Exploring crystal field influences on optical spectra of 3d7 (Ni3+ vs Co2+) ions in cubic pyrochlores A2Ti2O7 (A = Y, Gd) and layered-hexagonal MCl2 (M = Cd, Mg) for potential optoelectronic applications

Abstract

Herein we utilize computational methods for crystal field (CF) parameter (CFP) modelling to analyse experimental energy levels obtained from optical spectra of transition metal (TM) ions doped in crystalline hosts. These methods enable assessing the influence of structural distortions and chemical environments on the CF energy levels and thus optical properties of host-dopant systems. As a case study, we selected Ni³⁺ and Co²⁺(3d⁷) ions doped into: (a) cubic pyrochlores A₂Ti₂O₇ (A = Y, Gd), and (b) layered-hexagonal MCl₂ (M = Cd, Mg). The interplay of octahedral and trigonal CF, and spin-orbit coupling leads to low-spin ²E_g for Ni³⁺ and high-spin ⁴T_1g ground states for Co²⁺, impacting the optical properties of doped hosts. Semiempirical approaches: exchange charge model (ECM) and superposition model (SPM), are employed. For CFP modelling in titanates, the symmetry adapted axis system is adopted, whereas in chlorides - the modified crystallographic axis system. The predicted CF energy levels and g-factors compare well with experimental data. Ni³⁺ ion shows stronger cubic CF with larger CFPs and smaller distortions than Co²⁺ ion. For Co²⁺ ions across hosts, the SPM predicts more accurate CFPs and thus the CF energy levels than ECM. The behaviour of Co²⁺ differs substantially from that of other TM ions in doped hosts. Due to strong absorption/emission in the visible and infrared wavelength region, these hosts are promising for fabrication of infrared laser and sensor devices. This work highlights importance of fine-tuning structural properties and chemical environment, thus offering deeper insights into the structural and optical properties of the studied systems. The results may be useful for designing other TM ions doped hosts tailored for specific technological applications.