The beta-LiNH4SO4 is ferroelectric from 12 to 187 degrees C. Mixed crystals of the general formula LiNH4(SO4)(1-x)(SeO4)(x) (0 < x less than or equal to 0.04), Li(NH4)(1-x)KxSO4 (0 < x less than or equal to 0.03), and Li(NH4)(1-x)RbxSO4 (0 < x less than or equal to 0.50), in which the orthorhombic P2(1)cn symmetry of beta-LiNH4SO4 is maintained, were studied. Differential scanning calorimetry, X-ray powder diffraction, and N-15 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 beta-LiNH4SO4 from 195 degrees C to liquid nitrogen temperature. The ferroelectric beta-LAS, Li(NH4)(0.97)K0.03SO4, Li(NH4)(0.80)Rb0.20SO4, and Li(NH4)(0.50)Rb0.50SO4 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.