  It is known that covalently bonded materials undergo nonthermal structure
transformations upon ultrafast excitation of an electronic system, whereas
metals exhibit phonon hardening. Here we study how ionic bonds react to
electronic excitation. Density-functional molecular dynamics predicts that
ionic crystals may melt nonthermally, however, into an electronically
insulating state, in contrast to covalent materials. We demonstrate that the
band gap behavior during nonthermal transitions depends on a bonding type: it
is harder to collapse the band gap in more ionic compounds, which is
illustrated by transformations in Y2O3 vs. NaCl, LiF and KBr.
