Structures and thermal stability of the α-LiNH₄SO₄ polytypes doped with Er³⁺ and Yb³⁺

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• 作者：N. P. Sabalisck ; C. Guzmá ; n-Afonso ; C. Gonzá ; lez-Silgo ; M. E. Torres ; J. Pasá ; n ; J. del-Castillo ; D. Ramos-Herná ; ndez ; A. Herná ; ndez-Suá ; rez and L. Mestres
• 关键词：polytypism ; polymorphism ; phase transitions ; lithium ammonium sulfate
• 刊名：Acta Crystallographica Section B
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：73
• 期：1
• 页码：122-133
• 全文大小：1952K
• ISSN：2052-5206

文摘

In order to clarify the polymorphism in the lithium sulfate family, LiRE_x(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 α₂-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.