Recently, there's been some talk of potential lab results in an attempt to create pure, solid hydrogen in a stable form, which, if true, would be a first. Much of the excitement seems to be about the fact that this 'metallic hydrogen' is meta-stable, specifically at normal temperatures and pressures. Whether this is true, or the researchers are simply observing some silica from the surface coating on the diamond anvil, remains to be seen.

What is it about an element or material that makes it meta-stable rather than just stable? Is it the shape of its atoms (orderly lattice vs random alignment)? Or an energy state of a material?

I've read these other questions on the recent solid hydrogen report, especially the last one, but I'm not sure whether they address my question about what causes metallic hydrogen, or another material, to be meta-stable in general.

Recently, there's been some talk of potential lab results in an attempt to create pure, solid hydrogen in a stable form, which, if true, would be a first. Much of the excitement seems to be about the fact that this 'metallic hydrogen' is meta-stable, specifically at normal temperatures and pressures. Whether this is true, or the researchers are simply observing some silica from the surface coating on the diamond anvil, remains to be seen.

What is it about an element or material that makes it meta-stable rather than just stable? Is it the shape of its atoms (orderly lattice vs random alignment)? Or an energy state of a material?

I've read these other questions on the recent solid hydrogen report, especially the last one, but I'm not sure whether they address my question about what causes metallic hydrogen, or another material, to be meta-stable in general.