in the zeolite framework due to the assumption of the equivalence of Si and A1 ions and their environments,16 and (3) the relatively small fraction of threefold axis scattering matter due to the zero-coordinate cation, make this determination the least definitive of the three, and worthy of further concern. However, the detailed agreement of the positional and thermal parameters of the zero-coordinate cation found here with those found in the previous two structures, coupled with newer results in dehydrated TI12-A,17 affirms the present result and allows it to constitute a reconfirmation of the existence of zero coordination.
ConclusionZero-coordinate cations, as have been found in dehydrated K12-A,1 dehydrated Rbi|NarA,3•4 and now in dehydrated CS7K5-A, occur when well-defined conditions involving the zeolite and the cations are satisfied. Briefly, zero coordination occurs when all of the coordination sites available to large cations in the zeolite framework are filled before all anionic charges of the framework are balanced. By difference, one cation per unit cell remains uncoordinated. This has been discussed earlier3•4 in greater detail.As a working definition, similar to the distance criterion used to decide whether a significant hydrogen bonding interaction exists, an ion is considered not coordinated to another ion if the distance between them exceeds the sum of their corresponding radii by more than 1.0 Á.3 At least, the bond order is much less than one. On this basis, the ion at K(3) is termed zero coordinate, zero being the sum of integers, all zero, describing its bond orders to its nearest neighbors.3 For this particular structure, the distance discrepancy, 1.75 Á, is substantially larger that the value of 1.0 Á used in the criterion.It appears that zero-coordinate cations can be found in any dehydrated sample of KrRb, Cs,-A, where r + s + t = 12 for charge balance. As in Rbi ¡Na-, zero coordination may persist when one or more smaller exchangeable cations are present per unit cell.
