Cation and Anion Substitution in β-LiNH4SO4: Stabilization of the Ferroelectric Phase

The β-LiNH4SO4 is ferroelectric from 12 to 187°C. Mixed crystals of the general formula LiNH4(SO4)1−x(SeO4)x (0 < x ≤ 0.04), Li(NH4)1−xKxSO4 (0 < x ≤ 0.03), and Li(NH4)1−xRbxSO4 (0 < x ≤ 0.50), in which the orthorhombic P21cn symmetry of β-LiNH4SO4 is maintained, were studied. Differential scanning calorimetry, X-ray powder diffraction, and 15N CP MAS nuclear magnetic resonance studies suggest that the anion substitution hardly modifies the temperature range of the existing ferroelectric phase. In contrast, the cation substitution stabilizes the ferroelectric phase of β-LiNH4SO4 from 195°C to liquid nitrogen temperature. The ferroelectric β-LAS, Li(NH4)0.97K0.03SO4, Li(NH4)0.80Rb0.20SO4, and Li(NH4)0.50Rb0.50SO4 are stable indefinitely at room temperature, but the structure of LiNH4(SO4)0.96(SeO4)0.04 changes and the ferroelectric phase disappears after several months.