# Optical and structural study of the pressure-induced phase transition of CdWO$_4$

## TL;DR

This study investigates the pressure-induced phase transition of CdWO$_4$, revealing structural changes, band gap evolution, and stability of high-pressure phases through experimental and theoretical methods up to 23 GPa.

## Contribution

It provides the first detailed structural determination of the high-pressure phase of CdWO$_4$ and analyzes its electronic and vibrational properties under pressure.

## Key findings

- Phase transition occurs at 19.5 GPa with symmetry change from P2/c to P2_1/c.
- The band gap collapses by 0.7 eV at the transition.
- The high-pressure phase is more stable than the low-pressure phase at 18 GPa.

## 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.