Acidic Salt of the Decaoxodiperiodic Acid: NiH 4 I 2 O 10 ×6H 2 O, ZnH 4 I 2 O 10 ×6H 2 O and MgH 4 I 2 O 10 ×6H 2 O; Crystal Structures, Vibrational Spectra and Thermal Decomposition.