Two crystals of doubly protonated melaminium salts: melaminium bis(trifluoroacetate) trihydrate and melaminium bis(trichloroacetate) dihydrate.

Crystals of 2,4,6-triamino-1,3,5-triazine-1,3-dium bis(trifluoroacetate) trihydrate, C(3)H(8)N(6)(2+)x2CF(3)COO(-)x3H(2)O, and 2,4,6-triamino-1,3,5-triazine-1,3-dium bis(trichloroacetate) dihydrate, C(3)H(8)N(6)(2+)x2CCl(3)COO(-)x2H(2)O, both contain doubly protonated melamine rings that lie on crystallographic twofold axes. In the former structure, one water molecule also lies on a twofold axis. While the trifluoroacetate compound crystallizes in a centrosymmetric space group, the trichloroacetate is non-centrosymmetric, so it is useful as a material for non-linear optics. The efficiency of second harmonic generation is about three times greater than that of KDP (KH(2)PO(4)). A combination of ionic and donor-acceptor hydrogen-bond interactions link the melaminium(2+) residues with trifluoroacetate or trichloroacetate ions and water molecules to form a three-dimensional network.