Date: 16.00am-18.00am, Friday, 15 June 2018
Place: 3F, Lecture room, Main Chemistry Build.
場 所： 化学本館3階講義室
Title: Pressure-induced Phase Transition of Alkali Metal Hydrogen Carbonates
Alkali metal hydrogen carbonates belong to series of inorganic substances that contain HCO3- as anions and alkali metal ions as cations. At ambient condition, two (HCO3-) groups connected by H-bonds in KHCO3, RbHCO3 and CsHCO3 form (HCO3-)2 dimers, composing the basis of crystal structures (Thomas 1974; Kim 1969; Kaduk 1993). Under high pressure condition, the distances between oxygen atoms in the typical O-H···O configurations will decrease, leading to dramatic changes of the bond angles and bond energies of hydrogen bonds in dimers. This phenomenon which is called as proton transfer plays an important part in pressure-induced phase transition.
Previous researches showed that there are two different high-pressure phases for KHCO3. One of them occurs at around 3.2 GPa, room temperature, and the crystal structure of this high-pressure phase was determined by a powder neutron diffraction and powder X-ray diffraction research (Allan et al. 2007). The crystal structure of another high-pressure phase occurring at around 2.8 GPa is similar to the ambient phase of CsHCO3 based on a single crystal X-ray diffraction research (Komatsu et al. 2007). In this research, the coexistence of both high-pressure phase was also observed.
Since the similarity in crystal structures of KHCO3, RbHCO3 and CsHCO3, it is reasonable to propose the high-pressure phases for RbHCO3 and CsHCO3. Research on this field can reveal the pressure-induced phase transition principle for those minerals. The mechanism of the coexistence of the two high-pressure phases of KHCO3 is still in debate, study on other similar minerals can also help us to understand this problem. Moreover, direct observation of hydrogen atoms can also reveal the role of proton transfer during pressure-induced phase transition.
• Observe the pressure-induced phase transition of RbHCO3 and CsHCO3.
• Determine the crystal structure of high-pressure phases of RbHCO3 and CsHCO3.
• Conclude the universal principle for alkali metal hydrogen carbonates.
In this research, in-situ powder X-ray diffraction (XRD) was applied to determine the lattice parameters. In-situ Raman spectroscopy was applied to observe the shifts of molecular vibration during phase transition. Pressures were estimated by the ruby fluorescence gauge. Moreover, powder neutron diffraction is also applied to determine the position of hydrogen atoms in hydrogen carbonates.
The main pressure apparatus applied for XRD and Raman spectroscopy is diamond anvil cell (DAC). It can generate pressure above 100 GPa (Ruoff et al. 1990). The sample was loaded into a hole on a stainless gasket. 4:1 methanol-ethanol mixture worked as the pressure medium to keep a hydrostatic pressure up to 10 GPa (Piermarini et al. 1973). As for neutron diffraction, sample was loaded into a Ti-Zr encapsulating gasket with deuterated 4:1 methanol-ethanol pressure medium in a Paris-Edinburgh press.
For RbHCO3, there are two high-pressure phases have been observed and their crystal structures were determined. One of these two phase is similar to the high-pressure phase of KHCO3 discovered by Komatsu, which is named as high-pressure phase type-K; and another phase is similar to the high-pressure phase of KHCO3 discovered by Allan, which is named as high-pressure phase type-A. Both phase transition occurs at around 0.3-0.5 GPa and both high-pressure phases can keep stable up to around 6 GPa independently. The high-pressure phase A-type could transform into K-type at around 0.6-0.9 GPa, and the coexistence of them was also observed at around 0.9 GPa. This phenomenon supports the coexistence of two high-pressure phases for KHCO3 in Komatsu’s research. Among these three phases of RbHCO3, the phase group of ambient phase and high-pressure phase type-K is monoclinic, while that of high-pressure phase type-A is triclinic. This difference comes from the hydrostatic pressure conditions for type-K and type-A. Under hydrostatic pressure, high-pressure phase type-K are dominant, while high-pressure phase type-A are dominant when RbHCO3 powders suffered a shear stress. During these three phase transitions, there are only the lattices distortive displacements like contraction (ambient phase to high-pressure type-K) and shear (ambient phase to high-pressure phase type-A). Atoms still maintain their relative spatial relationships without the long-range diffusion. As for CsHCO3, its ambient phase can keep stable and there is no high-pressure phase up to 5 GPa.