Structural properties of A(I)M(III)P2O7-type diphosphates are compared. All compounds were prepared by a "wet" method. A comparative study of the three structural forms i.e. NaFeP2O7-I type (low temperature form), NaFeP2O7-II type (high temperature form) and LiFeP2O7 (structure III), is reported. X-ray diffraction and IR spectroscopy turn out to be the simplest methods to characterize the three structural families. Linear correlations between specific unit cell volume and the volume of cations as (r(A)I)3 and (r(M)III)3 are shown to depend on the type of structure and on the nature of the trivalent ion.