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.

The photoluminescence properties of Cr3+-doped LaGaO3 perovskites are investigated by high-pressure spectroscopy. The pressure-induced phase transition from orthorhombic (Pbnm) to rhombohedral (R3̅c) at around 2 GPa is confirmed by Raman spectroscopy. Cr3+-doped LaGaO3 shows deep-red emission peaks around 730 nm due to the zero-phonon line (R-line) and the phonon sidebands, which correspond to Cr3+: 2Eg → 4A2g transitions in the ideal octahedral site and the Cr–Cr pair luminescence (N-line) under ambient condition. Under a high pressure, the R-line shifts to a lower energy at a rate of −13 cm–1/GPa. From the pressure dependence of photoluminescence excitation (PLE) spectra, it is suggested that the redshift of the R-line is caused by the decrease of Racah parameters B and C. Moreover, the N-line luminescence becomes stronger relative to the R-line with increasing pressure and the N-line/R-line can be used to monitor the phase transition pressure. Under a high pressure, the tilt angle of the GaO6 octahedral unit becomes smaller. It implies that the enhanced N-line luminescence is caused by the stronger superexchange interaction between Cr3+ ions due to the increased Cr–O–Cr bond angle closer to 180°.

We report a new material, BaCN2:Eu 2+ for a very sensitive optical pressure sensor, 50 times more sensitive than ruby. Photoluminescence spectra of the BaCN2:Eu 2+ phosphor was measured under hydrostatic pressures from ambient pressure to 5.34 GPa at room temperature. The peak wavelength of the luminescence was drastically red-shifted at a rate of 19 nm/GPa, which is approximately 50 times larger than that of the ruby, most commonly used as a pressure sensor in the high-pressure experiments. This large shift of the luminescence wavelength is suitable for application in optical pressure sensors for the highpressure experiments without a high-resolution monochromator.

The increasing attention on the unique properties of oxyhydride materials motivates the exploration of their potential applications in optical fields, and the theoretical studies of their luminescence properties are still under progress. Here, we report the experimental and theoretical high-pressure photoluminescence (PL) studies on Eu-activated Sr3– x AxAlO4H ( A = Ca and Ba; x = 0 and 1) oxyhydride materials. Under hydrostatic pressures from ambient pressure up to 6.41 GPa, the luminescence band in all the samples exhibits redshift with increasing pressure and the highest energy-shift rate of −101.85 cm−1/GPa was observed in Sr3AlO4H:Eu2+. The asymmetric bands were deconvoluted into two peaks corresponding to the two Eu sites with different coordination environments. Although the shift rates of Eu2+ centers in Sr3AlO4H are not remarkable as expected for the large compressibility of hydride ion ligands, their pressure-dependences in opposite directions were successfully reproduced by constrained density functional theory calculations using the advanced on-site Coulomb interaction parameter ( U) determination method. The lower shift rate as seen in conventional oxide phosphors indicates that Eu-4 f and 5 d level positions are determined by the interaction with less compressive oxide ion ligands. Therefore, the high shift rate required for pressure sensing applications is expected in more hydrogen-rich oxyhydrides and related hydride compounds.