Nd3+-doped Ca3Ga2Ge3O12 garnet: A new optical pressure sensor

Abstract

A pressure-induced shift of the emission spectrum corresponding to the near infrared 4F3/2 → 4I9/2 transition of Nd3+ ions in a calcium gadolinium germanium garnet was obtained in the interval from ambient conditions up to 23 GPa in order to test its suitability as an optical pressure sensor. Although several Nd3+ non-equivalent centers are present in this garnet, which complicates the assignation of the optical transitions, the R1,R2 → Z5 transitions are unequivocally characterised and fit the requirements of an ideal optical pressure sensor. Results obtained for these emission peaks indicate large pressure coefficients of -8.8 and -10.8 cm -1 GPa-1; meanwhile, the rest of the R1,R 2 → Z1-4 emissions remain almost unchanged under pressure. This behaviour is ascribed to the influence of the crystal-field at high pressure on the Z5 Stark level of the ground state and can be easily reproduced exclusively by varying the cubic term of fourth rank of the crystal-field Hamiltonian, which accounts for the Nd3+ ions and is related to medium Nd3+-oxygen distances. These coefficients are larger than those found for the R-lines of Cr3+ in ruby, -7.56 cm-1 GPa-1, suggesting that this system may be a good candidate for a luminescence pressure sensor.