Fig. 4: The thermodynamic metastability of Am-SP.

a, b The evolution of the XRD patterns and Raman spectra during the compression of Am-SP at room temperature. c The isobaric variable temperature Raman scatterings of Am-SP under the pressure of 5 GPa. The Am-SP is stable at a wide range of temperatures from 5 to 573 K. d The evolution of the Raman spectra and XRD patterns during the compression of Am-SP at 300 K.