Optical and structural study of the pressure-induced phase transition of CdWO4

Abstract

The optical absorption of CdWO4 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 CdWO4 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 P 21/c in the highpressure 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 CdWO4 has been reevaluated with powder x-ray diffraction up to 15 GPa finding a bulk modulus of B0 = 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 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 P 21/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.

Brief Review of the Effects of Pressure on Wolframite-Type Oxides

“…They have been assigned to 19 Ag + 17 Bg modes. Out of them, only twenty-six modes have been observed [14,18]. The frequency of these modes and the pressure coefficients are summarized in Table IV.…”

“…Considering that the Cd-O distances are abnormally large in the wolframite-type structure, the coordination increase associated to the phase transition, relates well with CdWO4 which finds a more compact and stable structure that differs more from the wolframite-type structure than the high-pressure phase of the wolframite-type tungstates that keep the cationic coordination. This coordination change is directly related to a band-gap energy drop associated to the phase transition as we shall show in section IV [14].…”

“…However, no definitive conclusion can be stated on the structure of the HP phase of ZnWO4 or MgWO4 only from Raman measurements. Indeed, the accurate assignation of the modes of the HP phases of ZnWO4 and MgWO4 is waiting for HP single-crystal experiments similar to those already carried out in MnWO4 and CdWO4 [13,14].…”

“…Studies beyond the pressure range of stability have been carried out for CdWO4 [14]. In this compound optical-absorption studies under quasi-hydrostatic conditions have been performed up to 23 GPa.…”

“…However, during the last decade the interest on the behavior under high pressure of wolframites has boosted many works that have contributed to improve the understanding of their structural [12 -16], vibrational [15 -21], and electronic [14,22] properties under compression.…”