Cation and Anion Substitution in β-LiNH 4SO 4: Stabilization of the Ferroelectric Phase

Abstract

The β-LiNH 4SO 4 is ferroelectric from 12 to 187°C. Mixed crystals of the general formula LiNH 4(SO 4) 1−x(SeO 4) x (0 < x ≤ 0.04), Li(NH 4) 1−xK xSO 4 (0 < x ≤ 0.03), and Li(NH 4) 1−xRb xSO 4 (0 < x ≤ 0.50), in which the orthorhombic P2 1 cn symmetry of β-LiNH 4SO 4 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-LiNH 4SO 4 from 195°C to liquid nitrogen temperature. The ferroelectric β-LAS, Li(NH 4) 0.97K 0.03SO 4, Li(NH 4) 0.80Rb 0.20SO 4, and Li(NH 4) 0.50Rb 0.50SO 4 are stable indefinitely at room temperature, but the structure of LiNH 4(SO 4) 0.96(SeO 4) 0.04 changes and the ferroelectric phase disappears after several months.