Stress dependence of Tc for the incommensurate phase transition in K2SeO4

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 T_c. 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. T_c 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.