Why was the structure of the niobium silicate AM-11 so difficult to solve?

The synthesis of the polycrystalline niobium silicate catalyst AM-11 was first reported in 1998 [1], but its structure proved to be elusive. In 2007 we received two samples from the Aveiro group. At the time, we were looking for a material suitable for the application of the texture method of structure solution, and AM-11 seemed to be ideal for this purpose. One of the samples had needle and the other platelet morphology, and textured samples could be prepared in both cases. The conventional powder diffraction pattern could be indexed on a hexagonal, an orthorhombic or a monoclinic unit cell, so this was the first issue to be resolved. The texture measurements quickly revealed that the crystal system was orthorhombic, but the structure resisted solution. We then tried applying the precession electron diffraction technique in combination with high-resolution powder diffraction data, but beyond confirming the orthorhombic symmetry, these data did not help us to solve the structure. Another attempt was made with a new sample and an improved texture setup, but to no avail. Rotation electron diffraction data and high-resolution transmission electron microscopy images showed that some disorder was present and helped to define the space group, but the structure remained a mystery. The powder charge-flipping routine in Superflip [2], yielded tantalizingly clear electron density maps, but they could not be interpreted sensibly. The unit cell parameters were seen to be related to those of the titanium silicate zorite [3] (one axis doubled in AM-11), so the problem was taken up once again last year. By starting with a simplified zorite framework structure with Nb in place of Ti, and performing what amounts to manual Fourier recycling, the surprisingly simple structure (1Nb, 3 Si, 9 O), which is significantly different from zorite, finally revealed itself. There is some stacking disorder, but the structure is otherwise innocuous. What made it so difficult to solve?

When two bad data sets are better than one

High-resolution synchrotron X-ray powder diffraction (SXPD) data alone are sometimes not enough to solve the structure of a complex polycrystalline material. Such was the case for the high-silica zeolites SSZ-61 and SSZ-87, where combining data from different sources, in particular XPD and electron microscopy, was vital to success. For SSZ-61, the SXPD data feature broad peaks and a resolution of ca. 1.2 Å. Although the pattern could be indexed, structure determination failed both with the charge flipping routine in SUPERFLIP [1] and with the zeolite-specific program FOCUS [2]. The unit cell parameters and HRTEM images indicated a relationship with ZSM-12 (MTW) and SSZ-59 (SFN), so several models derived from these two frameworks were built. Eventually, after considering Si-29 MAS NMR data and the size of the organic structure directing agent (SDA), a framework model that fits all the data emerged. To complete the structure, the SDA was included as a rigid-body, and its location and orientation optimized using simulated annealing. Subsequent Rietveld refinement confirmed the structure. In contrast to SSZ-61, the SXPD pattern for SSZ-87 was quite good, and it could be indexed with a C-centered cell. However, structure solution failed, probably because of the very high degree of reflection overlap (93%). Therefore, rotation electron diffraction (RED) data [3] were collected, but they proved to be of low resolution and poor quality. Only 2 of the 7 data sets could be indexed, and these had different unit cells. Neither fit the XPD pattern directly. The problem was traced to large errors in the RED cell parameters, and eventually one RED cell could be transformed to one similar to the SXPD cell. The RED data with this cell was only 15% complete up to a resolution of 1.22 Å. Even so, the structure could be solved using a recently developed version of FOCUS that works with ED data. The SDA was found as for SSZ-61, and the structure then confirmed by Rietveld refinement.

