Crystallography, materials and computation

The birth of crystallography 100 years ago was in the determination of the structures of inorganic materials. And materials continue to pose some of the most fascinating challenges in our discipline. Moreover, structural studies in materials science and indeed in all areas are increasingly supported by computation which now permeates all aspects of crystallography

Challenges in the structural science of materials

Articles published recently in IUCrJ continue to exemplify the developments and challenges in the structural science of materials

Computational investigation of CO adsorbed on Aux, Agx and (AuAg)x nanoclusters (x = 1-5, 147) and monometallic Au and Ag low-energy surfaces

Density functional theory calculations have been performed investigating the use of CO as a probe molecule for determining the structure and composition of Au, Ag AuAg nanoparticles. For very small nanoclusters (x = 1-5), vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles showed a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are calculated for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, and this proves to be energetically favourable only on atop and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies for the CO molecule red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that vibrational frequencies cannot be relied upon solely in order to obtain accurate compositional analysis, but we do believe that elemental rearrangement in the nanocluster from Ag@Au (or Au@Ag) to an alloy would result in a shift in the vibrational frequencies that indicate the change in the surface composition

Recent developments in the structural science of materials

This Editorial surveys the current status and recent developments in the structural science of materials as exemplified by the articles recently published in IUCrJ

Bulk electronic, elastic, structural, and dielectric properties of the Weyl semimetal TaAs

We present results of electronic structure calculations of the bulk properties of the Weyl semimetal TaAs. The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. Many of the physical properties of the material, however, are unknown. We have calculated the structural parameters, dielectric function, elastic constants, phonon dispersion, electronic band structure, and Born effective charges using density functional theory within the generalized gradient approximation, including spin-orbit coupling where necessary. Our results provide essential information on the material; and our calculations agree well with the relatively small number of experimental data available. Moreover, we have determined the relative stability of the ground state body-centered tetragonal phase with respect to other common binary phases as a function of pressure at the athermal limit, predicting a transition to the CsCl cubic structure at 23.3 GPa. Finally, we have determined the band structure using an unbiased hybrid density functional that includes 25% exact exchange, in order to refine the previously determined positions in k space of the Weyl points

Double bubble secondary building units used as a structural motif for enhanced electron-hole separation in solids

A structural motif designed for enhancing electron–hole separation in semiconducting composite materials, the so-called double bubble, is introduced. The addition of silicon carbide in the construction of heterogeneous double bubble systems, along with zinc oxide and gallium nitride, yields electronic structures that are favourable for electron–hole separation. The standard formation enthalpies of such systems are comparable with those of fullerenes, suggesting that these systems would be achievable and of direct benefit to photovoltaic and electrochemical applications such as water splitting; with the (SiC)12@(ZnO)48 proving to be the most promising building block for future functional composite materials

The reaction of formic acid with RaneyTM copper

The interaction of formic acid with RaneyTM Cu proves to be complex. Rather than the expected generation of a monolayer of bidentate formate, we find the formation of a Cu(II) compound. This process occurs by direct reaction of copper and formic acid; in contrast, previous methods are by solution reaction. This is a rare example of formic acid acting as an oxidant rather than, as more commonly found, a reductant. The combination of diffraction, spectroscopic and computational methods has allowed this unexpected process to be characterized

On the synthesis and performance of hierarchical nanoporous TS-1 catalysts

Hierarchical TS-1 zeolite was successfully prepared using chitosan as a sacrificial template. The X-ray diffraction showed that the presence of chitosan with the synthesis precursor had no deleterious effect on the crystallinity and phase purity of this zeolite. X-ray absorption spectroscopy at the Ti K-edge, FTIR and Raman spectroscopies revealed the titanium ions in the zeolite structure have predominantly tetrahedral coordination. However, it appears that the higher chitosan content in the synthesis gel imparted some hydrophilic character to the TS-1 system. Furthermore, the technique adopted for the preparation of the synthesis gel – e.g partially dried or fully dried – appears to affect the amount of framework titanium in the zeolite structure. The calcined form of the chitosan templated TS-1 zeolites exhibited higher cyclohexene conversion compared to the TS-1 material synthesised without this template, but these catalysts showed lower selectivity for cyclohexene epoxide

The reactivity of CO2 on the MgO(100) surface

We investigate the adsorption of CO2 over an MgO(001) terrace, as calculated using an embedded cluster method. We find adsorbed geometries for CO2 on the perfect surface with energies which differ appreciably from previous studies, and observe that it is polarization of the surface rather than the inclusion of electron correlation which leads to this discrepancy. Our results suggest that both monodentate and tridentate carbonate formation on the MgO(001) surface are favourable processes, with the monodentate structure being of lower energy. Adsorption of CO2 is found to be favourable at both F0 and F+ terrace sites, but not at F2+. We also find that chemisorption at oxygen vacancy sites with a single localized electron (F+) could provide a route for the conversion of CO2 to other products, and that this system may be a useful model for other, more effective catalysts