The strontium tantalate Sr₂Ta₂O₇ is a ceramic with an orthogonal Cmcm space group. Due to its potential applications in the field of multiferroics, it has become a research hotspot in recent years. However, the regulatory mechanism and phase transition behavior of hydrostatic pressure on its complex lattice structure remain unclear, which, to some extent, limits the in-depth understanding of the “structure-property” relationship of this material. This study systematically investigates the lattice dynamic response characteristics of orthogonal Cmcm Sr₂Ta₂O₇ under high pressure up to 30 GPa by using in-situ high-pressure Raman spectroscopy, marking the highest pressure on this system to date. The results manifest that when the pressure reaches 5 GPa, significant changes occur in the Raman vibrational modes of the material. This phenomenon is attributed to a commensurate-to-incommensurate structural transition induced by symmetry breaking, consistent with previous research findings on Sr₂Ta₂O₇. As the pressure further increasing to 20 GPa, a second phase transition may occur, which is identified as a first-order phase transition closely related to lattice disordering. However, the specific crystal structure of this high-pressure phase remains to be further confirmed in future studies. The Raman spectroscopy analysis suggests that the structural distortion of this high-pressure phase may follow a transformation pathway from orthogonal to monoclinic.

It has been shown that NaI undergoes a pressure-induced phase transition which is different from other alkali metal halides such as NaCl. Previous X-ray diffraction experiments show that NaI will experience a structure phase transition from B1 to B33 phase under high pressure. However, there may exist pressure difference of the structure phase transition in experiments due to the lack of pressure transmit medium and the high water absorption of NaI. In this study, we predict the structural phase transition from B1−B33 phase at 20 GPa based on density functional theory of the first principles calculation method. The results of the theoretical calculations confirm the previous reported data, but the calculated pressure of the phase transition is slightly lower than the experimental value. In addition, detailed evolution of physical properties in NaI under high pressure was investigated. The band gap of NaI shows a gradual closure with increasing pressure, while its brittleness and ultraviolet light reflectance exhibit an enhancement. This work establishes a theoretical foundation for exploring the potential applications of alkali metal halides under extreme conditions.