What is it about?
Lithium ammonium sulfate is a kind of material that presents open networks that favors the generation of polymorphisms with interesting optical, electrical, ferroelectric, ferroelastic and others physical properties. Otherwise, rare earth doping is used to enhance these properties. In this work we perform a complete structural and thermal study of lithium ammonium sulfate doped with Rare earth (Er+3 and Yb+3). Two alpha polytypes are obtained by a slow evaporation process between 298 and 313 K. The results obtained by X-ray diffraction, thermal analysis and dielectric techniques used in this study demonstrate that the alpha-polytypes are stable over a wide range of temperatures and show an irreversible phase transition to the beta-polytype paraelectric. 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.
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Why is it important?
The varied polymorphism and its phase transition sequence for lithium ammonium sulfate (LAS) were not exempt from controverse. The alpha-LAS polytypes usually crystallizes below 293 K and they present a complex correlated structural disorder that can be represented by, at least, three different polytypes. In this work two of their were obtained, above room temperature, by doped with Er3+ and Yb3+. When the ionic radius of the rare earth decreases, the substitution of the ammonium ion favors the more open layered structure. Both structures belong to the space group P21=c and a displacive distortion has been calculated between them. On the other hand, by use of the use of thermal analysis, X-ray diffraction and dielectric techniques we have clarified most of the controversy over the stability of alpha-polytypes. We also offer information that helps explain the nature of alpha-polytypes (with two new structures) and the key to explaining the reconstructive alpha-beta phase transition, through the common structural motif between both structural modifications.
Perspectives
The lithium sulfate family with the chemical formula ALiSO4 (A = Li, Na, K, Rb, Cs, NH4 or Tl) and related compounds presents great flexibility that makes possible thermal rearrangements of their LiO4 and SO4 tetrahedra, involving a variety of polymorphs and phases and phase transitions that are largely influenced by the chemical pressure. Throughout this work it has been seen that doping with rare earth facilitates the formation of more open and stratified structures, in addition to improving its thermal stability. Using this procedure a great diversity of phases based on LiO4 and SO4 tetrahedra, constructed by chains, layers or three-dimensional connections, could be obtained, which have important applications in the study of the ferroic phases, nonlinear and transport properties. Another point to note is the use of the Jonscher parameter (universal dielectric response) for the study of the evolution of order-disorder in phase transition processes. This is a novel contribution for the alternative treatment of this type of processes.
Dr Nanci N. S. P. Sabalisck
Universidad de La Laguna, Seccion Ciencia, Dep. Fisica
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This page is a summary of: Structures and thermal stability of the α-LiNH4SO4 polytypes doped with Er3+ and Yb3+, Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, January 2017, International Union of Crystallography,
DOI: 10.1107/s2052520616019028.
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