Predicting the Optical Pressure Sensitivity of ²E → ⁴A₂ Spin-Flip Transition in Cr³⁺-Doped Crystals

Understanding the role played by the material chemistry to increase the pressure sensitivity of new optical pressure probes is of great scientific interest. After almost 50 years from the first proposal as an optical pressure sensor, the R-line emission of ruby (α-Al2O3:Cr3+) is still the standard pressure probe used for the diamond anvil cell experiments in worldwide laboratories. Besides the fundamental importance of developing new materials able to discriminate pressure variations with high sensitivity, the ability to predict the potentials of new materials is still a huge challenge. In this view, the pressure dependence of the R-lines in mullite-type Bi2M4O9:Cr3+ (M = Ga, Al) systems is exploited as a case study. Despite the promising performances as a pressure sensor, the mixing between 4T2 and 2E hinders the applicability of Bi2Ga4O9:Cr3+, while Bi2Al4O9:Cr3+ is characterized by a linear trend in the whole pressure range explored and a remarkable sensitivity higher than ruby. The analysis of the Cr3+-based pressure sensors in terms of crystal field, nephelauxetic effect, and bulk modulus led to a universal relationship between the pressure sensitivity and the ambient pressure 2E energy of Cr3+-doped phosphors, allowing the prediction of highly sensitive optical pressure sensors.