The curious case of the unexpected neutron diffraction impurity

Le résumé fourni par la source

As a beamline scientist one gets handed mysteries, and just once in a while they have clean and clear solutions. Along with Judy Stalick and Qingzheng Huang, our job was to look after the care and feeding of the NCNR BT-1 diffractometer and to assist the users delivering those samples. Once a user brought a sample of Ca2IrD5 that had been demonstrated to be pure from an x-ray diffraction pattern collected (with difficulty, as the sample decomposes in air) at the user's home institution. We took a "quick" scan of the sample on BT-1 and there were a huge number of large and unexpected peaks. These peaks were from the CaD2 starting material (which interestingly has a much lower symmetry structure than the product). The sample was made by mixing the CaD2 material with Ir metal and heating in the presence of D2 gas. But, why were we seeing unreacted starting materials in the neutron pattern not present in the x-ray pattern? The solution to the mystery and how the problem was solved will be presented at the conference. Once that was complete, we went on to study the Ca2(Rh,Ir)D5 solid solution. That gave a clean diffraction pattern that could be well-fit in Fm3m at ambient temperatures with the expected partial occupancies, but at lower temperatures partially transformed to I4/mmm, but the Fm3m structure persisted. Again, plenty of head-scratching ensued until this mystery too was solved, as will be also presented.

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