Storing data for a really really long time
If you plan to store something that long, why are you using a rewritable medium at all? Rewriting = reversible = inherently won't last as long. At least go for permanently fusible ROM to store the data which gives you a fighting chance.
And just powering on FRAM (or flash, or any other rewritable medium for that matter) won't do anything. You'd have to power it up, reread and rewrite it all so all that stuff has to survive too, plus power source.
Now, a plain, undoped, silicon wafer, which is just a giant crystal, with something etched into might do just as well as a properly chosen metal alloy with something engraved into it. But to rely on the semiconductor properties? No. Higher complexity = more to go wrong. I believe I read that dopants drift which is a huge Achilles' heel for semiconductors.
But if you do go the semiconductor route, the only thing that even remotely has a fighting chance are permanently fusible links (which in the end is not too different from just permanently etching some gold traces into a silicon depending on how you go about it...for the data storage part at least...not the mechanism to read it out).
Something like a pressed CD seems like it would last a long time...or even better...the glass master that was used to make it. And like permanently fusible links, it only addresses the data storage part, not the data readout mechanism.