Combinatorial Vibration-Mode Assignment for the FTIR Spectrum of Crystalline Melamine: A Strategic Approach toward Theoretical IR Vibrational Calculations of Triazine-Based Compounds

Although polymeric graphitic carbon nitride (g-C₃N₄) has been widely studied as metal-free photocatalyst, the description of its structure still remains a great challenge. Fourier transform infrared (FTIR) spectroscopy can provide complementary structural information. In this paper, we reconsider the representative crystalline melamine and develop a strategic approach to theoretically calculate the IR vibrations of this triazine-based nitrogen-rich system. IR calculations were based on three different models: a single molecule, a 4-molecule unit cell, and a 32-molecule cluster, respectively. By this comparative study the contribution of the intermolecular weak interactions were elucidated in detail. An accurate and visualized description on the experimental FTIR spectrum has been further presented by a combinatorial vibration-mode assignment based on the calculated potential energy distribution of the 32-molecule cluster. The theoretical approach reported in this study opens the way to the facile and accurate assignment for IR vibrational modes of other complex triazine-based compounds, such as g-C₃N₄

Direct Band Gap Silicon Allotropes

Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in solar cell industry. The great demand of society for new clean energy and the shortcomings of the current silicon solar cells are calling for new materials that can make full use of the solar power. In this paper, six metastable allotropes of silicon with direct or quasidirect band gaps of 0.39–1.25 eV are predicted by <i>ab initio</i> calculations at ambient pressure. Five of them possess band gaps within the optimal range for high converting efficiency from solar energy to electric power and also have better optical properties than the Si-I phase. These Si structures with different band gaps could be applied to multiple p–n junction photovoltaic modules

Si₁₀: A sp³ Silicon Allotrope with Spirally Connected Si₅ Tetrahedrons

Si₁₀: A sp³ Silicon Allotrope with Spirally Connected Si₅ Tetrahedron

Low-Temperature Diffusion of Oxygen through Ordered Carbon Vacancies in Zr₂C_<i>x</i>: The Formation of Ordered Zr₂C_<i>x</i>O_<i>y</i>

Investigations are performed on low-temperature oxygen diffusion in the carbon vacancy ordered ZrC_0.6 and thus induced formation of the oxygen atom ordered ZrC_0.6O_0.4. Theoretically, a superstructure of Zr₂CO can be constructed via the complete substitution of carbon vacancies with O atoms in the Zr₂C model. In the ordered ZrC_0.6, the consecutive arrangement of vacancies forms the vacancy channels along some zone axes in the C sublattice. Through these vacancy channels, the thermally activated oxygen diffusion is significantly facilitated. The oxygen atoms diffuse directly into and occupy the vacancies, producing the ordered ZrC_0.6O_0.4. Relative to the ordered ZrC_0.6, the Zr positions are finely tuned in the ordered ZrC_0.6O_0.4 because of the ionic Zr–O bonds. Because of this fine adjustment of Zr positions and the presence of oxygen atoms, the superstructural reflections are always observable in a selected area electron diffraction (SAED) pattern, despite the invisibility of superstructural reflections in ZrC_0.6 along some special zone axes. Similar to the vacancies in ordered ZrC_0.6, the ordering arrangement of O atoms in the ordered ZrC_0.6O_0.4 is in nanoscale length, thus forming the nano superstructural domains with irregular shapes

Tetragonal Allotrope of Group 14 Elements

Group 14 elements (C, Si, and Ge) exist as various stable and metastable allotropes, some of which have been widely applied in industry. The discovery of new allotropes of these elements has long attracted considerable attention; however, the search is far from complete. Here we computationally discovered a tetragonal allotrope (12 atoms/cell, named T12) commonly found in C, Si, and Ge through a particle swarm structural search. The T12 structure employs sp³ bonding and contains extended helical six-membered rings interconnected by pairs of five- and seven-membered rings. This arrangement results in favorable thermodynamic conditions compared with most other experimentally or theoretically known sp³ species of group 14 elements. The T12 polymorph naturally accounts for the experimental <i>d</i> spacings and Raman spectra of synthesized metastable Ge and Si-XIII phases with long-puzzling unknown structures, respectively. We rationalized an alternative experimental route for the synthesis of the T12 phase via decompression from the high-pressure Si- or Ge-II phase