structure at high-pressure shows that amorphization above 22 GPa at room
temperature might be caused by the mechanical instability of the cubic
bixbyite-type structure which is theoretically predicted above 23.5 GPa.
