Title: Optical and structural study of the pressure-induced phase transition of CdWO$_4$,
Abstract: The optical absorption of CdWO$_4$ is reported at high pressures up to 23
GPa. The onset of a phase transition was detected at 19.5 GPa, in good
agreement with a previous Raman spectroscopy study. The crystal structure of
the high-pressure phase of CdWO$_4$ was solved at 22 GPa employing
single-crystal synchrotron x-ray diffraction. The symmetry changes from space
group $P$2/$c$ in the low-pressure wolframite phase to $P2_1/c$ in the
high-pressure post-wolframite phase accompanied by a doubling of the unit-cell
volume. The octahedral oxygen coordination of the tungsten and cadmium ions is
increased to [7]-fold and [6+1]-fold, respectively, at the phase transition.
The compressibility of the low-pressure phase of CdWO$_4$ has been reevaluated
with powder x-ray diffraction up to 15 GPa finding a bulk modulus of $B_0$ =
123 GPa. The direct band gap of the low-pressure phase increases with
compression up to 16.9 GPa at 12 meV/GPa. At this point an indirect band gap
crosses the direct band gap and decreases at -2 meV/GPa up to 19.5 GPa where
the phase transition starts. At the phase transition the band gap collapses by
0.7 eV and another direct band gap decreases at -50 meV/GPa up to the maximum
measured pressure. The structural stability of the post-wolframite structure is
confirmed by \textit{ab initio} calculations finding the post-wolframite-type
phase to be more stable than the wolframite at 18 GPa. Lattice dynamic
calculations based on space group $P2_1/c$ explain well the Raman-active modes
previously measured in the high-pressure post-wolframite phase. The
pressure-induced band gap crossing in the wolframite phase as well as the
pressure dependence of the direct band gap in the high-pressure phase are
further discussed with respect to the calculations.