Structures and thermal stability of the a-LiNH 4 SO 4 polytypes doped with Er 3 + and Yb 3 +

In order to clarify the polymorphism in the lithium sulfate family, LiREx(NH4)1 xSO4 (0.5 x 4.0 mol%, nominal value; RE = Er, Yb and Dy) 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, and , of LiNH4SO4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two new polytypes, which we have named 1and 2-LAS. They present the same space group P21/c and the following relation among their lattice parameters: a2 = c1, b2 = b1, c2 = 2a1 c1. In order to evaluate the stability of the new 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 -polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric -phase above 500 K. In addition, a comparative study of and polytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive ! transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an -habit.