6 . These vacancies compensate for half the negative charge introduced by substituting M(II) for M(III). The remaining charge is compensated by interstitial ammonium cations, which occupy two different sites, the first between two non-defective layers, and the second as part of the large 18 atom water cluster. This water cluster, like that in 2, shows significant disorder, in part because each cluster lies across a centre of symmetry but contains only one NH + 4 cation. The presence of the NH + 4 cation was inferred from charge balance constraints, and which of the electron density peaks was NH + 4 was decided on the basis of two factors: as it is half-occupied it will be the source of the disorder and it will be a better hydrogen bond donor than other water atoms. As we were unable to locate the H-atoms, definitive assignment of which molecule is NH + 4 was not possible. This water cluster also coordinates to the Co sites which would be otherwise undercoordinated due to the Bi(SCN) vacancies.