and which occur mainly due to the static or dynamic distortions from the regular octahedral geometry, which partially removes the inversion center. Moreover, those could also be magnetic dipole allowed. As a consequence, their
oscillator strengths are several orders of magnitude lower than that of tetrahedral. The excitation spectrum of 0.2 mol% Ni2+:ZPB glass has been given in Fig. 10 with an emission lemis ¼ 670 nm. A sharp excitation band at 420 nm has been obtained which coincides with the absorption transition 3A2g (F) - 3T1g (P) of Ni2+-doped samples. The CT excitation bands at 374, 343, 321 nm for 0.2 mol% Ni2+-doped samples are observed. Fig. 11 presents the emission spectra of Ni2+ samples at an excitation with 420 nm. It reveals emissions at the green (577 nm) and at the red (670 nm) wavelengths. Following the energy levels of Ni2+ ions in octahedral sites, these emissions in green and red regions are assigned to 1T2g (D)- 3A2g (F) and 1T2g (D)- 3T2g (F) electronic transitions, respectively. At this excitation (420 nm), both the samples show emission peaks at the same position. However, the relative intensities of these peaks are changing considerably with Ni2+ ions concentration change. The green broad (577 nm) emission intensity is low compared to the sharp red band