Influence of inversion level on the optical absorption spectra of Ti‐doped transparent MgGa₂O₄ ceramics

Abstract

Ti‐doped (0.08, 0.30, and 1.00 atomic% [at.%]) transparent MgGa2O4 ceramics (possessing a high inversion level; i up to 0.8) were fabricated by pulsed electric current sintering, at 950°C, under vacuum for 30–90 min. Optical transmission, emission, and electron paramagnetic resonance spectra were recorded. The maximal transmission level was ∼70% (820 nm), for a thickness of ∼1 mm, which, while not very high, permitted the observation of the optical absorption bands location and profile. Interpretation of the fluorescence spectra suggests that some Ti4+ cations (mostly hexacoordinated) were accommodated by the host despite the scarcity of oxygen in the atmosphere during the sintering process. The Ti3+ cations substitute native ions located in tetrahedral sites, distorting the original Td symmetry toward a D2d symmetry. Comparing the Ti‐doped MgGa2O4 (high inversion) and MgAl2O4 (low inversion) spinels, spectral characteristics revealed that a significant increase in the inversion level drives Ti3+ cations from octahedral toward tetrahedral sites. This is reflected in the optical absorption spectra by the disappearance of the band at ∼20 000 cm−1 (detectable in MgAl2O4) in MgGa2O4; the two d–d bands, of MgA2O4, in MgGa2O4 are reduced to a single one, located at 11 800 cm−1. These results, for MgGa2O4, strongly support a similar assignment—of the strong band at 12 800 cm−1, in Ti‐doped MgAl2O4—to a tetracoordinated Ti3+. Thus, while in MgAl2O4, Ti3+ appears in both octahedral and tetrahedral coordination and in MgGa2O4 only the latter state is stable. In both spinels, Ti dopant speciates into Ti3+ and Ti4+ cations.