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Structures and thermal stability of the α‐LiNH 4 SO 4 polytypes doped with Er 3+ and Yb 3+
Author(s) -
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.
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520616019028
Subject(s) - crystallite , crystallography , diffraction , materials science , x ray crystallography , thermal stability , phase transition , analytical chemistry (journal) , ionic radius , doping , crystal structure , lithium sulfate , dielectric , crystal (programming language) , polymorphism (computer science) , atmospheric temperature range , ion , chemistry , ionic bonding , condensed matter physics , physics , optics , thermodynamics , programming language , biochemistry , optoelectronics , organic chemistry , chromatography , computer science , genotype , gene
In order to clarify the polymorphism in the lithium sulfate family, LiRE x (NH 4 ) 1 − x SO 4 (0.5 ≤ x ≤ 4.0 mol%, nominal value; RE = Er 3+ , Yb 3+ and Dy 3+ ) 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 LiNH 4 SO 4 (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 α 1 ‐ and α 2 ‐LAS. They present the same space group P 2 1 / c and the following relation among their lattice parameters: a 2 = − c 1 , b 2 = − b 1 , c 2 = −2 a 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.