“…According to our structural results, there are two classes of such compounds: the triclinic group (P1) containing the diphosphates with Ni [21] and Mg [23], and orthorhombic group (Pnma) for those with Mn [18], Co [19], and Cu, Ni and Zn [22]. Numerous other mixed hydrogen-and dihydrogen-diphosphates have been reported in the literature during the last few years ( [24] and references herein) and many aspects, related to the synthesis methods and physico-chemical properties of diphosphates have been intensively investigated [25][26][27]. The present work is a continuation of our investigations in the system (A, M) x (H 2 P 2 O 7 ) y ·nH 2 O (A and M, as previously mentioned).…”