The α phase is derived from the parent phase by the incorporation of Zn 2+ interstitials in half of all layers. More completely, it is built from two different layers, a Prussian Blue-like layer and CdI 2 -like layer, which stack in an alternating fashion along the a axis to form a three-dimensional framework [Fig. ]. The Prussian Blue-like layer has the composition ZnBi(SCN) 2 and can be described as a {100} layer of a hypothetical 'ZnBi(NCS) 6 ' analogue of the Fe[Bi(NCS) 6 ] structure [Fig. ]. As the Zn is four-coordinated, its bonding requirements are saturated within the layer, and so the layer is connected to the layers above and below through the Bi atom. The second layer has composition Zn 2 Bi(NCS) 6 • NCS and a pseudo-CdI 2 structure, where the Bi ('Cd') atoms approximately form a triangular lattice, in which the interstices are occupied by Zn ('I') atoms [Fig. ]. In this layer, the Bi atoms are saturated by bonds within the layer, and so the connection to the other layer occurs through the Zn atoms, which are bound to an additional thiocyanate The predominant size of ring in the β -Zn 3 Bi 2 (NCS) 12 phase is the sixteen-membered Zn 2 Bi 2 (NCS) 4 ring (four-membered ring, considering only metal cations) common to the M[Bi(SCN) 6 ] perovskite. The ring-statistics are otherwise very different [Fig. ] and the structure cannot be easily decomposed into simple crystal-chemical units. The coordination geometry for both α and β phases remain similar, and there is only slightly larger variation in the octahedral coordination of BiS 6 distortions in the β phase.