New phase of amorphous ice formed by ball milling

Water keeps getting stranger. To the many forms of ice already known, researchers at University College London (UCL), UK, have now added one more ¹. The new phase particularly complicates the picture because it is not crystalline but amorphous and, with a similar density to water itself, looks rather like a frozen snapshot of the liquid state. That contrasts with water’s peculiar expansion when it freezes into regular ice (denoted ice Ih, with a hexagonal structure). The discovery throws a cat among the pigeons for the conventional view of how liquid water is related to its frozen forms.

It already seemed absurdly profligate of nature to give the deceptively simple H₂O molecule at least 22 ways to arrange itself in the solid state, depending on the conditions of temperature and pressure. Changing these parameters alters the balance of enthalpies (due to intermolecular bonding) and entropies (due to ordering) associated with different configurations – but that many possibilities suggests a remarkably delicate balance of such factors.