Evidence for site-specific strong and weak pinning of the modulation wave in the incommensurate phases off randomly quenched Rb2ZnCl4 systems

The influence of randomly quenched disorder in the incommensurate phases of Rb-2(Zn1-xCux)Cl-4 (for x = 0.03), Rb-2(Zn1-xCdx)Cl-4 (for x = 0.03 and 0. 05), Rb-2(Zn1-xHgx)Cl-4 (for x = 0.03 and 0.05) and Rb2Zn(Cl1-xBrx)(4) (for x = 0.01 and 0.03) is investigated via the amplitudon and phason dynamics using Cl-35 nuclear quadrupole resonance studies. Defect pinning at the met al and halogen sites in the prototype compound Rb2ZnCl4? has been attempted for the first time and has yielded novel results. Quenched randomness at t he metal site (Zn) in Rb2ZnCl4 induced strong pinning of the modulation wav e (irrespective of the size of the dopant compared to the host). This is ev ident from a temperature-independent Delta(phi) and consequently T-1 phi un like the case for impurity pinning at the other sites (cation and anion). T he effect is enhanced with increasing concentration of the dopant. This res ult is contrasted with defect pinning at the halogen site (Cl) in Rb2ZnCl4 with Br substitution which induced weak pinning of the modulation wave (tem perature-dependent Delta(phi) and consequently T-1 phi) similarly to substi tution at the cation site as seen from earlier studies. Furthermore, the im purities have been categorized as random-field or random-potential type by evaluating the symmetry parameter (m) associated with the impurity. It is s een that Cu, Cd and Hg are random-field-type impurities inducing strong pin ning of the modulation wave (m < 6; m = 6 for Rb2ZnCl4) while the Br impuri ty is of a random-potential type inducing a weak pinning of the modulation wave.