In order to clarify the polymorphism in the lithium sulfate family, LiRE _x (NH₄)_{1 − x} SO₄ (0.5 ≤ x ≤ 4.0 mol%, nominal value; RE = Er³⁺, Yb³⁺ and Dy³⁺) crystals were grown from aqueous solution by slow evaporation between 298 and 313 K. The doping of the samples allowed us to obtain two polymorphic forms, α and β, of LiNH₄SO₄ (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two new α-polytypes, which we have named α₁- and α₂-LAS. They present the same space group P2₁/c and the following relation among their lattice parameters: a ₂ = −c ₁, b ₂ = −b ₁, c ₂ = −2a ₁ − c ₁. In order to evaluate the stability of the new α-polytypes, we performed thermal analysis, X-ray diffraction and dielectric spectroscopy on single crystals and polycrystalline samples over the cyclic temperature range: 190 → 575 → 190 K. The results obtained by all the techniques used in this study demonstrate that α-polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric β-phase above 500 K. In addition, a comparative study of α- and β-polytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive α → β transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an α-habit.