Optical Spectroscopy and Crystal Field Analysis of Ni²⁺ Centres in Lithium Tetraborate Glass

The Ni-doped Li₂B₄O₇ (Li₂O-2B₂O₃) glass was prepared by the conventional melt-quenching method. Structural and spectroscopic properties of the glass were investigated using X-ray diffraction (XRD), optical absorption, and photoluminescence spectroscopy, including excitation, emission, and decay kinetics. The absence of sharp diffraction peaks in the XRD pattern confirms the disordered glassy structure of the investigated glass. The optical absorption spectrum shows three broad bands associated with spin-allowed transitions of Ni²⁺ (3d⁸) ions. The excitation and emission spectra of the Li₂B₄O₇:Ni glass reveal several spin-allowed and spin-forbidden 3d-3d transitions of Ni²⁺ ions. In particular, the Li₂B₄O₇:Ni glass exhibits broad green and red emissions centred at approximately 525 and 687 nm, respectively, as well as weak near-infrared emission bands in the 900-1100 and 1500-1750 nm regions. The analysis of the crystal field splitting for Ni²⁺ ions in the Li₂B₄O₇:Ni glass is carried out, and the Racah parameters (B and C) and the crystal field strength (Dq) are calculated. The Tanabe-Sugano diagram for the octahedrally coordinated Ni²⁺ ions in the Li₂B₄O₇:Ni glass is built and discussed.