Electronic structure of FeWO4 and CoWO4 tungstates: First-principles FP-LAPW calculations and X-ray spectroscopy studies

First-principles LCAO study of phonons in NiWO4

“…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]. 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 calculated electronic structure in NiWO 4 can be compared to that in other wolframite-type monoclinic (P2/c) tungstates as CdWO 4 [36], FeWO 4 [37], CoWO 4 [37], and ZnWO 4 [22,23] as well as in triclinic CuWO 4 [38]. 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 .…”

“…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].…”

Effect of partial preferential orientation and distortions in octahedral clusters on the photoluminescence properties of FeWO4 nanocrystals

“…Several synthesis methods are used in the preparation of FeWO 4 nanocrystals, but the conventional hydrothermal (CH) method has most often been employed and reported in the literature. [27][28][29][30] However, the CH method has some inconveniences, such as a long time period (typically one-half to several days) because of slow heat transfer due to the conduction and convection transport mechanism and high electric power usage (over a thousand watts). The solution to these problems is the microwave-hydrothermal (MH) method; this method facilitates the attainment of this oxide material in a shorter time and with lower energy consumption 31 which is possible due to efficient internal heating (in-core volumetric heating) by direct coupling of microwave irradiation with solvent molecules, ions and hydroxides in the reaction mixture.…”

Effect of Zn2+ ions on the structure, morphology and optical properties of CaWO4 microcrystals

“…The second group have a wolframite-type monoclinic structure, where both the W and (Me = Mn 2? , Fe 2? , Ni 2? , and Zn 2? ions) are coordinated to six oxygen atoms forming octahedral [WO 6 ]/ [MeO 6 ] clusters [15][16][17][18].…”