Hi Kyle,
This appears to be using an older scheme than Villars based on Pettifor. You can read more information in Pettifor, D. G. (1984). A chemical scale for crystal-structure maps. Solid State Communications , 51 (1), 31-34 and also mentioned in Pettifor, D. G. (1986). The structures of binary compounds. I. Phenomenological structure maps. Journal of Physics C: Solid State Physics , 19 (3), 285.
I’m not sure why this wasn’t referenced appropriately, I didn’t add the data. Thanks for bringing this to my attention, I’ll make sure a reference is added.
Best,
Matt
e.g. see this illustration from from The Structures of Binary Compounds, Hafner et al.
