Microscopic optical and photoelectron measurements of MWO4 (M=Mn, Fe, and Ni)

First-principles LCAO study of phonons in NiWO4

“…The conduction band is formed by the empty W5d states with an admixture of the empty Ni3d(e g ↓) states at the bottom of the band. These results agree well with those obtained recently by optical and ultraviolet photoelectron spectroscopy in [8]. The calculated phonon frequencies (Table 2) are compared with the experimental Raman signal measured at room temperature in Fig.…”

“…According to our calculations, NiWO 4 has an indirect band gap (E→ Γ or A→ Γ). 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]. It seems that the low quality measurements performed in a limited energy range from 1.4 eV to 3.4 eV on NiWO 4 thin films in [9] have led to confusion of the band gap with the absorption bands due to the d-d transitions at Ni 2+ ions, being responsible for the color of the compound [8,35].…”

“…Nickel tungstate (NiWO 4 ) has been extensively studied in the past because of its use for catalysts [1][2][3][4], humidity [5] and gas sensors [6], in microwave devices [7], and photoanodes [8,9]. The application of mixed nickeltungsten oxide thin films in electrochromic systems has also been proposed [10].…”

UV-VUV synchrotron radiation spectroscopy of NiWO4

“…2). They differ from that for ZnWO 4 from [32] due to the difference of about 1 eV in the band gaps: E g = 3.6 eV [46] for NiWO 4 , but E g = 4.6 eV for ZnWO 4 [47]. As a result, the strong excitonic band clearly visible at ~4.5 eV for ZnWO 4 [32] is not observed in the present data for NiWO 4 due to the spectrometer range limitations: the excitonic band is expected to be located below 3.7 eV.…”