Abstract
Studies of the structure of hydroxides under pressure using neutron diffraction reveal that the high concentration of hydrogen is distributed in a disordered network. The disorder in the hydrogen-bond network and possible phase transitions are reported to occur at pressures within the range accessible to experiments for layered calcium hydroxides, which are considered to be exemplary prototype materials. In this study, the static and dynamical properties of these layered hydroxides are investigated using a quantum approach describing nuclear motion, shown herein to be required particularly when studying diffusion processes involving light hydrogen atoms. The effect of high-pressure on the disordered hydrogen-bond network shows that the protons tunnel back and forth across the barriers between three potential minima around the oxygen atoms. At higher pressures the structure has quasi two-dimensional layers of hydrogen atoms, such that at low temperatures this causes the barrier crossing of the hydrogen to be significantly rarefied. Furthermore, for moderate values of both temperature and pressure this process occurs less often than the usual mechanism of proton transport via vacancies, limiting global proton diffusion within layers at high pressure.
Highlights
The hydroxide group of compounds continues to stimulate wide interest in view of the prospect of their use in broadening the scope of neutron diffraction techniques at high pressure[1]
Is there the formation of a remarkable two-dimensional layer of protons at high pressure? How are the properties of protons modified when constrained and confined by pressure? In essence, we found that the kinetics of proton transport depends on tunnelling and the effects of zero point energy
Calcium hydroxides crystallize in a layered phase, known as the mineral portlandite, with the hexagonal space group P-3m119, in which the hydroxyl groups are found aligned along the c axis
Summary
The hydroxide group of compounds continues to stimulate wide interest in view of the prospect of their use in broadening the scope of neutron diffraction techniques at high pressure[1]. The pivotal feature of hydroxide crystals is that protons take an important role in the structure – hydrogen bonds form the link between sheets in the layered structure of many hydroxides. The hydrogen-bond network becomes disordered due to geometrical frustration[6], because protons tend to stick to three different sites around each oxygen atom[7, 8]. Hydrogen, such as mineral hydroxides, in order to clarify the dynamic properties of protons in their layered and disordered structure. Temperatures ranging from 100 K to 500 K, our simulations address proton disorder and dynamical properties in the context of global transport schemes. We argue that this limit on hydroxides might be observable experimentally using neutron scattering at high pressures
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