The sample ZT exhibits a weak light absorption in the violet region, probably connected with a metal–ligand charge transfer (CT) analogue to the well-known Ti4+–O2 CT in rutile [12]. However, comparing the two spectra (Fig. 5), it is clear that the position of the CT band of zirconium titanate is shifted to higher energies as well as the band slope is much less inclined, thus playing minor effects on the visible range. At all events, some overlappings of CT and CF effects may occur, making more difficult the quantitative interpretation of optical spectra with measurement of CF strength (Dq) as well as interelectronic repulsion Racah B parameter. In order to enhance the resolution of CF peaks in the blue to UV
range, the CT contribution was removed by systematic subtraction of the spectrum of undoped zirconium titanate.
The wavenumbers of CF peaks and band width (d), the inferred values of average (cubic) Dq and Racah B35 as well
as the calculated CFSE and nephelauxetic ratio (b35) are listed in Table 3. For simplicity sake, CF peaks are
described as the symmetry of octahedral site would be cubic, despite the point symmetry of the (Zr,Ti)O6 is C2.
Just in case of a conspicuous peak splitting, the terminology is referred to a lower symmetry group.