Pressure effects on the electronic and optical properties of AWO4 wolframites (A = Cd, Mg, Mn, and Zn): The distinctive behavior of multiferroic MnWO4

The electronic band-structure and band-gap dependence on the d character of A(2+) cation in AWO(4) wolframite-type oxides is investigated for different compounds (A = Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled d levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled d electron shells, give rise to direct and wide band gaps, Mn, with a half-filled d shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO4, ZnWO4, and CdWO4 blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO4 red-shifts at -22 meV/GPa. Finally, in MnWO4, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO6 complexes and how it affects their band-gap closure.