[en] The structure of the alkali-metal hexaiodozirconates(IV) and hexaiodohafnates(IV) is distinguished by its great diversity. The high-symmetry phases with regular [MI₆]^2- anion octahedrons are present, however, in different temperature ranges for all the compounds except the potassium salts. Phase transitions of the first and second kinds were discovered in the hexaiodometalates of sodium and rubidium. In the case of the sodium salts, the high-temperature phases have a lower symmetry than do the low-temperature phases, and the phase transitions are characterized by extensive hysteresis regions