Phase transition in GeF2driven by change of type of intermolecular interaction

A first-order phase transition from the low-temperature P2${}_{1}$2${}_{1}$2${}_{1}$ ($\ensuremath{\alpha}$) phase to the high-temperature Pnma ($\ensuremath{\beta}$) phase of the GeF${}_{2}$ crystal is studied experimentally and theoretically. GeF${}_{2}$ consists of covalently bonded chains, which arrange to the three-dimensional crystal due to noncovalent intermolecular forces. In addition to relatively strong electrostatic and van der Waals interactions, the $\ensuremath{\alpha}$ phase features anisotropic ``electrostatic bonds'' (reminiscent of hydrogen bonds). During the phase transition these electrostatic bonds melt, which causes expansion of the lattice, driven by excess exchange repulsion, until compensated by the remaining interchain interactions.