Study On The Structural Phase Transitions Of Two Iodine-based Ionic Crystals At Extreme Conditions

Ionic bonds are susceptible to structural phase transitions by applying external temperature and pressure,and the phase transitions are often accompanied by changing of properties.In this paper,the phase transition process and the characteristics before and after the phase transition of two representative iodine-based ionic crystals under extreme conditions of ultra-high pressure and variations of temperature were studied to provide support for the design of new functional materials.Firstly,a high-pressure measurement of potassium iodide was carried out using a diamond anvil combined with a four-circle single crystal X-ray diffractometer.The phase transition was found to occur at 1.2 GPa and 2.0 GPa,respectively.The crystal structure of the intermediate phase at 1.2 GPa was determined for the first time by the X-ray diffraction pattern changes before and after the phase transition combined with first-principles calculations.And it is confirmed that the potassium atoms and the iodine atoms move in a zigzag shape during the phase transition.Meanwhile,elastic constants and phonon spectrum calculations also showed that the phase was stable.Secondly,the high-quality silver iodide single crystal obtained by heat treatment was used for in-situ high-temperature XRD test,and combined with DSC test and in-situ high-temperature resistance test.It was determined that the phase transition point of the single crystal was 430 k,which was 10 k higher than that of the powder sample.At the same time,the atomic displacement before and after the phase transition was analyzed by the construction of supercell method.Finally,the phase transition process of silver iodide single crystal from room temperature to 90 k was studied by in-situ single crystal X-ray diffraction technique.Combined with the DSC test and in-situ low temperature resistance test,it was found that the silver iodide sample still could maintain structural stability at the extremely low temperature of 90 k,which clarified the previous controversy about whether the phase transition occurs at 150 k.