Crystal structure, hydrogen bonds, and lattice dynamics in kanemite from first-principles calculations

The crystal structure and vibrational spectrum of kanemite $\left({\text{NaHSi}}_2{\text{O}}_5\cdot 3{\text{H}}_2\text{O}\right)$ were studied using the density-functional theory. The calculations were carried out in a lower space group $\left(Pna{2}_1\right)$ in order to eliminate the finite-temperature disordered configurations of protons in hydrogen bonds. It was found that the ${\text{SiO}}_4$ tetrahedra are linked by a short strong $\text{O-H}\cdots \text{O}$ hydrogen bond with a single asymmetric potential well at low temperatures. Two energetically equivalent sites were obtained for the position of the H atom. Apart from the hydrogen bonds reported in earlier studies, additional moderate $\text{O-H}\cdots \text{O}$ hydrogen bond was found to stabilize the hydrated Na layers within the silicon layers at low temperatures. The vibrational spectrum was analyzed in detail in the entire energy range (0–500 meV), focusing on the vibrations of hydrogen atoms. A perfect correlation appears to exist between the OH (stretching and torsional) frequencies and donor-acceptor distances of the respective hydrogen bonds.

Figure 1

(Color online) Comparison of hydrogen bonds geometry in the experimental (left) and theoretical (right) structures visualized in the $bc$ plane and constrained by $Pbcn$ and $Pna{2}_1$ space groups, respectively.

Figure 2

(Color online) Low-temperature structure of kanemite shown in the $bc$ plane as obtained from the present DFT calculations.

Figure 3

(Color online) Hydrogen bonds in the optimized structure of kanemite.

Figure 4

Energy potential of H2A in the short hydrogen bond $\text{O}2\text{A-H}2\text{A}\cdots \text{O}2\text{B}$ calculated for the low temperature structure of kanemite.

Figure 5

Total and partial phonon densities of states in kanemite.

Figure 6

Energies of the OH (a) stretching and (b) torsion modes for H2A, H4A, H4B, H5A, and H5B hydrogen atoms.