Optical spectroscopy of Al₂O₃:Ti³⁺ single crystal under hydrostatic pressure: The influence on the Jahn-Teller coupling

This work investigates the effect of hydrostatic pressure on the excitation and emission spectra, as well as on the lifetime, of Al2O3:Ti3+ at room temperature. The aim is to establish correlations between the pressure-induced band shifts and the corresponding local structural changes undergone by the TiO6 complex. A blue-shift of 8.52 and 6.86 cm−1 kbar−1 was found for the lower (E1) and upper (E2) energy components of the excitation band at 17 760 and 20 500 cm−1, respectively, and blue-shifts of 5.93 and 5.40 cm−1 kbar−1 for the two overlapping bands of the emission spectrum located at 12 680 and 14 210 cm−1. We explain these results on the basis of a reduction of the TiO6 Jahn–Teller distortion upon increasing the pressure. In contrast, the increase of the overall Stokes shift, which is mainly associated with electron–vibration coupling to the totally symmetric a1g vibration, is explained by the increase of the excited-state stabilization energy, Sa1g hωa1g , with increasing pressure. The luminescence lifetime is also found to be pressure dependent, varying from 2.6 μs at ambient conditions to 3.2 μs at 80 kbar. This increase is caused by a diminution of the transition oscillator strength that is related to the odd vibration assistance mechanism. The softening of the transition mechanism is interpreted in terms of the blue-shift experienced by the O2− -> Ti3+ charge transfer transition energy upon increasing the pressure.