It should be noted that unlike for the bimetallic tetrametaphosphates discussed above, which could be thought of as solid solutions between two known M 2 (PO 3 ) 4 compounds, the pure Ca 2 (PO 3 ) 4 compound in this structure is not known, perhaps because Ca(II) is much larger than any of the other M(II) ions listed above that can form this structure. Despite this, it might be possible that Ca(II) could enter into the (M,M') 2 (PO 3 ) 4 structure, and could order in the way seen in reported compounds. However, it is also possible that in fact no Ca has entered the tetrametaphosphate phase, as the PXRD pattern of the bimetallic phases are almost identical to that of the single metal analogue. For example, the proposed CaNi(PO 3 ) 4 simulated PXRD is almost identical to that of the known compound Ni 2 (PO 3 ) 4 .