Thermal hysteresis of the dielectric constant and anomalous behaviour of the raman soft mode damping of Cd2Nb2O7

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Abstract
Between 46 K > T > 80 K the dielectric constant of Cd2Nb2O7 shows an anomalous thermal hysteresis which is a characteristic feature of the incommensurate phase. In the Raman spectrum below Ttr = 80 K there appears a soft mode. Decline of its damping slows down between 46 K > T > 80 K, whereas its integrated intensity drops discontinuously at Ttr = 46 K. This soft mode of Cd2Nb2O7 behaves like the soft mode in the incommensurate phase of K2SeO4 and Rb2ZnBr4, and such behaviour of the soft mode damping and integrated intensity is not observed at usual structural phase transitions. These experimental facts suggest the existence of the incommensurate phase in Cd2Nb2O7 between Tc = 46 K and Ti = 80 K. Anomalous behaviour of the soft mode is believed to be due to the contributions of the first-order Raman scattering from the amplitudon (q = 0) and the second-order Raman scattering involving the amplitudon (q = 0) and the phason (q = Ki). When the amplitudon damping is taken as TA ∼ (Ti — T)-1, the relative contribution of the phason (q = Ki) to the soft mode damping may be estimated as ∼6 cm-1 for Cd2Nb2O7 and ∼3 cm-1 for K2SeO4 and Rb2ZnBr4 well above Tc. In the C phase the soft mode merges into the amplitudon (q = 0) of the first-order Raman spectrum at T > (Tc-26 K) for Cd2Nb2O7, T > (Tc - 13 K) for K2SeO4 and T > (Tc - 163 K) for Rb2ZnBr4.

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