Phase transitions in the mixed crystals LiRb_1-x(NH₄)_xSO₄

Abstract

Abstract The successive phase transitions of mixed crystal systems of LiRbSO4-LiNH4SO4 were studied by X-ray diffraction, dielectric measurement, thermal analysis and SHG detection as functions of temperature and ammonium concentration x. Although the ammonium ion is almost of the same size as the rubidium ion, a small amount of ammonium reduces the transition temperatures and the incommensurate and the fivefold commensurate phases of LiRbSO4 fade out for x>0.1. With further replacing rubidium by ammonium, the antiferroelectric phase of LiRbSO4 does not appear and the phase sequence is similar to LiNH4SO4 for x>0.25. The phase diagram and the modulated structure are discussed in reference to an Ising model with long range interactions.