Contribution to the theoretical and experimental study of the dielectric material Rb-0.7(NH4)(0.3)HSO4

The mixed sulphate acid Rb-0.7(NH4)(0.3)HSO4 presents at room temperature a phase with a structure similar to the one of the parent compounds RbHSO4 a nd NH4HSO4. Different techniques of the study of phase transitions show tha t this compound exhibits five phases with varying temperature and in partic ular the appearance of a new phase (noted IV) in comparison of the phase di agram of parent compounds, this phase takes place in the range of temperatu re T between 330 and 435 K. The symmetry of the mixed compound at room temp erature (phase III) has been performed again by X-ray diffraction on single crystals. The crystallographic characterization of the phase IV is made by X-ray diffraction on powder sample. Crystallographic parameters in this ph ase are then obtained whereas group theory considerations gives the most pr obable space group of this phase. In this work, the phase transitions are s tudied by dielectric measurements. Theoretical study of the behaviour of di electric susceptibility based on LANDAU theory of phase transitions shows a n improper ferroic character of the transition occurring at 330 K with a fa intness index f equal to 2.