Today's paper summary comes from David Sholl.
Far more is known about defects in zeolites than about defects in MOFs, although the structural similarities between ZIFs and zeolites strongly hint that defects should be common in ZIFs and, by extension, other kinds of MOFs. Defects are typically thought of as introducing disorder or randomness into crystals, but this does not always have to be the case. Baerlocher and colleagues give a nice example of a high silica zeolite structure that includes an ordered set of defects in the crystal structure of the zeolite SSZ-74 (Baerlocher et al. Nature Materials, DOI: 10.1038/nmat2228) . Much of their paper deals with the technical details of solving the crystal structure of this complicated zeolite, but the main conclusion is that the structure includes a set of ordered Si vacancies in each unit cell. It is important to note that a perfectly plausible model of a defect-free silica material with the same framework could be constructed, so the existence of the ordered vacancies is a consequence of the complex crystallization process that generates SSZ-74.
This paper doesn't give a direct example of how the Si vacancies in the structure affect the performance or properties of the material. There are examples, however, of point defects such as oxygen vacancies giving zeolites properties such as luminescence that are not possible with defect-free materials (Bai et al., J. Luminesence, DOI:10.1016/j.jlumin.2013.08.01) .
Far more is known about defects in zeolites than about defects in MOFs, although the structural similarities between ZIFs and zeolites strongly hint that defects should be common in ZIFs and, by extension, other kinds of MOFs. Defects are typically thought of as introducing disorder or randomness into crystals, but this does not always have to be the case. Baerlocher and colleagues give a nice example of a high silica zeolite structure that includes an ordered set of defects in the crystal structure of the zeolite SSZ-74 (Baerlocher et al. Nature Materials, DOI: 10.1038/nmat2228) . Much of their paper deals with the technical details of solving the crystal structure of this complicated zeolite, but the main conclusion is that the structure includes a set of ordered Si vacancies in each unit cell. It is important to note that a perfectly plausible model of a defect-free silica material with the same framework could be constructed, so the existence of the ordered vacancies is a consequence of the complex crystallization process that generates SSZ-74.
This paper doesn't give a direct example of how the Si vacancies in the structure affect the performance or properties of the material. There are examples, however, of point defects such as oxygen vacancies giving zeolites properties such as luminescence that are not possible with defect-free materials (Bai et al., J. Luminesence, DOI:
I looked up reference 6 to look at the charge flipping scheme, but I still don't quite understand what exactly is going on. Is someone able to explain the structure-factor amplitudes and the phase in Layman's terms? I think if I had a better grasp of where these terms are coming from and what they represent physically that I could follow the scheme better.
ReplyDeleteDoes the user defined threshold have to be set to a small positive number? It seems that the main purpose of the threshold value is the make the negative electron densities positive. So why not set it to be zero so that only the sign of all the negative electron densities are flipped?
Also, is the inverse FT an approximation? I thought it would need to be an integral over all possible values?
One more thing, does anyone know if it is possible to type symbols into these comments? Similar to something one would do in a LaTeX document? I tried $\delta$ (among other variations), but as you can see it does not work.