Fig. 1: Ice rules on the pyrochlore lattice.
a Hydrogen-bond orientations in water ice and b magnetic moment orientations in rare-earth spin-ices both obey the same ‘two-in-two-out’ rule for each tetrahedral unit of the pyrochlore lattice (black lines). The same rules are thought to apply to cyanide ion orientations in Cd(CN)2, the crystal structure of which is represented in c. Cd atoms shown as green spheres and CN– ions as ellipsoids; the unit cell (outlined in red) corresponds to one octant of the underlying pyrochlore lattice (outlined in black). For clarity, only one of the two interpenetrating pyrochlore lattices is shown. In the average structure of Cd(CN)2, CN– orientations are disordered (grey). This disorder is not random: there is a preference for each Cd to bind two C atoms (white hemiellipsoids) and two N atoms (black hemiellipsoids) in an ice-like ‘two-in-two-out’ arrangement.
