Abstract
In this work, the temperature and high-pressure behaviors of Mn3+-doped garnet-type Ca3Ga2Ge3O12 single crystals have been investigated by means of photoluminescence and Raman spectroscopy, respectively. The Jahn–Teller stabilization energy in the 5E ground state was found to be 1630 cm–1, being as much as 6 times greater than that in the 5T2 excited state, that is, 237 cm–1. The room-temperature emission spectrum is dominated by the spin-allowed 5T2 → 5E transition at 670 nm upon 532 nm excitation. The temperature dependences of photoluminescence spectra and time decays reveal strong thermalization between 5T2 and 1T2 levels. Cooling to 50 K empties the 5T2 level by virtue of multiphonon transition to the lower-lying 1T2 level. As a result, the 1T2 → 3T1/5E electric-dipole transitions are induced by coupling to odd-parity phonons. The existence of a small amount of Mn4+ can be ascertained by lowering the temperature or applying high pressure. Upon compression up to 100 kbar, the 5T2 → 5E transition of Mn3+ undergoes a blue shift at a rate of ∼10.5 cm–1/kbar. Since both 5T2 and 1T2 have inverse crystal field dependences, the quenching process from the 5T2 level becomes stronger under high pressure. Cryogenic luminescence ratiometric thermometry based on the diverse thermal quenching behaviors of Mn3+ and Mn4+ was explored. Furthermore, theoretical calculations employing the exchange-charge model of crystal field for Mn3+ and Mn4+ ions in Ga3+ octahedral sites (C3i) in garnet-type Ca3Ga2Ge3O12 remain in perfect agreement with the experimental data.
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