10:15 AM - 10:30 AM
[SMP29-05] Low-temperature phase transition of keatite
Keywords:keatite, silica polymorphs, phase transition, differential scanning calorimeter, density functional theory calculation
Keatite is a polymorph of silica that is extremely rare in nature, but it is relatively easy to produce using hydrothermal synthesis with amorphous silica as the starting material. Keatite has several things in common with quartz and cristobalite. All of them have a helical structure made up of SiO4 tetrahedra, and they all exhibit a negative thermal expansion coefficient. However, the negative thermal expansion coefficient is only exhibited by the high-temperature phases of quartz and cristobalite. Although keatite shows a negative thermal expansion coefficient from room temperature to around 250 oC, there have been no reports of temperature-induced phase transitions. Kanzaki (Mineralogical Society of Japan Annual Meeting 2023) reported the possibility of a phase transition in keatite at low temperatures and high pressures based on density functional theory calculations and classical MD calculations. If there is a phase transition below room temperature, then the room-temperature form of keatite is actually the high-temperature phase, and the negative coefficient of thermal expansion can be understood as a unified behavior of the high-temperature phase, in the same way as quartz and cristobalite. However, the phase transition has not been experimentally verified until now. We previously conducted synchrotron radiation powder X-ray diffraction at -100 oC, but the transition did not occur. Therefore, it is expected to occur at even lower temperatures. In this study, we attempted to detect the phase transition by performing differential scanning calorimetry (DSC) measurements on keatite at low temperatures. We also performed detailed density functional vibration frequency calculations.
We synthesized keatite for the DSC measurement, etc. Using silicic acid and deionized water as the starting materials, we performed hydrothermal treatment at 150 MPa, 600 oC, and for 24 hours. The product was identified by micro-Raman spectroscopy. We obtained sample that was almost entirely composed of keatite. Since we did not have access to a low-temperature DSC at our university or nearby, we asked Japan Thermal Consulting to measure it for us. They used a TA Instruments DSC2920. The sample, which weighed 11.42 mg, was measured while being heated from liquid nitrogen temperature to room temperature at a rate of 10 oC/min. An endothermic peak was observed at around -152 oC, and there was also a change in the baseline. From the analysis of the endothermic peak, the transition point was determined to be -161 oC. In addition, the presence of an endothermic peak suggests that this is a first-order phase transition.
Kanzaki (2023) found a soft mode from the density functional theory calculation, but when the pressure change of this B1mode was investigated in more detail using DFT calculations, it was found that this mode was actually in pressure-induced resonance with another B1 mode on the higher frequency side. As a result, it was found that the other B1 mode was actually behaving like a soft mode. However, the atomic displacements of this B1 mode did not correspond to the displacement of the phase transition. Therefore, it could not be called a soft mode, and the original picture of a second-order transition could no longer be established. On the other hand, it was no longer inconsistent with the endothermic peak seen in DSC.
At present, we are attempting to measure the Raman shift of keatite at temperatures close to liquid nitrogen in order to investigate the transition further. However, it is difficult to measure at low temperatures, so we have not yet succeeded. In addition, the transition is expected at high pressure even at room temperature. We have previously conducted high-pressure Raman measurements and observed changes that appear to be related to the transition, but we are still conducting further experiments. We will report on the results of these experiments on the meeting.
We synthesized keatite for the DSC measurement, etc. Using silicic acid and deionized water as the starting materials, we performed hydrothermal treatment at 150 MPa, 600 oC, and for 24 hours. The product was identified by micro-Raman spectroscopy. We obtained sample that was almost entirely composed of keatite. Since we did not have access to a low-temperature DSC at our university or nearby, we asked Japan Thermal Consulting to measure it for us. They used a TA Instruments DSC2920. The sample, which weighed 11.42 mg, was measured while being heated from liquid nitrogen temperature to room temperature at a rate of 10 oC/min. An endothermic peak was observed at around -152 oC, and there was also a change in the baseline. From the analysis of the endothermic peak, the transition point was determined to be -161 oC. In addition, the presence of an endothermic peak suggests that this is a first-order phase transition.
Kanzaki (2023) found a soft mode from the density functional theory calculation, but when the pressure change of this B1mode was investigated in more detail using DFT calculations, it was found that this mode was actually in pressure-induced resonance with another B1 mode on the higher frequency side. As a result, it was found that the other B1 mode was actually behaving like a soft mode. However, the atomic displacements of this B1 mode did not correspond to the displacement of the phase transition. Therefore, it could not be called a soft mode, and the original picture of a second-order transition could no longer be established. On the other hand, it was no longer inconsistent with the endothermic peak seen in DSC.
At present, we are attempting to measure the Raman shift of keatite at temperatures close to liquid nitrogen in order to investigate the transition further. However, it is difficult to measure at low temperatures, so we have not yet succeeded. In addition, the transition is expected at high pressure even at room temperature. We have previously conducted high-pressure Raman measurements and observed changes that appear to be related to the transition, but we are still conducting further experiments. We will report on the results of these experiments on the meeting.