Search

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.

The sequence of phase transitions in Cs2ZnI4 was studied by several methods. The incommensurate one-dimensional modulation with the wave vector q Δ = (1/2 + Δ)a* was revealed in the interval between 120–108 K. The nature of incommensurate and lock-in transitions are discussed on the basis of experimental data and numerical calculations of the line shape of the resonance spectra.

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs2CdI4 and Cs2ZnI4. There is the following sequence of phases: commensurate orthorhombic Pnma ↔ incommensurate ↔ monoclinic ferroelastic P21/n ↔ triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs2ZnI4 crystal in a broad pre-transition region (T - T i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).

The sequences of the phase transitions in Cs2CdI4 and Cs2ZnI4 were investigated with the help of the NQR, NMR and X-ray methods. The dielectric and second harmonic generation measurements were used as complementary techniques. Incommensurate phases were found in both substances between the orthorhombic and monoclinic phases. For the first time the complete NQR spectrum was observed in the incommensurate phase of a Cs-containing compound of the β -K2SO4 family. Near the normal-incommensurate transition point (Ti) an unusual behaviour of the NQR line was observed.