Phase Identification by Rotation Electron Diffraction from Multiphasic Samples

Electron Crystallography is an important technique for studying micro- and nano-sized crystals[1]. Crystals considered as powder by X-ray diffraction behave as single crystals by electron diffraction. Recently we developed a new method, Rotation Electron Diffraction (RED) for three-dimensional diffraction data collection by combining electron beam tilt with goniometer tilt on a transmission electron microscope (TEM)[2]. Here we apply the RED method on an unknown oxide sample in a Ni-Se-Cl-O system, which may show special physical properties, for example magnetic properties. The crystals in the sample were less than a few micrometers in sizes. Powder X-ray diffraction patterns of the sample could not be indexed by existing known phases. The sample was thus studied by TEM. Five 3D RED datasets were collected from five crystals with different morphologies using the software package RED. The data processing was also performed using the software RED-processing. The unit cell and space groups of all the five phases were obtained using RED and the structures of four of five phases were solved. Nearly all peaks in the powder X-ray diffraction pattern could be indexed using these five phases. To conclude, five phases from a powder sample have been identified using RED. RED is a powerful method for phase identification of multiphasic samples with nano-sized crystals.

Correlated defects in group 4 metal-organic frameworks

The importance of defects for inorganic functional framework materials is well established, being crucial for properties from relaxor ferroelectricity to superconductivity. The corresponding study of defects in metal-organic frameworks (MOFs) is still however in its infancy. Recent studies have established that ligand-absence defects can be controllably introduced into frameworks and that these defects can drastically improve the material properties, but have so far shown no evidence of correlation between defects. Much of this research has focussed on UiO-66, a zirconium dicarboxylate MOF that was amongst the first very stable MOFs to be discovered.[1] As a result of its stability, it and its derivatives have been investigated for a wide range of properties including photo- and Brønsted acid catalysis, sensing and gas sorption properties. The ability to introduce defects has been demonstrated to substantially enhance both the sorption and catalytic properties of UiO-66.[2][3] We have demonstrated, using a combination of powder X-ray diffraction, total scattering and electron diffraction measurements, that UiO-66 can be engineered, under the appropriate synthetic conditions, to accommodate correlated defect nanodomains. These correlations offer exciting opportunities for manipulating the physical properties, including mass transport, chemical activity and mechanical flexibility.

Quasicrystal approximants solved by Rotation Electron Diffraction (RED)

We have developed single crystal electron diffraction for powder-sized samples, i.e. < 0.1μm in all dimensions. Complete 3D electron diffraction is collected by Rotation Electron Diffraction (RED) in about one hour. Data processing takes another hour. The crystal structures are solved by standard crystallographic techniques. X-ray crystallography requires crystals several micrometers big. For nanometer sized crystals, electron diffraction and electron microscopy (EM) are the only possibilities. Modern transmission EMs are equipped with the two things that are necessary for turning them into automatic single crystal diffractometers; they have CCD cameras and all lenses and the sample stage are computer-controlled. Two methods have been developed for collecting complete (except for a missing cone) 3D electron diffraction data; the Rotation Electron Diffraction (RED) [1] and Automated Electron Diffraction Tomography (ADT) by Kolb et al. [2]. Because of the very strong interaction between electrons and matter, an electron diffraction pattern with visible spots is obtained in one second from a submicron sized crystal in the EM. By collecting 1000-2000 electron diffraction patterns, a complete 3D data set is obtained. The geometry in RED is analogous to the rotation method in X-ray crystallography; the sample is rotated continuously along one rotation axis. The data processing results in a list of typically over 1000 reflections with h,k,l and Intensity. The unit cell is typically obtained correctly to within 1%. Space group determination is done as in X-ray crystallography from systematically absent reflections, but special care must be taken because occasionally multiple electron diffraction can give rise to very strong forbidden reflections. At +/-60° tilt with 0.1° steps, a complete data collection will be some 1200 frames. With one second exposures this takes about one hour. There is no need to align the crystal orientation. The reciprocal lattice can be rotated and displayed at any direction of view. Sections such as hk0, hk1, hk2, h0l and so on can easily be cut out and displayed. We have solved over 50 crystal structures by RED in one year. These include the most complex zeolites ever solved and quasicrystal approximants, such as the pseudo-decagonal approximants PD2 [3] and PD1 in AlCoNi. Observed and calculated sections of reciprocal space (cut at 1.0Å) are shown in Fig. 1. Notice the 10-fold symmetry of strong reflections.