Fig. 1

a Temperature-dependent Raman spectra of Sr₃Ir₂O₇ at ambient pressure. The direction of decreasing temperature is indicated by the dashed arrow line. Five major vibrational modes are identified, and their atomic motions in the polyhedral are represented at the top of the figure. b Raman spectra of Sr₃Ir₂O₇ for selected pressures at 10 K. The dotted lines are guides to the eye, showing the evolution of three A_1g modes, respectively, at 152, 288 and 595 cm⁻¹. The dashed curve tracks the pressure dependence of the one-magnon peak. Since the two-magnon mode merges with the first order Raman mode of diamond, we cannot study its high-pressure behavior. Two additional phonon modes appearing at 14.4 GPa are indicated by dots. Additional modes occurring at 1.9 GPa close to the A_1g (187 cm⁻¹) and B_2g (403 cm⁻¹) modes are indicated by arrow and asterisk, respectively