Pressure-induced luminescence of cerium-doped gadolinium gallium garnet crystal

Studies of the spectroscopic properties of Ce³⁺ dopant in bulk Gd₃Ga₅O₁₂:Ce crystal under pressure are presented. In spite of strong intershell 4f→5d absorption bands at ambient pressure, the cerium luminescence in Gd₃Ga₅O₁₂ is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the 5d→4f radiative transitions. Further increase of pressure improves the emission efficiency. This effect is analyzed in terms of two possible phenomena: (i) by pressure-induced electronic crossover of the excited 5d energy level of the Ce³⁺ with the conduction band bottom of the host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reduction of the Stokes shift and nonradiative transitions between the low vibrational levels of the 5d state and high vibrational levels of the ground 4f state. The results of high-pressure absorption and luminescence measurements point out that the ambient-pressure luminescence quenching is caused by the donor-like charge transfer processes due to the resonant location of the Ce³⁺ 5d electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce³⁺ to Ce⁴⁺ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4f state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5d energy level of the Ce³⁺ results in mixing between conduction band states and the 5d state, and the broadband luminescence from this mixed state is observed at pressures from 30 up to about 60 kbar. Then, for pressures exceeding 60 kbar, this luminescence is replaced by the classic Ce³⁺0.16em0ex5d→4f transitions. Using a simple anticrossing model and configuration coordinate model in the appropriate pressure range, quantitative description of the system has been carried out, and the pressure-induced decrease of electron-lattice coupling has been shown.