Figure 1 (a) The typical Na-centered cage structure and the Al-centered tetrahedron cluster of Al and O. (b) The snapshot of total atomic configuration for amorphous (NaAl11O17)10 at 300K. (c) The connection between the Na-centered cage structures. (d) The typical tube structure surrounds Na atoms. The green, red, and pink balls represent Na, O and Al atoms, respectively.
Recently, the amorphous Sodium beta-alumina (SBA: Na-Beta-Al2O3) [1] with high-dielectric-constant and novel frequency-dependence transport properties has been regarded as a potential candidate for the gate dielectrics of field-effect transistors (FETs). As an anisotropic two-dimensionally conductive material, the crystalline SBA can provide high capacitance perpendicular to the conductive planes, while causing negligible leakage current owing to the lack of electron carriers and limited mobility of sodium ions through the aluminum oxide layers. But, the related atomic-level mechanism of those novel properties for amorphous SBA (Na-Beta-Al2O3) is still unclear.
In order to understand the transport and dielectronic properties of amorphous Na-Beta-Al2O3, the atomic structure should be extracted for further investigations. By using first-principle molecular dynamics in a plane-wave pseudopotential formulation within state-of-the-art density-functional theory methods, the realistic models of amorphous X-Beta-Al2O3 could be generated in “melt-and-quench” fashion. The calculations were carried out with canonical NVT (constant atom number, volume and temperature) ensembles were accomplished by using the Vienna ab initio simulation package (VASP). The ensemble experienced a stepwise quenching process to derive the final glass structure and the cooling rate is ~1014K/s. Obviously, the cooling rate is far beyond the fastest cooling rate that can be obtained experimentally by laser techniques, as it is limited by the short tine interval accessible to simulations. Nevertheless this technique is well accepted and widely used for the generation of amorphous samples.
The results regarding the observed structure are shown in Figure 1. Our results thus far suggest that the segregation of Na can be observed, and the network structure which is composed of Na-centered cages is shown in Figure 1(c). The results suggest that the Al-O cages construct the tube structure shown in Figure 1(d) which can be regarded as the highway for Na ion transmission.
According to this structural information, the dielectronic and vibrational properties of SBA will be analyzed using the linear response method based on the density functional perturbation theory. Based on this approach, the computed Born effective charges and the mode eigenvectors of lattice vibration can be used to decompose the lattice dielectric susceptibility tensor into contribution arising from individual infrared-active phonon modes. Then, we can perform a spectral decomposition of the lattice constant and find where are the strongest contributions come from. low-frequency or high frequency, Na ions or O/Al ions. All these results will help us for understanding the dielectronic properties of SBA.
We are also interested in how the cation (Na+, K+ ….) affects the dielectric properties of this kind of amorphous material.
[1] Pal, B. N. et al. Nature Materials 8, 898 (2009)