Heat capacity measurement of the successive phase transitions in Cs2ZnI4

The heat capacity of a Cs2ZnI4 single crystal has been measured by means of AC calorimetry. Three anomalies have been detected corresponding to a normal to inconmensurate phase transition at TI = 116.9±0.1 K, an incommensurate to commensurate lock-in transition at TL = 107.58±0.05 K, and a structural transition from a commensurate orthorhombic to a monoclinic phase at TC = 93.77±0.05 K with thermal hysteresis. The lock-in transition has the peculiarity of being discontinuous but not showing any measurable hysteresis. After comparison with the Landau model predictions for the heat capacity, derived for the k Δ = ( 1 2 + Δ) a ∗ case, it is concluded that Cs2ZnI4 lies close to the Lifschitz point of coexistence of the normal, incommensurate and commensurate phases.