Comparing structural failure of crystal to cell contents??

[ChrisW]

Hello,
I have a compound that when placed in a diamond anvil cell always fails in the same way and at the same pressure. The crystals have a very distinctive shape, a hexagonal prism with one elongated axis, and the fractures always occur perpendicular to the elongated axis.

I think that it is likely that the crystals crack due to defects within the crystal but I have noted that there is a phase change above the pressure that causes the crystal cracking.

I was wondering what is the best way to correlate the crystal cracking to the unit cell contents and back to the original crystal morpholgy?

Any advice would be greatly appreciated.
Chris
p.s. I'm definately not cracking the crystal with the DAC

[Dave]

If the crystal is failing in the same way at the same pressure it could well be undergoing a first-order phase transition with a relatively large accompanying volume change. Alternatively, it could well be that the pressure-transmitting medium is freezing and applying non-hydraustatic pressure to the sample. What fluid are you using and at what pressure does the fracturing occur?

[ChrisW]

Hello Dave,
thanks for the response! I am fairly certain that it is not being caused by the pressure medium as the fracturing occurs at 1.1GPa using the standard methanol: ethanol mix and had been tested on multiple other samples.

I think that a first order phse transition is likely although it is hard to tell conclusively as it is difficult to collect enough data points at this low end of the pressure range, made more difficult by the fact that I have only managed to get 1 crystal to survive the transition. So therefore it is tricky to see if there is a significant volume reduction but it does appear that there is one.

But going back to the orginal point, what is the best way to correlate crystal morphology to the unit cell contents and would this be viable in this case to try and compare structural failure of the crystal to the molecular packing environment?

Many thanks
Chris

[johnewarren]

But going back to the orginal point, what is the best way to correlate crystal morphology to the unit cell contents and would this be viable in this case to try and compare structural failure of the crystal to the molecular packing environment?

Are you thinking about some sort of shear plane? Which allows your structure to cleave?

You may also access that kind of effect by straining the crystal with temperature. If it is a weak point then it just needs the correct motivation.

So relating structure to the crystal there are a couple of ways to do that.

An easy one is to stick the crystal on the OD/Nonius Kappa and get the unit cell (no DAC) then do face indexing. You can now relate the crystal morphology to the unit cell and you already know the unit cell contents and how they relate to the structure. From this you can tell that if the crystal cleaves along a particular direction and that direction relates back to a VdW or weak packing interaction etc.

Does that make any sense?

Also you could try cleaving a crystal on the microscope and see if it reproducibly breaks in the same direction.

If it was me I would:

1. Smush some of my nice crystals under the micrscope with a big blunt scalpel blade to see if they repeated cleave and or break in a similar direction
2. Glue mount a crystal to a tip - glue mount as you want to use the glued side to strain the crystal in the next step
3. Do a variable temperature study on the crystal remembering to take good video/photos to look for evidence of changes and to cross reference any peak splitting in the diffraction against the images.

To disprove the phase change do a powder high pressure study as if it just the material cleaving but not changing into a new form the powder pattern will tell you. May also be possible to do a Raman experiment with powder if no X-ray diffraction available. On your OD you can do powder data but use FIT2D to convert the images to powder patterns, etc.

[ChrisW]

Hello John,
Yes I am looking for a sheer plane to possibly explain why the crystal cleaves in such a manner.

What I had planned to do was to get a nice single crystal, mark it with a red pen for future orientation purposes, mount it on our Diffractometer and collect a data set as well as a video so that I may face index the crystal.

I was then going to load the crystal into a DAC and slowly increase pressure while watching for the failure. Once the crystal fails I was going to relate this back to the original crystal using the marks on the crystal as a reference and get the plane of failure through the crystal.

This seems very similar to your suggestions John so I will give it a go and I might come back later with some more questions.

Many thanks
Chris

[johnewarren]

Sounds good.

I was not certain of the size of same or stability but we use to use nail varnish sometimes to make faces for identification especially when we were breaking up crystals. A different colour on each face then you can track back to the fragment. Great fun under a microscope (smells much better than the normal solvents). Always a risk that the polish/varnish will damage the crystal and or protect it!

[ChrisW]

that is an excellent idea.
Will give it a try.
Thanks for the help John, hope all is well with you!

Chris

[ChrisW]

Hello again,
just thought I would ask few more questions on this topic. I have performed this experiment discussed above i.e. I found a crystal, marked it, collected a structure, face indexed it and then fractured the crystal deliberately in a DAC and worked out the miller indices of the fractures using the earlier crystal data.
Interestingly and as I had hoped the cracks correspond to what I percieve as a weak point in the unit cell. Would this be described as a shear plane? and if so does any one know of any work investigating shear planes in molecular crystals?

Any help would be greatly appreciated
Chris