Structures And Thermal Stability Of The Alpha-Linh4so4 Polytypes Doped With Er3+ And Yb3+

In order to clarify the polymorphism in the lithium sulfate family, LiREx(NH4)(1) -xSO4 (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, alpha and beta, of LiNH4SO4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two new alpha polytypes, which we have named alpha(1)- and alpha(2)-LAS. They present the same space group P2(1)/c and the following relation among their lattice parameters: a(2) = -c(1), b(2) = -b(1), c(2) = -2a(1) - c1. In order to evaluate the stability of the new alpha-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 alpha-polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric beta-phase above 500 K. In addition, a comparative study of alpha- and beta- polytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive alpha -> beta transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an alpha-habit.