Carbon nitride compounds with high N : C ratios and graphitic to polymeric structures are being investigated as potential next-generation materials for incorporation in devices for energy conversion and storage as well as for optoelectronic and catalysis applications. The materials are built from C- and N-containing heterocycles with heptazine or triazine rings linked via sp²-bonded N atoms (N(C)₃ units) or –NH– groups. The electronic, chemical and optical functionalities are determined by the nature of the local to extended structures as well as the chemical composition of the materials. Because of their typically amorphous to nanocrystalline nature and variable composition, significant challenges remain to fully assess and calibrate the structure–functionality relationships among carbon nitride materials. It is also important to devise a useful and consistent approach to naming the different classes of carbon nitride compounds that accurately describes their chemical and structural characteristics related to their functional performance. Here we evaluate the current state of understanding to highlight key issues in these areas and point out new directions in their development as advanced technological materials.

Because of its electrically conducting properties combined with excellent thermal stability and transparency throughout the visible spectrum, tin oxide (SnO₂) is extremely attractive as a transparent conducting material for applications in low-emission window coatings and solar cells, as well as in lithium-ion batteries and gas sensors. It is also an important catalyst and catalyst support for oxidation reactions. Here, we describe a novel nonaqueous sol–gel synthesis approach to produce tin oxide nanoparticles (NPs) with a low NP size dispersion. The success of this method lies in the nonhydrolytic pathway that involves the reaction between tin chloride and an oxygen donor, 1-hexanol, without the need for a surfactant or subsequent thermal treatment. This one-pot procedure is carried out at relatively low temperatures in the 160–260 °C range, compatible with coating processes on flexible plastic supports. The NP size distribution, shape, and dislocation density were studied by powder X-ray powder diffraction analyzed using the method of whole powder pattern modeling, as well as high-resolution transmission electron microscopy. The SnO₂ NPs were determined to have particle sizes between 3.4 and 7.7 nm. The reaction products were characterized using liquid-state ¹³C and ¹H nuclear magnetic resonance (NMR) that confirmed the formation of dihexyl ether and 1-chlorohexane. The NPs were studied by a combination of ¹³C, ¹H, and ¹¹⁹Sn solid-state NMR as well as Fourier transform infrared (FTIR) and Raman spectroscopy. The ¹³C SSNMR, FTIR, and Raman data showed the presence of organic species derived from the 1-hexanol reactant remaining within the samples. The optical absorption, studied using UV–visible spectroscopy, indicated that the band gap (E_g) shifted systematically to lower energy with decreasing NP sizes. This unusual result could be due to mechanical strains present within the smallest NPs perhaps associated with the organic ligands decorating the NP surface. As the size increased, we observed a correlation with an increased density of screw dislocations present within the NPs that could indicate relaxation of the stress. We suggest that this could provide a useful method for band gap control within SnO₂ NPs in the absence of chemical dopants.

The high-pressure behavior of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ is studied up to $40\mathrm{GPa}$ using synchrotron x-ray diffraction and Raman spectroscopy in diamond anvil cells. A phase transformation from the monoclinic $\beta$-phase $(C2∕m)$ is observed at a pressure above 20–$22\mathrm{GPa}$. The high-pressure polymorph is identified as the $\alpha$-phase that is isostructural with corundum ($R\overline{3}c$ symmetry) from the x-ray diffraction data. However, considerable anion disorder is indicated by the appearance of broad bands in the Raman spectra. The experimental results are complemented by ab initio theoretical calculations of the energetics of the two structures and the lattice dynamics.