Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

Abstract

We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO 4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn).

First-principles LCAO study of phonons in NiWO4

“…The calculated band gap value E g = 3 7 eV is in good agreement with the experimental one equal to 3.6 eV [8]. Note that the smaller value of the band gap in NiWO 4 (2.28 eV in [9] and 2.52 eV in [34]) has been also reported. We believe that these two numbers are underestimated due to incorrect band gap determination in [9].…”

“…Previous band structure calculations for these tungstates allows one to conclude that FeWO 4 [37] and CoWO 4 [37] have an indirect band-gap as we found also for NiWO 4 , whereas CdWO 4 [36] and ZnWO 4 [22,23] have a direct band-gap. Note that the indirect band-gap has been also found experimentally from optical absorption spectra in CuWO 4 [34,41]. The difference between the two groups of tungstates is connected with the contribution of partially filled 3d states, provided by Fe 2+ , Co 2+ , Ni 2+ , and Cu 2+ ions, into the band structure.…”

“…Most previous first-principles calculations were based on DFT functionals and underestimate the band gap values: E g = 2 9 eV versus E g (exp) = 3 8 − 4 09 eV in CdWO 4 [36]; E g = 1 78 eV versus E g (exp) = 2 0 eV in FeWO 4 [37]; E g = 1 36 eV [37] versus E g (exp) = 2 0 eV [39] in CoWO 4 ; E g = 1 9 eV [38] versus E g (exp) = 2 06 eV [40] and 2.3 eV [34,41] in CuWO 4 . However, the use of hybrid Hamil-tonians within the LCAO method allowed us to obtain for ZnWO 4 the band gap energy E g = 4 6 eV [22] and 5.4 eV [23] in better agreement with experimental values E g (exp) = 4 6 eV [42] and 4.9 eV [43].…”

Ab initio LCAO study of the atomic, electronic and magnetic structures and the lattice dynamics of triclinic CuWO4

“…The dispersion curves are plotted along nine different symmetry directions in the Brillouin zone. According to our calculations, CuWO 4 has an indirect band gap, which agrees with the experimental results of optical absorption spectroscopy [19,45]. The band gap values E g calculated for different functionals are given in Table 1.…”

A combined theoretical and experimental study of electronic structure and optical properties of β-ZnMoO4 microcrystals

“…According to the literature, the isostructural ZnWO 4 crystals exhibit an optical absorption spectrum governed by direct electronic transitions [73][74][75]. In this phenomenon, after the electronic absorption process, the electrons located in the maximum-energy states in the valence band revert to minimum-energy states in the conduction band under the same point in the Brillouin zone [75,76].…”