Strong electron–phonon coupling of Cr3+ ion provides an opportunity for superior sensitivity cryogenic sensing

•Reporting a novel cryogenic sensing material with superior sensitivity.•Achieving a high relative sensitivity (Sr) of 2.62 %·K−1 and super low temperature uncertainty of 0.064 K at 77 K.•Deeply studying the interaction of Cr3+-Cr3+ and their electron–phonon coupling effect.•Providing a train of thought for exploring new materials for optical cryogenic thermometer.