Abstract: Since the crystal-field strength at the Cr3 + site is very close to the excited-state crossover (ESCO), this work investigates the optical properties of Cr3 + -doped Gd3Ga5O12 (GGG) nanoparticles as a function of temperature and pressure in order to establish the effect of the ESCO on the optical behaviour of nanocrystalline GGG. Luminescence, time-resolved emission and lifetime measurements have been performed on GGG:0.5% Cr3 + nanoparticles in the 25?300 K temperature range, as well as under hydrostatic pressure up to 20 GPa. We show how low temperature and high pressure progressively transforms broadband emission into a ruby-like luminescence. This behaviour together with the lifetime dependence on pressure and temperature are explained on the basis of the spin-orbit interaction between the 4T2 and 2E states of Cr3 + .

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