Fluorescence pressure sensors: Calibration of ruby, Sm²⁺: SrB₄O₇, and Sm³⁺: YAG to 55 GPa and 850 K

In this work, a calibration of ruby, samarium-doped strontium tetraborate (Sm2+: SrB4O7), and samarium-doped yttrium aluminum garnet (Sm3+: YAG) using Raman and fluorescence spectra was conducted within the temperature range of 296-850 K and pressure range of 0-55 GPa. The obtained calibration can be applied independently for high-temperature or high-pressure conditions and described as the unit form of P = (A '/B ') x [(lambda/lambda(T))(B ') - 1] with A ' = A (296 K) + A(1 )x (T - 296) + A(2 )x (T - 296)(2), B ' = B(296 K) + B(1 )x (T - 296), and lambda(T) = lambda(T) (296 K) + Delta lambda(T), where the specific parameters are provided in the main text. It was observed that for the lambda(1) line (D-5(0) -> F-7(0) transition, about 685.2 nm under ambient conditions, also known as the 0-0 line) of Sm2+: SrB4O7, the neglect of the temperature effect on the pressure coefficient may lead to an underestimation of pressure above 35-40 GPa, with a maximum deviation of approximately 2.5 GPa within the range of 55 GPa and 850 K. For Sm3+: YAG, it may introduce significant errors under the whole high temperature and high pressure range if the effect of temperature is ignored, that is, about 3.9 GPa for Y-1 line ((4)G(5/2) -> H-6(7/2) transition, about 617.8 nm under ambient conditions) and 4.6 GPa for Y-2 line ((4)G(5/2) -> H-6(7/2) transition, about 616.0 nm under ambient conditions) at 850 K. Comparing the three fluorescence pressure sensors, the ruby has the strongest signal intensity and highest temperature sensitivity, and the Sm2+: SrB4O7 and the Sm3+: YAG possess lower temperature sensibility, wider used temperature range, and better spectral quality under high temperature and high pressure (HTHP), especially Sm2+: SrB4O7, which has a sharp high-intensity single peak lambda(1), perhaps the most promising sensor for high P-T experiments. Therefore, in view of the potential deflections of fluorescence peaks of each pressure sensor under HTHP, we recommend utilizing the HTHP-corrected relationships for pressure calibration.