Structural Principles of Semiconducting Group 14 Clathrate Frameworks

Abstract

We have performed a comprehensive theoretical investigation of the structural principles of semiconducting clathrate frameworks composed of the Group 14 elements carbon, silicon, germanium, and tin. We have investigated the basic clathrate frameworks, together with their polytypes, intergrowth clathrate frameworks, and extended frameworks based on larger icosahedral building blocks. Quantum chemical calculations with the PBE0 hybrid density functional method provided a clear overview of the structural trends and electronic properties among the various clathrate frameworks. In agreement with previous experimental and theoretical studies, the clathrate II framework proved to be the energetically most favorable, but novel hexagonal polytypes of clathrate II also proved to be energetically very favorable. In the case of silicon, several of the studied clathrate frameworks possess direct and wide band gaps. The band structure diagrams and simulated powder X-ray patterns of the studied frameworks are provided and systematic preliminary evaluation of guest-occupied frameworks is conducted to shed light on the characteristics of novel, experimentally feasible clathrate compositions.

Novel silicon phases and nanostructures for solar energy conversion

“…Recently, Si based clathrates have drawn attention for photovoltaic applications as a result of their tunable quasi-direct band gap in the range needed for optimal performance according to the Shockley-Queisser (SQ) limit. [21][22][23][24] Si-based clathrates without guest atoms in the cages are neutral, and thus they are intrinsic semiconductors. The guest-free low density phases are metastable at ambient conditions.…”

Semiconducting Clathrates Meet Gas Hydrates: Xe24[Sn136]

“…Finally, while the clathrate-II framework is the energetically most stable among all empty group 14 clathrate frameworks, the noble gas adsorption could be energetically even more favourable for some other clathrate frameworks with different constituent cages. [11] Concerning the applied theoretical methods, the adsorption energy trends obtained at the LMP2 and DFT-B3LYP-D3 levels of theory are generally in good agreement, the only cases of qualitative disagreement being Ar 24 [Ge 136 ] and Kr 24 [Ge 136 ], where LMP2 predicts adsorption to be favourable by a small margin, while DFT-B3LYP-D3 predicts the adsorption to be unfavourable. Also for Kr 24 [Si 136 ], where the adsorption is clearly unfavourable with both methods, the quantitative difference of the adsorption energies is rather large.…”

“…In the following comparisons, the relative energies of the empty clathrate-II frameworks with respect to the bulk α-phases have been obtained at the DFT-PBE0/SVP level of theory, and they are in good agreement with previous theoretical studies. [11] In the case of Xe 24 [Sn 136 ], the LMP2 adsorption energy of -52 kJ/mol/guest is equivalent to -1248 kJ/mol/unit cell or -9.2 kJ/mol/Sn atom. Considering that the hypothetical guest-free Sn clathrate-II framework is only 2.6 kJ/mol/Sn less stable than the diamond-like α-Sn, the additional stabilization resulting from the noble gas adsorption is very significant.…”

“…[7] However, in the case of cubic clathrate-II framework (Figure 1), practically "guest-free" modifications have been synthetized, [8][9][10] in line with computational studies showing the clathrate-II framework to be the energetically the most favorable one within various experimentally observed and hypothetical clathrate frameworks. [11] Recent synthesis of alloyed, guest-free SiGe clathrates is a promising step in the direction of tunable band gap materials for optoelectronic applications. [12,13] In direct analogy to the gas hydrates, the existence of guest-free, uncharged semiconducting clathrates suggests that neutral gaseous species such as noble gas atoms or methane could be encapsulated within the semiconducting clathrates.…”

Predicted Siliconoids by Bridging Si9 Clusters through sp3-Si Linkers

“…All quantum chemical calculations for the polymers were carried out using the CRYSTAL09 program package [50] with a hybrid DFT functional after Perdew, Burke, and Ernzerhof (DFT-PBE0) [51,52]. For silicon, a modified split-valence + polarization (SVP) basis set [53] was applied. The shrinking factor (SHRINK) for generating the Monkhorst-Packtype grid of k points in the reciprocal space was set to 4, resulting in three k-points in the irreducible Further, it was shown that such clusters can serve as seed crystals for the synthesis of Si nanoparticles and nanoscale materials [45,46].…”