Effect of impurities on the successive phase transitions in (Cs1-xRbx)2ZnI4 compounds

The heat capacity, nuclear quadrupole resonance (NQR) and x-ray diffraction of (Cs1-xRbx)2ZnI4 single crystals have been measured, for x=0, 0.001, 0.005, 0.01, 0.025 and 0.05. The normal to incommensurate (N-Inc) phase transition at TI, the incommensurate to commensurate (Inc-C) lock-in transition at TL and the structural commensurate monoclinic to triclinic transition at TLT, observed in the parent compound (x=0), takes place for x=0, 0.001, 0.005 and 0.01. For x=0.025 only TI and TL are detected, while for x=0.05 no transitions were observable. The values of TI and TL increase with x while TLT decreases and disappears at the concentration x=0.025. The effect of defects, besides modifying the transition temperatures, is that of broadening and lowering the heat capacity anomaly at the lock-in transition until its total quenching for x=0.05. No observable hysteresis is detected in this transition. NQR and x-ray diffraction data show the Inc-C transition up to the highest concentration. We conclude that this phenomenology is caused by weak interaction of the incommensurate modulation with point defects even in the region close to the Inc-C transition