Actually, Intel has been almost totally opaque about what 3D XPoint is, except it's not a phase change media like that is used in optical disks, and per https:

Actually, Intel has been almost totally opaque about what 3D XPoint is, except it's not a phase change media like that is used in optical disks, and per https://news.ycombinator.com/item?id=10366907 https://news.ycombinator.com/item?id=10366907 "We learned that 3D XPoint used a bulk material property change (rather than a stored charge) to address each cell."