The addition of small amounts of Ba, starting from the parent compound Sr 2 CdP 2 (x = 1), causes a sharp increase in b, which is also the layer stacking direction (Figure ). This is explained by the fact that under Ba-poor conditions, Ba occupies the interlayer positions (site 1), a site preference which has also been observed in the Sb analogue of these quaternary structures . This introduction of the larger Ba cation into the interlayer site (site 1) forces the [CdP 2 ] polyanionic layers further apart from one another to accommodate the larger cation. The direct evidence for this expansion can be found in the increase in the Ba1/Sr1-P1/P2 bond lengths (Table ), which represent the base of the square pyramidal coordination.