Isolated and isothermal susceptibilities of a pseudospin-phonon system

A study is made of the model of a phonon system coupled to a two-level subsystem containing a substitutional impurity. It is shown that the isolated and isothermal susceptibilities may have different forms in both the ferrophase and the paraphase. As a result, there is an additional concentration dependence of the energy of the coupled pseudospin-phonon excitations and a central peak in the neutron or light scattering cross sections. The results are discussed for the example of the ferroelectric $\operatorname{K(H_{1-x}D_x)_2PO_4}$.