  Recently, evidence for positronium (Ps) in a Bloch state in self-assembled
metal--organic frameworks (MOFs) has been reported [Dutta et al., Phys. Rev.
Lett. 110, 197403 (2013)]. In this paper, we study Ps emission into vacuum from
four different MOFs crystals: MOF-5, IRMOF-8, FMA and IRMOF-20. Our
measurements of Ps yield and emission energy into vacuum provide definitive
evidence of Ps delocalization. We determine with a different technique Ps
diffusion lengths in agreement with the recently published results.
Furthermore, we measure that a fraction of the Ps is emitted into vacuum with a
distinctly smaller energy than what one would expect for Ps localized in the
MOFs' cells. We show that a calculation assuming Ps delocalized in a
Kronig--Penney potential reproduces the measured Ps emission energy.
