Effect of impurities on the successive phase transitions in (Cs1−xRbx)2ZnI4compounds

Abstract

The heat capacity, nuclear quadrupole resonance (NQR) and x-raydiffraction of (Cs1−xRbx)2ZnI4 single crystals have been measured, forx = 0, 0.001,0.005, 0.01, 0.025 and 0.05. The normal to incommensurate (N–Inc) phase transitionat TI,the incommensurate to commensurate (Inc–C) lock-in transition atTLand the structural commensurate monoclinic to triclinic transition atTLT, observed in theparent compound (x = 0),takes place for x = 0, 0.001,0.005 and 0.01. For x = 0.025only TI andTL are detected,while for x = 0.05no transitions were observable. The values ofTI andTL increasewith x whileTLTdecreases and disappears at the concentrationx = 0.025.The effect of defects, besides modifying the transition temperatures, is that of broadeningand lowering the heat capacity anomaly at the lock-in transition until its total quenchingfor x = 0.05.No observable hysteresis is detected in this transition. NQR andx-ray diffraction data show the Inc–C transition up to the highest concentration.We conclude that this phenomenology is caused by weak interaction of theincommensurate modulation with point defects even in the region close to theInc–C transition.