The incommensurate phase of Cs2ZnI4

The incommensurate state and the incommensurate-commensurate (IC-C) transition in a minimal Landau model for Cs2ZnI4 are investigated using rigorous analytical and numerical techniques. The Euler-Lagrange equations are solved numerically using parameters for Cs2ZnI4; the results differ significantly from those obtained using the constant-amplitude approximation. The minimal model predicts that the IC-C transition is first order, in agreement with experiment; the predicted discontinuity of the wavenumber q at the transition is 42% of the maximum q, rather than the measured 50%. The predicted temperature dependence of the IC wavenumber is too strong, and the predicted entropy change at the transition is too small, suggesting that the transition in Cs2ZnI4 is more strongly first order than predicted by the minimal model. Some general results for the asymptotic interaction of discommensurations are also obtained. (author)