Stress dependence of Tc for the incommensurate phase transition in K2SeO4

Abstract

Abstract We have measured the uniaxial stress dependence of the frequency of the zone center A1 soft mode in the low temperature phase of K2SeO4. The frequency shows a strong decrease with stress. We infer from this a similar decrease of the incommensurate phase transition temperature Tc. This behavior is explained by detailed lattice dynamical calculations for K2SeO4 which reveal the unusual balance between Coulomb and short-range forces in this structure. Our calculations show that this balance is changed by stress in such a way that the quasi-harmonic normal mode frequencies harden drastically. Tc is reduced by this purely volume dependent effect. This unusual balance between Coulumb and short-range forces also provides a natural explanation for the occurence of an incommensurate phase transition in K2SeO4. We infer that similar transitions are likely in isomorphous structures.