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- Title
Structures and thermal stability of the α-LiNH<sub>4</sub>SO<sub>4</sub> polytypes doped with Er<sup>3+</sup> and Yb<sup>3+</sup>.
- Authors
Sabalisck, N. P.; Guzmán-Afonso, C.; González-Silgo, C.; Torres, M. E.; Pasán, J.; del-Castillo, J.; Ramos-Hernández, D.; Hernández-Suárez, A.; Mestres, L.
- Abstract
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, 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 β2-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 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.
- Subjects
CRYSTAL structure; THERMAL stability; POLYMORPHISM (Crystallography)
- Publication
Acta Crystallographica Section B: Structural Science, Crystal Engineering & Materials, 2017, Vol 73, Issue 1, p122
- ISSN
2052-5192
- Publication type
Article
- DOI
10.1107/S2052520616019028