High Conducting Heterovalent Substituted NASICON-like Phases in ScPO 4 -Na 3 PO 4 Quasibinary System

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High Conducting Heterovalent Substituted NASICON-like Phases in ScPO4-Na3PO4 Quasibinary System Anna Potapova1, Mariya Zhuravleva1, Irina Smirnova1, Felix Spiridonov2, Galina Zimina1, and Andrey Novoselov1 1 Department of Chemistry and Chemical Engineering for Rare and Dispersed Elements, Lomonosov Moscow State Academy of Fine Chemical Technology, Moscow, 119571, Russian Federation 2 Department of Chemistry, Lomonosov Moscow State University, Moscow, 119899, Russian Federation ABSTRACT Phase equilibria in quasibinary system ScPO4-Na3PO4 and formation of heterovalent Zrsubstituted solid solutions (up to 10 mol%) for Sc3+ in Na3Sc2(PO4)3 complex phosphate were studied by ceramic technique at 1050°C. Obtained samples were investigated with X-ray powder diffraction and impedance spectroscopy. Zr-substituted (10 mol%) Na3Sc2(PO4)3 has ionic conductivity of 3.18⋅10-1 S/cm at 300°C. INTRODUCTION The discovery of Na1+xZr2SixP3−xO12 (later named as NASICON-NAtrium SuperIonic CONductor) by Hong, Goodenough and Kafalas in middle 1970th [1,2] made a tremendous contribution to development of solid-state ionics. The concept of high ionic conductivity based on migration of Na cation through three-dimensional system of channels has been working well already 30 years and was leading to discovery of a huge list of new compounds. Internet search using for example Google search engine [3] gives over 18000 references, even though big part of the materials reported does not contain Na [4] and some of them do not belong to super ionic conductors at all [5]. High alkali ion-based conductivity and good cation-exchange properties of NASICON family members, as well as fast growing market of potential applications such as battery electrodes, fuel cells and chemical sensors, have motivated advances in NASICON structurally related compounds. There is a big group of NASICON-like ionic conductors of general composition M(I)3M(III)2(PO4)3 where M(I) = Li and Na, and M(III) = Sc, Cr, Fe and In [6,7]. The wellknown representative of this group, Na3Sc2(PO4)3, has significant ionic conductivity σ = 10-1 S/cm achieved only at elevated temperature of over 300°C thus limiting its practical application. Another complication is complex polymorphism of Na3Sc2(PO4)3, it has 3 polymorphous modifications: α → β at ~50°C and β → γ at ~168°C [8]. It should be also noted that high price of scandium-containing compounds does not serve well their wide application. We have demonstrated the possibility to stabilize crystal structure of Na3Sc2(PO4)3 by solid solutions formation in ScPO4-Na3PO4-Li3PO4 quasiternary system [9]. Nevertheless, the obtained conductivity data were not so outstanding. On the other hand, we have recently demonstrated that heterovalent substitution is a viable approach to obtain compositions with high ionic conductivity [10].

In this paper, we will present the results on phase equilibria in quasibinary system ScPO4Na3PO4 and heterovalent Zr-substitution into Na3Sc2(PO4)3 trying to find a way to obtain promising conducting materia