Crystals with incommensurate Cs2CdI4 and Cs2ZnI4 phases

Polarized-light investigations and measurements were made of the birefringence of Cs 2 CdI 4 and Cs 2 ZnI 4 crystals in the course of consecutive phase transitions Pnma ↔ IC ↔ P2 1 /n ↔ P1 (Cs 2 CdI 4 ) and Pnma ↔ IC ↔ P2 1 /m ↔ P1 (Cs 2 ZnI 4 ), where IC is an incommensurate phase. A pretransition state appeared in the orthorhombic phases of both crystals and this influenced both the temperature dependence of the birefringence and the profiles of lines in nuclear quadrupole resonance spectra. In the case of Cs 2 ZnI 4 it was concluded that in a wide pretransition temperature interval (T−T 1 ≃ 100 K) there were precursor clusters responsible for the coexistence of the lines due to two phases in the resonance spectra and for the deviation of the temperature dependence of the birefringence from linearity